% Building the JDK

## TL;DR (Instructions for the Impatient)

If you are eager to try out building the JDK, these simple steps work most of
the time. They assume that you have installed Git (and Cygwin, MSYS2 or WSL if
running on Windows), and want to clone the main-line JDK repository.

 1. [Get the complete source code](#getting-the-source-code): \
    `git clone https://git.openjdk.org/jdk`

 2. [Run configure](#running-configure): \
    `bash configure`

    If `configure` fails due to missing dependencies (to either the
    [toolchain](#native-compiler-toolchain-requirements), [build tools](
    #build-tools-requirements), [external libraries](
    #external-library-requirements) or the [boot JDK](#boot-jdk-requirements)),
    most of the time it prints a suggestion on how to resolve the situation on
    your platform. Follow the instructions, and try running `bash configure`
    again.

 3. [Run make](#running-make): \
    `make images`

 4. Verify your newly built JDK: \
    `./build/*/images/jdk/bin/java -version`

 5. [Run basic tests](#running-tests): \
    `make test-tier1`

If any of these steps failed, or if you want to know more about build
requirements or build functionality, please continue reading this document.

## Introduction

The JDK is a complex software project. Building it requires a certain amount of
technical expertise, a fair number of dependencies on external software, and
reasonably powerful hardware.

If you just want to use the JDK and not build it yourself, this document is not
for you. See for instance [OpenJDK installation](https://openjdk.org/install)
for some methods of installing a prebuilt JDK.

## Getting the Source Code

Make sure you are getting the correct version. At the [OpenJDK Git
site](https://git.openjdk.org/) you can see a list of all available
repositories. Commonly used repositories are:

* The [JDK Project](https://openjdk.org/projects/jdk) (the main-line currently
  in development): https://git.openjdk.org/jdk

* The [JDK Updates Project](https://openjdk.org/projects/jdk-updates/), which
  has one repository per update release, e.g. https://git.openjdk.org/jdk17u
  for JDK 17.

If you want to build an older version, e.g. JDK 17, it is strongly recommended
that you use the JDK Updates repository, e.g. the `jdk17u`, which contains
incremental updates, instead of the JDK Project repository `jdk17`, which was
frozen at JDK 17 GA.

If you are new to Git, a good place to start is the book [Pro
Git](https://git-scm.com/book/en/v2). The rest of this document assumes a
working knowledge of Git.

### Special Considerations

For a smooth building experience, it is recommended that you follow these rules
on where and how to check out the source code.

* Do not check out the source code in a path which contains spaces. Chances are
  the build will not work. This is most likely to be an issue on Windows
  systems.

* Do not check out the source code in a path which has a very long name or is
  nested many levels deep. Chances are you will hit an OS limitation during the
  build.

* Put the source code on a local disk, not a network share. If possible, use an
  SSD. The build process is very disk intensive, and having slow disk access
  will significantly increase build times. If you need to use a network share
  for the source code, see below for suggestions on how to keep the build
  artifacts on a local disk.

* UTF-8 support is needed to compile the JDK. On Unix systems, this typically
  means that the `C.UTF-8` or `en_US.UTF-8` locale needs to be available. For
  Windows users, please see the section on [Locale
  Requirements](#locale-requirements) below.

* On Windows, extra care must be taken to have a smooth building experience:

  * Make sure that all relevant paths have short names. Short names are used by
    the build system to create space-free alternative paths. Short name
    creation is enabled per volume. The default setting can be checked with the
    command: `fsutil 8dot3name query`. If short name creation was turned off
    when a directory was created, it will not have a short name. Whether a
    short name exists can be checked by running `dir /X` in the containing
    directory (in cmd.exe). If a short path is present you should see something
    like 'ASDF~1' being displayed in one of the columns of the ouput. If a
    directory is missing a short name, the safest way to get one is to enable
    short names for that particular volume with `fsutil 8dot3name set <drive
    letter>: 0` (note that you need to run as administrator for this), and then
    re-create the particular directory. A short name should be generated
    automatically then. Another option is to manually assign a short name to
    the directory using `fsutil file setShortName <path> <short name>`.

  * If using [Cygwin](#cygwin), you must make sure the file permissions and
    attributes between Windows and Cygwin are consistent. It is recommended
    that you follow this procedure:

    * Create the directory that is going to contain the top directory of the
      JDK clone by using the `mkdir` command in the Cygwin bash shell. That is,
      do *not* create it using Windows Explorer. This will ensure that it will
      have proper Cygwin attributes, and that it's children will inherit those
      attributes.

    * Do not put the JDK clone in a path under your Cygwin home directory. This
      is especially important if your user name contains spaces and/or mixed
      upper and lower case letters.

    Failure to follow these procedures might result in hard-to-debug build
    problems.

  * You need to install a git client. You have two choices, Cygwin git or Git
    for Windows. Unfortunately there are pros and cons with each choice.

    * The Cygwin `git` client has no line ending issues and understands Cygwin
      paths (which are used throughout the JDK build system). However, it does
      not currently work well with the Skara CLI tooling. Please see the [Skara
      wiki on Git clients](
      https://wiki.openjdk.org/display/SKARA/Skara#Skara-Git) for up-to-date
      information about the Skara git client support.

    * The [Git for Windows](https://gitforwindows.org) client has issues with
      line endings, and do not understand Cygwin paths. It does work well with
      the Skara CLI tooling, however. To alleviate the line ending problems,
      make sure you set `core.autocrlf` to `false` (this is asked during
      installation).

## Build Hardware Requirements

The JDK is a massive project, and require machines ranging from decent to
powerful to be able to build in a reasonable amount of time, or to be able to
complete a build at all.

We *strongly* recommend usage of an SSD disk for the build, since disk speed is
one of the limiting factors for build performance.

### Building on x86

At a minimum, a machine with 2-4 cores is advisable, as well as 2-4 GB of RAM.
(The more cores to use, the more memory you need.) At least 6 GB of free disk
space is required.

Even for 32-bit builds, it is recommended to use a 64-bit build machine, and
instead create a 32-bit target using `--with-target-bits=32`.

Note: The 32-bit x86 port is deprecated and may be removed in a future release.

### Building on aarch64

At a minimum, a machine with 8 cores is advisable, as well as 8 GB of RAM. (The
more cores to use, the more memory you need.) At least 6 GB of free disk space
is required.

If you do not have access to sufficiently powerful hardware, it is also
possible to use [cross-compiling](#cross-compiling).

#### Branch Protection

In order to use Branch Protection features in the VM,
`--enable-branch-protection` must be used. This option requires C++ compiler
support (GCC 9.1.0+ or Clang 10+). The resulting build can be run on both
machines with and without support for branch protection in hardware. Branch
Protection is only supported for Linux targets.

### Building on 32-bit ARM

This is not recommended. Instead, see the section on [Cross-compiling](
#cross-compiling).

## Operating System Requirements

The mainline JDK project supports Linux, macOS, AIX and Windows. Support for
other operating system, e.g. BSD, exists in separate "port" projects.

In general, the JDK can be built on a wide range of versions of these operating
systems, but the further you deviate from what is tested on a daily basis, the
more likely you are to run into problems.

This table lists the OS versions used by Oracle when building the JDK. Such
information is always subject to change, but this table is up to date at the
time of writing.

| Operating system  | Vendor/version used                |
| ----------------- | ---------------------------------- |
| Linux/x64         | Oracle Enterprise Linux 6.4 / 8.x  |
| Linux/aarch64     | Oracle Enterprise Linux 7.6 / 8.x  |
| macOS             | macOS 14.x                         |
| Windows           | Windows Server 2016                |

The double version numbers for Linux are due to the hybrid model used at
Oracle, where header files and external libraries from an older version are
used when building on a more modern version of the OS.

The Build Group has a wiki page with [Supported Build Platforms](
https://wiki.openjdk.org/display/Build/Supported+Build+Platforms). From time to
time, this is updated by contributors to list successes or failures of building
on different platforms.

### Windows

Windows XP is not a supported platform, but all newer Windows should be able to
build the JDK.

On Windows, it is important that you pay attention to the instructions in the
[Special Considerations](#special-considerations).

Windows is the only non-POSIX OS supported by the JDK, and as such, requires
some extra care. A POSIX support layer is required to build on Windows.
Currently, the supported such layers are Cygwin, MSYS2 and Windows Subsystem
for Linux (WSL). Of these, Cygwin is the one that has received the most
real-world testing and is likely to cause least trouble.

Internally in the build system, all paths are represented as Unix-style paths,
e.g. `/cygdrive/c/git/jdk/Makefile` rather than `C:\git\jdk\Makefile`. This
rule also applies to input to the build system, e.g. in arguments to
`configure`. So, use `--with-msvcr-dll=/cygdrive/c/msvcr100.dll` rather than
`--with-msvcr-dll=c:\msvcr100.dll`. For details on this conversion, see the
section on [Fixpath](#fixpath).

#### Locale Requirements

Building and testing the JDK requires a well-defined locale to be guaranteed to
run correctly. On non-Windows operating systems, this is achieved using the
`LC_*` variables, which propagate to all child processes of the build.
Unfortunately, there is no way to set the locale for a specific process like
this in Windows. Instead, changes to locale can only be made globally, which
will affect all applications run by the user. Furthermore, Windows makes a
difference between user locale and system locale, where the latter determines
e.g. the file path encoding. Both this locale settings affect building and
testing the JDK.

The **recommended** and **supported** way of building the JDK on Windows is to
set both the system locale and the user locale to **US English**. The system
setting can be changed by going to the Control Panel, choosing "Regional
Settings" -> "Administrative" and then pressing on the "Change System Locale"
button.

Since this is annoying for users who prefer another locale, we strive to get
the building and testing to work on other locales as well. This is on a "best
effort" level, so beware! You might get odd results in both building and
testing. If you do, remember that locales other than US English are not
supported nor recommended.

It is also imperative to install the US English language pack in Visual Studio.
For details, see [Microsoft Visual Studio](#microsoft-visual-studio).

#### Cygwin

Install [Cygwin](https://www.cygwin.com/) as instructed on the home page. It is
strongly recommended to use the 64-bit version of Cygwin.

**Note:** Cygwin has a model of continuously updating all packages without any
easy way to install or revert to a specific version of a package. This means
that whenever you add or update a package in Cygwin, you might (inadvertently)
update tools that are used by the JDK build process, and that can cause
unexpected build problems.

The JDK requires GNU Make 4.0 or greater in Cygwin. This is usually not a
problem, since Cygwin currently only distributes GNU Make at a version above
4.0.

Apart from the basic Cygwin installation, the following packages must also be
installed:

* `autoconf`
* `make`
* `zip`
* `unzip`

Often, you can install these packages using the following command line:

```
<path to Cygwin setup>/setup-x86_64 -q -P autoconf -P make -P unzip -P zip
```

Unfortunately, Cygwin can be unreliable in certain circumstances. If you
experience build tool crashes or strange issues when building on Windows,
please check the Cygwin FAQ on the ["BLODA" list](
https://cygwin.com/faq/faq.html#faq.using.bloda) and the section on [fork()
failures](https://cygwin.com/faq/faq.html#faq.using.fixing-fork-failures).

#### MSYS2

Install [MSYS2](https://www.msys2.org/) as instructed on the home page.

Apart from the basic MSYS2 installation, the following packages must also be
installed:

* `autoconf`
* `tar`
* `make`
* `zip`
* `unzip`

You can install these packages using the following command line:

```
pacman -S autoconf tar make zip unzip
```

#### Windows Subsystem for Linux (WSL)

WSL comes in two flavors, WSL1 and WSL2. These are drastically different under
the hood. WSL1 runs the binaries natively by translating Linux kernel calls
into Windows kernel calls, while WSL2 runs Linux in a virtual machine. Both
solutions have their pros and cons, and you might need to test both before
deciding which works best for you. Both WSL1 and WSL2 are supported, but to
varying degrees.

To use WSL for building the JDK, you will need Windows 10 version 1809 or
later, and you will need to install an Ubuntu guest.

It is possible to build both Windows and Linux binaries from WSL. To build
Windows binaries, you must use a Windows boot JDK (located in a
Windows-accessible directory). To build Linux binaries, you must use a Linux
boot JDK. The default behavior is to build for Windows. To build for Linux,
pass `--build=x86_64-unknown-linux-gnu
--openjdk-target=x86_64-unknown-linux-gnu` to `configure`.

If building Windows binaries, the source code must be located in a Windows-
accessible directory. This is because Windows executables (such as Visual
Studio and the boot JDK) must be able to access the source code. Also, the
drive where the source is stored must be mounted as case-insensitive by
changing either /etc/fstab or /etc/wsl.conf in WSL. Individual directories may
be corrected using the fsutil tool in case the source was cloned before
changing the mount options.

Note that while it's possible to build on WSL, testing is still not fully
supported.

### macOS

Apple is using a quite aggressive scheme of pushing OS updates, and coupling
these updates with required updates of Xcode. Unfortunately, this makes it
difficult for a project such as the JDK to keep pace with a continuously
updated machine running macOS. See the section on [Apple Xcode](#apple-xcode)
on some strategies to deal with this.

It is recommended that you use at least macOS 14 and Xcode 15.4, but
earlier versions may also work.

The standard macOS environment contains the basic tooling needed to build, but
for external libraries a package manager is recommended. The JDK uses
[homebrew](https://brew.sh/) in the examples, but feel free to use whatever
manager you want (or none).

### Linux

It is often not much problem to build the JDK on Linux. The only general advice
is to try to use the compilers, external libraries and header files as provided
by your distribution.

The basic tooling is provided as part of the core operating system, but you
will most likely need to install developer packages.

For apt-based distributions (Debian, Ubuntu, etc), try this:

```
sudo apt-get install build-essential autoconf
```

For rpm-based distributions (Fedora, Red Hat, etc), try this:

```
sudo yum groupinstall "Development Tools"
```

For Alpine Linux, aside from basic tooling, install the GNU versions of some
programs:

```
sudo apk add build-base bash grep zip
```

### AIX

Please consult the AIX section of the [Supported Build Platforms](
https://wiki.openjdk.org/display/Build/Supported+Build+Platforms) OpenJDK Build
Wiki page for details about which versions of AIX are supported.

## Native Compiler (Toolchain) Requirements

Large portions of the JDK consists of native code, that needs to be compiled to
be able to run on the target platform. In theory, toolchain and operating
system should be independent factors, but in practice there's more or less a
one-to-one correlation between target operating system and toolchain.

| Operating system   | Supported toolchain       |
| ------------------ | ------------------------- |
| Linux              | gcc, clang                |
| macOS              | Apple Xcode (using clang) |
| AIX                | IBM Open XL C/C++         |
| Windows            | Microsoft Visual Studio   |

Please see the individual sections on the toolchains for version
recommendations. As a reference, these versions of the toolchains are used, at
the time of writing, by Oracle for the daily builds of the JDK. It should be
possible to compile the JDK with both older and newer versions, but the closer
you stay to this list, the more likely you are to compile successfully without
issues.

| Operating system   | Toolchain version                            |
| ------------------ | -------------------------------------------- |
| Linux              | gcc 14.2.0                                   |
| macOS              | Apple Xcode 15.4 (using clang 15.0.0)       |
| Windows            | Microsoft Visual Studio 2022 version 17.13.2 |

All compilers are expected to be able to handle the C11 language standard for
C, and C++14 for C++.

### gcc

The minimum accepted version of gcc is 10.0. Older versions will not be accepted
by `configure`.

The JDK is currently known to compile successfully with gcc version 14.2 or
newer.

In general, any version between these two should be usable.

### clang

The minimum accepted version of clang is 13. Older versions will not be
accepted by `configure`.

To use clang instead of gcc on Linux, use `--with-toolchain-type=clang`.

### Apple Xcode

The oldest supported version of Xcode is 13.0.

You will need to download Xcode either from the App Store or specific versions
can be easily located via the [Xcode Releases](https://xcodereleases.com)
website.

When updating Xcode, it is advisable to keep an older version for building the
JDK. To use a specific version of Xcode you have multiple options:

* Use `xcode-select -s` before running `configure`, e.g.
`xcode-select -s /Applications/Xcode13.1.app`. The drawback is that the setting
is system wide and you may have to revert it after a JDK build.
* Use configure option `--with-xcode-path`, e.g.
`configure --with-xcode-path=/Applications/Xcode13.1.app` This allows using a
specific Xcode version for a JDK build, independently of the active Xcode
version by `xcode-select`.

If you have recently (inadvertently) updated your OS and/or Xcode version, and
the JDK can no longer be built, please see the section on [Problems with the
Build Environment](#problems-with-the-build-environment), and [Getting
Help](#getting-help) to find out if there are any recent, non-merged patches
available for this update.

### Microsoft Visual Studio

The minimum accepted version is Visual Studio 2019 version 16.8. (Note that
this version is often presented as "MSVC 14.28", and reported by cl.exe as
19.28.) Older versions will not be accepted by `configure` and will not work.
The maximum accepted version of Visual Studio is 2022.

If you have multiple versions of Visual Studio installed, `configure` will by
default pick the latest. You can request a specific version to be used by
setting `--with-toolchain-version`, e.g. `--with-toolchain-version=2022`.

If you have Visual Studio installed but `configure` fails to detect it, it may
be because of [spaces in path](#spaces-in-path).

You must install the US English locale, otherwise the build system might not be
able to interact properly with the compiler. You can add additional language
packs when installing Visual Studio.

If you have already installed Visual Studio without the US English language
pack, you can modify the installation to add this. You can either do this via a
GUI like this:

* Click on "Visual Studio Installer" in Start menu.
* Click "Modify".
* Select the tab "Language packs".
* Choose "English".
* Click "Modify".

or you can run it on the command line. For this to work, you need to start
`cmd.exe` using "Run as Administrator". Then execute the following line: (note
that the " characters are essential)

```
"C:\Program Files (x86)\Microsoft Visual Studio\Installer\vs_installer.exe" modify --channelId VisualStudio.16.Release --productId Microsoft.VisualStudio.Product.BuildTools --addProductLang en-us -p
```

`VisualStudio.16.Release` represent VS 2019, so adjust the version number
accordingly. If you have not installed the `BuildTools`, but e.g.
`Professional`, adjust the product ID accordingly.

### IBM Open XL C/C++

The minimum accepted version of Open XL is 17.1.1.4. This is in essence clang
15, and will be treated as such by the OpenJDK build system.

## Boot JDK Requirements

Paradoxically, building the JDK requires a pre-existing JDK. This is called the
"boot JDK". The boot JDK does not, however, have to be a JDK built directly
from the source code available in the OpenJDK Community. If you are porting the
JDK to a new platform, chances are that there already exists another JDK for
that platform that is usable as boot JDK.

The rule of thumb is that the boot JDK for building JDK major version *N*
should be a JDK of major version *N-1*, so for building JDK 18 a JDK 17 would
be suitable as boot JDK. However, the JDK should be able to "build itself", so
an up-to-date build of the current JDK source is an acceptable alternative. If
you are following the *N-1* rule, make sure you've got the latest update
version, since e.g. JDK 8 GA might not be able to build JDK 9 on all platforms.

Early in the release cycle, version *N-1* may not yet have been released. In
that case, the preferred boot JDK will be version *N-2* until version *N-1* is
available.

The `configure` scripts tries to locate a suitable boot JDK automatically, but
due to the lack of standard installation locations on most platforms, this
heuristics has a high likelihood to fail. If the boot JDK is not automatically
detected, or the wrong JDK is picked, use `--with-boot-jdk` to point to the JDK
to use.

### Getting JDK Binaries

An overview of common ways to download and install prebuilt JDK binaries can be
found on https://openjdk.org/install. An alternative is to download the [Oracle
JDK](https://www.oracle.com/java/technologies/downloads). Another is
[Adoptium](https://adoptium.net/), which publishes prebuilt binaries for
various platforms.

On Linux you can also get a JDK from the Linux distribution. On apt-based
distros (like Debian and Ubuntu), `sudo apt-get install openjdk-<VERSION>-jdk`
is typically enough to install a JDK \<VERSION\>. On rpm-based distros (like
Fedora and Red Hat), try `sudo yum install java-<VERSION>-openjdk-devel`.

## External Library Requirements

Different platforms require different external libraries. In general, libraries
are not optional - that is, they are either required or not used.

If a required library is not detected by `configure`, you need to provide the
path to it. There are two forms of the `configure` arguments to point to an
external library: `--with-<LIB>=<path>` or `--with-<LIB>-include=<path to
include> --with-<LIB>-lib=<path to lib>`.

The first variant is more concise, but require the include files and library
files to reside in a default hierarchy under this directory. In most cases, it
works fine. As a fallback, the second version allows you to point to the
include directory and the lib directory separately.

### FreeType

FreeType2 from [The FreeType Project](https://www.freetype.org/) is not
required on any platform. The exception is on Unix-based platforms when
configuring such that the build artifacts will reference a system installed
library, rather than bundling the JDK's own copy.

* To install on an apt-based Linux, try running `sudo apt-get install
libfreetype6-dev`.
* To install on an rpm-based Linux, try running `sudo yum install
freetype-devel`.
* To install on Alpine Linux, try running `sudo apk add freetype-dev`.
* To install on macOS, try running `brew install freetype`.

Use `--with-freetype-include=<path>` and `--with-freetype-lib=<path>` if
`configure` does not automatically locate the platform FreeType files.

### Fontconfig

Fontconfig from [freedesktop.org Fontconfig](https://fontconfig.org) is
required on all platforms except Windows and macOS.

* To install on an apt-based Linux, try running `sudo apt-get install
libfontconfig-dev`.
* To install on an rpm-based Linux, try running `sudo yum install
fontconfig-devel`.
* To install on Alpine Linux, try running `sudo apk add fontconfig-dev`.

Use `--with-fontconfig-include=<path>` and `--with-fontconfig=<path>` if
`configure` does not automatically locate the platform Fontconfig files.

### CUPS

CUPS, [Common UNIX Printing System](https://www.cups.org) header files are
required on all platforms, except Windows. Often these files are provided by
your operating system.

* To install on an apt-based Linux, try running `sudo apt-get install
libcups2-dev`.
* To install on an rpm-based Linux, try running `sudo yum install cups-devel`.
* To install on Alpine Linux, try running `sudo apk add cups-dev`.

Use `--with-cups=<path>` if `configure` does not properly locate your CUPS
files.

### X11

Certain [X11](https://www.x.org/) libraries and include files are required on
Linux.

* To install on an apt-based Linux, try running `sudo apt-get install
libx11-dev libxext-dev libxrender-dev libxrandr-dev libxtst-dev libxt-dev`.
* To install on an rpm-based Linux, try running `sudo yum install libXtst-devel
libXt-devel libXrender-devel libXrandr-devel libXi-devel`.
* To install on Alpine Linux, try running `sudo apk add libx11-dev libxext-dev
libxrender-dev libxrandr-dev libxtst-dev libxt-dev`.

Use `--with-x=<path>` if `configure` does not properly locate your X11 files.

### ALSA

ALSA, [Advanced Linux Sound Architecture](https://www.alsa-project.org/) is
required on Linux. At least version 0.9.1 of ALSA is required.

* To install on an apt-based Linux, try running `sudo apt-get install
libasound2-dev`.
* To install on an rpm-based Linux, try running `sudo yum install
alsa-lib-devel`.
* To install on Alpine Linux, try running `sudo apk add alsa-lib-dev`.

Use `--with-alsa=<path>` if `configure` does not properly locate your ALSA
files.

### libffi

libffi, the [Portable Foreign Function Interface Library](
https://sourceware.org/libffi) is required when building the Zero version of
Hotspot.

* To install on an apt-based Linux, try running `sudo apt-get install
libffi-dev`.
* To install on an rpm-based Linux, try running `sudo yum install
libffi-devel`.
* To install on Alpine Linux, try running `sudo apk add libffi-dev`.

Use `--with-libffi=<path>` if `configure` does not properly locate your libffi
files.

## Build Tools Requirements

### Autoconf

The JDK build requires [Autoconf](https://www.gnu.org/software/autoconf) on all
platforms. At least version 2.69 is required.

* To install on an apt-based Linux, try running `sudo apt-get install
autoconf`.
* To install on an rpm-based Linux, try running `sudo yum install autoconf`.
* To install on Alpine Linux, try running `sudo apk add autoconf`.
* To install on macOS, try running `brew install autoconf`.
* To install on Windows, try running `<path to Cygwin setup>/setup-x86_64 -q -P
autoconf`.

If `configure` has problems locating your installation of autoconf, you can
specify it using the `AUTOCONF` environment variable, like this:

```
AUTOCONF=<path to autoconf> configure ...
```

### GNU Make

The JDK build requires [GNU Make](https://www.gnu.org/software/make). No other
flavors of make are supported.

At least version 3.81 of GNU Make must be used. For distributions supporting
GNU Make 4.0 or above, we strongly recommend it. GNU Make 4.0 contains useful
functionality to handle parallel building (supported by `--with-output-sync`)
and speed and stability improvements.

Note that `configure` locates and verifies a properly functioning version of
`make` and stores the path to this `make` binary in the configuration. If you
start a build using `make` on the command line, you will be using the version
of make found first in your `PATH`, and not necessarily the one stored in the
configuration. This initial make will be used as "bootstrap make", and in a
second stage, the make located by `configure` will be called. Normally, this
will present no issues, but if you have a very old `make`, or a non-GNU Make
`make` in your path, this might cause issues.

If you want to override the default make found by `configure`, use the `MAKE`
configure variable, e.g. `configure MAKE=/opt/gnu/make`.

### GNU Bash

The JDK build requires [GNU Bash](https://www.gnu.org/software/bash). No other
shells are supported.

At least version 3.2 of GNU Bash must be used.

### Graphviz and Pandoc

In order to build man pages and the full docs (see the `--enable-full-docs`
configure option) [Pandoc](https://pandoc.org) is required. For full docs also
[Graphviz](https://www.graphviz.org) is required. Any recent versions should
work. For reference, and subject to change, Oracle builds use Graphviz
9.0.0 and Pandoc 2.19.2.

## Running Configure

To build the JDK, you need a "configuration", which consists of a directory
where to store the build output, coupled with information about the platform,
the specific build machine, and choices that affect how the JDK is built.

The configuration is created by the `configure` script. The basic invocation of
the `configure` script looks like this:

```
bash configure [options]
```

This will create an output directory containing the configuration and setup an
area for the build result. This directory typically looks like
`build/linux-x64-server-release`, but the actual name depends on your specific
configuration. (It can also be set directly, see [Using Multiple
Configurations](#using-multiple-configurations)). This directory is referred to
as `$BUILD` in this documentation.

`configure` will try to figure out what system you are running on and where all
necessary build components are. If you have all prerequisites for building
installed, it should find everything. If it fails to detect any component
automatically, it will exit and inform you about the problem.

Some command line examples:

* Create a 32-bit build for Windows with FreeType2 in `C:\freetype-i586`:

  ```
  bash configure --with-freetype=/cygdrive/c/freetype-i586 --with-target-bits=32
  ```

* Create a debug build with the `server` JVM and DTrace enabled:

  ```
  bash configure --enable-debug --with-jvm-variants=server --enable-dtrace
  ```

### Common Configure Arguments

Here follows some of the most common and important `configure` argument.

To get up-to-date information on *all* available `configure` argument, please
run:

```
bash configure --help
```

(Note that this help text also include general autoconf options, like
`--dvidir`, that is not relevant to the JDK. To list only JDK-specific
features, use `bash configure --help=short` instead.)

#### Configure Arguments for Tailoring the Build

* `--enable-debug` - Set the debug level to `fastdebug` (this is a shorthand
  for `--with-debug-level=fastdebug`)
* `--with-debug-level=<level>` - Set the debug level, which can be `release`,
  `fastdebug`, `slowdebug` or `optimized`. Default is `release`. `optimized` is
  variant of `release` with additional Hotspot debug code.
* `--with-native-debug-symbols=<method>` - Specify if and how native debug
  symbols should be built. Available methods are `none`, `internal`,
  `external`, `zipped`. Default behavior depends on platform. See [Native Debug
  Symbols](#native-debug-symbols) for more details.
* `--with-version-string=<string>` - Specify the version string this build will
  be identified with.
* `--with-version-<part>=<value>` - A group of options, where `<part>` can be
  any of `pre`, `opt`, `build`, `major`, `minor`, `security` or `patch`. Use
  these options to modify just the corresponding part of the version string
  from the default, or the value provided by `--with-version-string`.
* `--with-jvm-variants=<variant>[,<variant>...]` - Build the specified variant
  (or variants) of Hotspot. Valid variants are: `server`, `client`, `minimal`,
  `core`, `zero`, `custom`. Note that not all variants are possible to combine
  in a single build.
* `--enable-jvm-feature-<feature>` or `--disable-jvm-feature-<feature>` -
  Include (or exclude) `<feature>` as a JVM feature in Hotspot. You can also
  specify a list of features to be enabled, separated by space or comma, as
  `--with-jvm-features=<feature>[,<feature>...]`. If you prefix `<feature>`
  with a `-`, it will be disabled. These options will modify the default list
  of features for the JVM variant(s) you are building. For the `custom` JVM
  variant, the default list is empty. A complete list of valid JVM features can
  be found using `bash configure --help`.
* `--with-target-bits=<bits>` - Create a target binary suitable for running on
  a `<bits>` platform. Use this to create 32-bit output on a 64-bit build
  platform, instead of doing a full cross-compile. (This is known as a
  *reduced* build.)

On Linux, BSD and AIX, it is possible to override where Java by default
searches for runtime/JNI libraries. This can be useful in situations where
there is a special shared directory for system JNI libraries. This setting can
in turn be overridden at runtime by setting the `java.library.path` property.

* `--with-jni-libpath=<path>` - Use the specified path as a default when
searching for runtime libraries.

#### Configure Arguments for Native Compilation

* `--with-devkit=<path>` - Use this devkit for compilers, tools and resources
* `--with-sysroot=<path>` - Use this directory as sysroot
* `--with-extra-path=<path>[;<path>]` - Prepend these directories to the
  default path when searching for all kinds of binaries
* `--with-toolchain-path=<path>[;<path>]` - Prepend these directories when
  searching for toolchain binaries (compilers etc)
* `--with-extra-cflags=<flags>` - Append these flags when compiling JDK C files
* `--with-extra-cxxflags=<flags>` - Append these flags when compiling JDK C++
  files
* `--with-extra-ldflags=<flags>` - Append these flags when linking JDK
  libraries

#### Configure Arguments for External Dependencies

* `--with-boot-jdk=<path>` - Set the path to the [Boot JDK](
  #boot-jdk-requirements)
* `--with-freetype=<path>` - Set the path to [FreeType](#freetype)
* `--with-cups=<path>` - Set the path to [CUPS](#cups)
* `--with-x=<path>` - Set the path to [X11](#x11)
* `--with-alsa=<path>` - Set the path to [ALSA](#alsa)
* `--with-libffi=<path>` - Set the path to [libffi](#libffi)
* `--with-jtreg=<path>` - Set the path to JTReg. See [Running Tests](
  #running-tests)

Certain third-party libraries used by the JDK (libjpeg, giflib, libpng, lcms
and zlib) are included in the JDK repository. The default behavior of the JDK
build is to use the included ("bundled") versions of libjpeg, giflib, libpng
and lcms. For zlib, the system lib (if present) is used except on Windows and
AIX. However the bundled libraries may be replaced by an external version. To
do so, specify `system` as the `<source>` option in these arguments. (The
default is `bundled`).

* `--with-libjpeg=<source>` - Use the specified source for libjpeg
* `--with-giflib=<source>` - Use the specified source for giflib
* `--with-libpng=<source>` - Use the specified source for libpng
* `--with-lcms=<source>` - Use the specified source for lcms
* `--with-zlib=<source>` - Use the specified source for zlib

On Linux, it is possible to select either static or dynamic linking of the C++
runtime. The default is static linking, with dynamic linking as fallback if the
static library is not found.

* `--with-stdc++lib=<method>` - Use the specified method (`static`, `dynamic`
  or `default`) for linking the C++ runtime.

### Configure Control Variables

It is possible to control certain aspects of `configure` by overriding the
value of `configure` variables, either on the command line or in the
environment.

Normally, this is **not recommended**. If used improperly, it can lead to a
broken configuration. Unless you're well versed in the build system, this is
hard to use properly. Therefore, `configure` will print a warning if this is
detected.

However, there are a few `configure` variables, known as *control variables*
that are supposed to be overridden on the command line. These are variables
that describe the location of tools needed by the build, like `MAKE` or `GREP`.
If any such variable is specified, `configure` will use that value instead of
trying to autodetect the tool. For instance, `bash configure
MAKE=/opt/gnumake4.0/bin/make`.

If a configure argument exists, use that instead, e.g. use `--with-jtreg`
instead of setting `JTREGEXE`.

Also note that, despite what autoconf claims, setting `CFLAGS` will not
accomplish anything. Instead use `--with-extra-cflags` (and similar for
`cxxflags` and `ldflags`).

## Running Make

When you have a proper configuration, all you need to do to build the JDK is to
run `make`. (But see the warning at [GNU Make](#gnu-make) about running the
correct version of make.)

When running `make` without any arguments, the default target is used, which is
the same as running `make default` or `make jdk`. This will build a minimal (or
roughly minimal) set of compiled output (known as an "exploded image") needed
for a developer to actually execute the newly built JDK. The idea is that in an
incremental development fashion, when doing a normal make, you should only
spend time recompiling what's changed (making it purely incremental) and only
do the work that's needed to actually run and test your code.

The output of the exploded image resides in `$BUILD/jdk`. You can test the
newly built JDK like this: `$BUILD/jdk/bin/java -version`.

### Common Make Targets

Apart from the default target, here are some common make targets:

* `hotspot` - Build all of hotspot (but only hotspot)
* `hotspot-<variant>` - Build just the specified jvm variant
* `images` or `product-images` - Build the JDK image
* `docs` or `docs-image` - Build the documentation image
* `test-image` - Build the test image
* `all` or `all-images` - Build all images (product, docs and test)
* `bootcycle-images` - Build images twice, second time with newly built JDK
  (good for testing)
* `clean` - Remove all files generated by make, but not those generated by
  configure
* `dist-clean` - Remove all files, including configuration

Run `make help` to get an up-to-date list of important make targets and make
control variables.

It is possible to build just a single module, a single phase, or a single phase
of a single module, by creating make targets according to these following
patterns. A phase can be either of `gensrc`, `gendata`, `copy`, `java`,
`launchers`, or `libs`. See [Using Fine-Grained Make Targets](
#using-fine-grained-make-targets) for more details about this functionality.

* `<phase>` - Build the specified phase and everything it depends on
* `<module>` - Build the specified module and everything it depends on
* `<module>-<phase>` - Compile the specified phase for the specified module and
  everything it depends on

Similarly, it is possible to clean just a part of the build by creating make
targets according to these patterns:

* `clean-<outputdir>` - Remove the subdir in the output dir with the name
* `clean-<phase>` - Remove all build results related to a certain build phase
* `clean-<module>` - Remove all build results related to a certain module
* `clean-<module>-<phase>` - Remove all build results related to a certain
  module and phase

### Make Control Variables

It is possible to control `make` behavior by overriding the value of `make`
variables, either on the command line or in the environment.

Normally, this is **not recommended**. If used improperly, it can lead to a
broken build. Unless you're well versed in the build system, this is hard to
use properly. Therefore, `make` will print a warning if this is detected.

However, there are a few `make` variables, known as *control variables* that
are supposed to be overridden on the command line. These make up the "make
time" configuration, as opposed to the "configure time" configuration.

#### General Make Control Variables

* `JOBS` - Specify the number of jobs to build with. See [Build
  Performance](#build-performance).
* `LOG` - Specify the logging level and functionality. See [Checking the Build
  Log File](#checking-the-build-log-file)
* `CONF` and `CONF_NAME` - Selecting the configuration(s) to use. See [Using
  Multiple Configurations](#using-multiple-configurations)

#### Test Make Control Variables

These make control variables only make sense when running tests. Please see
**Testing the JDK** ([html](testing.html), [markdown](testing.md)) for details.

* `TEST`
* `TEST_JOBS`
* `TEST_OPTS`
* `TEST_VM_OPTS`
* `JTREG`
* `GTEST`
* `MICRO`

#### Advanced Make Control Variables

These advanced make control variables can be potentially unsafe. See [Hints and
Suggestions for Advanced Users](#hints-and-suggestions-for-advanced-users) and
[Understanding the Build System](#understanding-the-build-system) for details.

* `SPEC`
* `CONF_CHECK`
* `COMPARE_BUILD`
* `JDK_FILTER`
* `SPEC_FILTER`

## Running Tests

Most of the JDK tests are using the [JTReg](https://openjdk.org/jtreg) test
framework. Make sure that your configuration knows where to find your
installation of JTReg. If this is not picked up automatically, use the
`--with-jtreg=<path to jtreg home>` option to point to the JTReg framework.
Note that this option should point to the JTReg home, i.e. the top directory,
containing `lib/jtreg.jar` etc.

The [Adoption Group](https://wiki.openjdk.org/display/Adoption) provides recent
builds of jtreg [here](
https://ci.adoptium.net/view/Dependencies/job/dependency_pipeline/lastSuccessfulBuild/artifact/jtreg/).
Download the latest `.tar.gz` file, unpack it, and point `--with-jtreg` to the
`jtreg` directory that you just unpacked.

Building of Hotspot Gtest suite requires the source code of Google Test
framework. The top directory, which contains both `googletest` and `googlemock`
directories, should be specified via `--with-gtest`. The minimum supported
version of Google Test is 1.14.0, whose source code can be obtained:

* by downloading and unpacking the source bundle from
  [here](https://github.com/google/googletest/releases/tag/v1.14.0), or
* by checking out `v1.14.0` tag of `googletest` project:
  `git clone -b v1.14.0 https://github.com/google/googletest`

To execute the most basic tests (tier 1), use:

```
make test-tier1
```

For more details on how to run tests, please see **Testing the JDK**
([html](testing.html), [markdown](testing.md)).

## Signing

### macOS

Modern versions of macOS require applications to be signed and notarized before
distribution. See Apple's documentation for more background on what this means
and how it works. To help support this, the JDK build can be configured to
automatically sign all native binaries, and the JDK bundle, with all the
options needed for successful notarization, as well as all the entitlements
required by the JDK. To enable `hardened` signing, use configure parameter
`--with-macosx-codesign=hardened` and configure the signing identity you wish
to use with `--with-macosx-codesign-identity=<identity>`. The identity refers
to a signing identity from Apple that needs to be preinstalled on the build
host.

When not signing for distribution with the hardened option, the JDK build will
still attempt to perform `adhoc` signing to add the special entitlement
`com.apple.security.get-task-allow` to each binary. This entitlement is
required to be able to dump core files from a process. Note that adding this
entitlement makes the build invalid for notarization, so it is only added when
signing in `debug` mode. To explicitly enable this kind of ad hoc signing, use
configure parameter `--with-macosx-codesign=debug`. It will be enabled by
default in most cases.

It's also possible to completely disable any explicit codesign operations done
by the JDK build using the configure parameter `--without-macosx-codesign`. The
exact behavior then depends on the architecture. For macOS on x64, it (at least
at the time of this writing) results in completely unsigned binaries that
should still work fine for development and debugging purposes. On aarch64, the
Xcode linker will apply a default "ad hoc" signing, without any entitlements.
Such a build does not allow dumping core files.

The default mode "auto" will try for `hardened` signing if the debug level is
`release` and either the default identity or the specified identity is valid.
If hardened isn't possible, then `debug` signing is chosen if it works. If
nothing works, the codesign build step is disabled.

## Cross-Compiling

Cross-compiling means using one platform (the *build* platform) to generate
output that can ran on another platform (the *target* platform).

The typical reason for cross-compiling is that the build is performed on a more
powerful desktop computer, but the resulting binaries will be able to run on a
different, typically low-performing system. Most of the complications that
arise when building for embedded is due to this separation of *build* and
*target* systems.

This requires a more complex setup and build procedure. This section assumes
you are familiar with cross-compiling in general, and will only deal with the
particularities of cross-compiling the JDK. If you are new to cross-compiling,
please see the [external links at Wikipedia](
https://en.wikipedia.org/wiki/Cross_compiler#External_links) for a good start
on reading materials.

Cross-compiling the JDK requires you to be able to build both for the build
platform and for the target platform. The reason for the former is that we need
to build and execute tools during the build process, both native tools and Java
tools.

If all you want to do is to compile a 32-bit version, for the same OS, on a
64-bit machine, consider using `--with-target-bits=32` instead of doing a
full-blown cross-compilation. (While this surely is possible, it's a lot more
work and will take much longer to build.)

Setting up a cross-compilation environment by hand is time-consuming and error
prone. It is highly recommended that you use one of the automated methods
described in [Cross compiling the easy way](#cross-compiling-the-easy-way).

### Specifying the Target Platform

You *must* specify the target platform when cross-compiling. Doing so will also
automatically turn the build into a cross-compiling mode. The simplest way to
do this is to use the `--openjdk-target` argument, e.g.
`--openjdk-target=arm-linux-gnueabihf`. or `--openjdk-target=aarch64-oe-linux`.
This will automatically set the `--host` and `--target` options for autoconf,
which can otherwise be confusing. (In autoconf terminology, the "target" is
known as "host", and "target" is used for building a Canadian cross-compiler.)

If `--build` has not been explicitly passed to configure, `--openjdk-target`
will autodetect the build platform and internally set the flag automatically,
otherwise the platform that was explicitly passed to `--build` will be used
instead.

### Boot JDK and Build JDK

When cross-compiling, make sure you use a boot JDK that runs on the *build*
system, and not on the *target* system.

To be able to build, we need a "Build JDK", which is a JDK built from the
current sources (that is, the same as the end result of the entire build
process), but able to run on the *build* system, and not the *target* system.
(In contrast, the Boot JDK should be from an older release, e.g. JDK 8 when
building JDK 9.)

The build process will create a minimal Build JDK for you, as part of building.
To speed up the build, you can use `--with-build-jdk` to `configure` to point
to a pre-built Build JDK. Please note that the build result is unpredictable,
and can possibly break in subtle ways, if the Build JDK does not **exactly**
match the current sources.

### Toolchain Considerations

You will need two copies of your toolchain, one which generates output that can
run on the target system (the normal, or *target*, toolchain), and one that
generates output that can run on the build system (the *build* toolchain).

If you are cross-compiling using gcc, it is recommended to use the gcc standard
where you prefix cross-compiling tools with the target denominator. If you
follow this standard, `configure` is likely to pick up the toolchain correctly.

The *build* toolchain will be auto-detected just the same way the normal
*build*/*target* toolchain will be auto-detected when not cross-compiling. If
this is not what you want, or if the auto-detection fails, you can specify a
devkit containing the *build* toolchain using `--with-build-devkit` to
`configure`, or by giving `BUILD_CC` and `BUILD_CXX` arguments.

It is often helpful to locate the cross-compilation tools, headers and
libraries in a separate directory, outside the normal path, and point out that
directory to `configure`. Do this by setting the sysroot (`--with-sysroot`) and
appending the directory when searching for cross-compilations tools
(`--with-toolchain-path`). As a compact form, you can also use `--with-devkit`
to point to a single directory, if it is correctly setup. (See
`make/autoconf/basics.m4` for details.)

### Native Libraries

You will need copies of external native libraries for the *target* system
present on the *build* machine while building.

Take care not to replace the *build* system's version of these libraries by
mistake, as that can render the *build* machine unusable.

Make sure that the libraries you point to (ALSA, X11, etc) are for the *target*
platform, not the *build* platform.

#### ALSA

You will need alsa libraries suitable for your *target* system. In most cases,
using Debian's pre-built libraries work fine.

Note that alsa is needed even if you only want to build a headless JDK.

* Go to [Debian Package Search](https://www.debian.org/distrib/packages),
  search for the `libasound2` and `libasound2-dev` packages for your *target*
  system, and download them to /tmp.

* Install the libraries into the cross-compilation toolchain. For instance:

  ```
  cd /tools/gcc-linaro-arm-linux-gnueabihf-raspbian-2012.09-20120921_linux/arm-linux-gnueabihf/libc
  dpkg-deb -x /tmp/libasound2_1.0.25-4_armhf.deb .
  dpkg-deb -x /tmp/libasound2-dev_1.0.25-4_armhf.deb .
  ```

* If alsa is not properly detected by `configure`, you can specify it by
  `--with-alsa`.

#### X11

You will need X11 libraries suitable for your *target* system. In most cases,
using Debian's pre-built libraries work fine.

Note that X11 is needed even if you only want to build a headless JDK.

* Go to [Debian Package Search](https://www.debian.org/distrib/packages),
  search for the following packages for your *target* system, and download them
  to /tmp/target-x11:

  * `libxi`
  * `libxi-dev`
  * `x11proto-core-dev`
  * `x11proto-input-dev`
  * `x11proto-kb-dev`
  * `x11proto-render-dev`
  * `x11proto-xext-dev`
  * `libice-dev`
  * `libxrender`
  * `libxrender-dev`
  * `libxrandr-dev`
  * `libsm-dev`
  * `libxt-dev`
  * `libx11`
  * `libx11-dev`
  * `libxtst`
  * `libxtst-dev`
  * `libxext`
  * `libxext-dev`

* Install the libraries into the cross-compilation toolchain. For instance:

  ```
  cd /tools/gcc-linaro-arm-linux-gnueabihf-raspbian-2012.09-20120921_linux/arm-linux-gnueabihf/libc/usr
  mkdir X11R6
  cd X11R6
  for deb in /tmp/target-x11/*.deb ; do dpkg-deb -x $deb . ; done
  mv usr/* .
  cd lib
  cp arm-linux-gnueabihf/* .
  ```

  You can ignore the following messages, since these libraries are not needed
  to successfully complete a full JDK build.

  ```
  cp: cannot stat `arm-linux-gnueabihf/libICE.so': No such file or directory
  cp: cannot stat `arm-linux-gnueabihf/libSM.so': No such file or directory
  cp: cannot stat `arm-linux-gnueabihf/libXt.so': No such file or directory
  ```

* If the X11 libraries are not properly detected by `configure`, you can point
  them out by `--with-x`.

### Verifying the Build

The build will end up in a directory named like
`build/linux-arm-normal-server-release`.

Inside this build output directory, the `images/jdk` will contain the newly
built JDK, for your *target* system.

Copy these folders to your *target* system. Then you can run e.g.
`images/jdk/bin/java -version`.

### Cross-Compiling the Easy Way

Setting up a proper cross-compilation environment can be a lot of work.
Fortunately there are ways that more or less automate this process. Here are
two recommended methods, using the "devkits" that can be generated by the JDK
build system, or by using the `debootstrap` command in Debian. The former works
on all Linux distributions, the latter only on Debian and derivatives. Both
solution only work for gcc.

The devkit method is regularly used for testing by Oracle, and the debootstrap
method is regularly used in GitHub Actions testing.

#### Using OpenJDK Devkits

The JDK build system provides out-of-the box support for creating and using so
called devkits. A `devkit` is basically a collection of a cross-compiling
toolchain and a sysroot environment which can easily be used together with the
`--with-devkit` configure option to cross compile the JDK. On Linux/x86_64, the
following command:

```
bash configure --with-devkit=<devkit-path> --openjdk-target=ppc64-linux-gnu && make
```

will configure and build the JDK for Linux/ppc64 assuming that `<devkit-path>`
points to a Linux/x86_64 to Linux/ppc64 devkit.

Devkits can be created from the `make/devkit` directory by executing:

```
make [ TARGETS="<TARGET_TRIPLET>+" ] [ BASE_OS=<OS> ] [ BASE_OS_VERSION=<VER> ]
```

where `TARGETS` contains one or more `TARGET_TRIPLET`s of the form described in
[section 3.4 of the GNU Autobook](
https://sourceware.org/autobook/autobook/autobook_17.html). If no targets are
given, a native toolchain for the current platform will be created. Currently,
at least the following targets are known to work:

| Supported devkit targets |
| ------------------------ |
| x86_64-linux-gnu         |
| aarch64-linux-gnu        |
| arm-linux-gnueabihf      |
| ppc64-linux-gnu          |
| ppc64le-linux-gnu        |
| s390x-linux-gnu          |

`BASE_OS` must be one of "OEL6" for Oracle Enterprise Linux 6 or "Fedora" (if
not specified "OEL6" will be the default). If the base OS is "Fedora" the
corresponding Fedora release can be specified with the help of the
`BASE_OS_VERSION` option (with "27" as default version). If the build is
successful, the new devkits can be found in the `build/devkit/result`
subdirectory:

```
cd make/devkit
make TARGETS="ppc64le-linux-gnu aarch64-linux-gnu" BASE_OS=Fedora BASE_OS_VERSION=21
ls -1 ../../build/devkit/result/
x86_64-linux-gnu-to-aarch64-linux-gnu
x86_64-linux-gnu-to-ppc64le-linux-gnu
```

Notice that devkits are not only useful for targeting different build
platforms. Because they contain the full build dependencies for a system (i.e.
compiler and root file system), they can easily be used to build well-known,
reliable and reproducible build environments. You can for example create and
use a devkit with GCC 7.3 and a Fedora 12 sysroot environment (with glibc 2.11)
on Ubuntu 14.04 (which doesn't have GCC 7.3 by default) to produce JDK binaries
which will run on all Linux systems with runtime libraries newer than the ones
from Fedora 12 (e.g. Ubuntu 16.04, SLES 11 or RHEL 6).

#### Using Debian debootstrap

On Debian (or a derivative like Ubuntu), you can create sysroots for foreign
architectures with tools provided by the OS. You can use `debootstrap` to
create a *target* system chroot directory, which would have the native
libraries and headers specific to that *target* system. After that, you can use
the cross-compiler on the *build* system, pointing into the chroot to get the
build dependencies right. This allows building for foreign architectures with
native compilation speed.

For example, cross-compiling to AArch64 from x86_64 could be done like this:

* Install cross-compiler on the *build* system:

  ```
  apt install g++-aarch64-linux-gnu gcc-aarch64-linux-gnu
  ```

* Create chroot on the *build* system, configuring it for *target* system:

  ```
  sudo debootstrap \
    --arch=arm64 \
    --verbose \
    --include=fakeroot,symlinks,build-essential,libx11-dev,libxext-dev,libxrender-dev,libxrandr-dev,libxtst-dev,libxt-dev,libcups2-dev,libfontconfig1-dev,libasound2-dev,libfreetype6-dev,libpng-dev,libffi-dev \
    --resolve-deps \
    buster \
    ~/sysroot-arm64 \
    https://httpredir.debian.org/debian/
  ```

* To create an Ubuntu-based chroot:

  ```
  sudo debootstrap \
    --arch=arm64 \
    --verbose \
    --components=main,universe \
    --include=fakeroot,symlinks,build-essential,libx11-dev,libxext-dev,libxrender-dev,libxrandr-dev,libxtst-dev,libxt-dev,libcups2-dev,libfontconfig1-dev,libasound2-dev,libfreetype6-dev,libpng-dev,libffi-dev \
    --resolve-deps \
    jammy \
    ~/sysroot-arm64 \
    http://ports.ubuntu.com/ubuntu-ports/
  ```

  Note that `symlinks` is in the universe repository.

* Make sure the symlinks inside the newly created chroot point to proper
  locations:

  ```
  sudo chroot ~/sysroot-arm64 symlinks -cr .
  ```

* Configure and build with newly created chroot as sysroot/toolchain-path:

  ```
  sh ./configure \
    --openjdk-target=aarch64-linux-gnu \
    --with-sysroot=~/sysroot-arm64
  make images
  ls build/linux-aarch64-server-release/
  ```

The build does not create new files in that chroot, so it can be reused for
multiple builds without additional cleanup.

The build system should automatically detect the toolchain paths and
dependencies, but sometimes it might require a little nudge with:

* Native compilers: override `CC` or `CXX` for `./configure`

* Freetype lib location: override `--with-freetype-lib`, for example
  `${sysroot}/usr/lib/${target}/`

* Freetype includes location: override `--with-freetype-include` for example
  `${sysroot}/usr/include/freetype2/`

* X11 libraries location: override `--x-libraries`, for example
  `${sysroot}/usr/lib/${target}/`

Architectures that are known to successfully cross-compile like this are:

| Target       | Debian tree  | Debian arch   | `--openjdk-target=...`   | `--with-jvm-variants=...` |
| ------------ | ------------ | ------------- | ------------------------ | ------------------------- |
| x86          | buster       | i386          | i386-linux-gnu           | (all)                     |
| arm          | buster       | armhf         | arm-linux-gnueabihf      | (all)                     |
| aarch64      | buster       | arm64         | aarch64-linux-gnu        | (all)                     |
| ppc64le      | buster       | ppc64el       | powerpc64le-linux-gnu    | (all)                     |
| s390x        | buster       | s390x         | s390x-linux-gnu          | (all)                     |
| mipsle       | buster       | mipsel        | mipsel-linux-gnu         | zero                      |
| mips64le     | buster       | mips64el      | mips64el-linux-gnueabi64 | zero                      |
| armel        | buster       | arm           | arm-linux-gnueabi        | zero                      |
| ppc          | sid          | powerpc       | powerpc-linux-gnu        | zero                      |
| ppc64be      | sid          | ppc64         | powerpc64-linux-gnu      | (all)                     |
| m68k         | sid          | m68k          | m68k-linux-gnu           | zero                      |
| alpha        | sid          | alpha         | alpha-linux-gnu          | zero                      |
| sh4          | sid          | sh4           | sh4-linux-gnu            | zero                      |
| riscv64      | sid          | riscv64       | riscv64-linux-gnu        | (all)                     |

### Considerations for Specific Targets

#### Building for ARM32

A common cross-compilation target is the ARM CPU. When building for ARM, it is
recommended to set the ABI profile. A number of pre-defined ABI profiles are
available using `--with-abi-profile`: `arm-vfp-sflt`, `arm-vfp-hflt`,
`arm-sflt`, `armv5-vfp-sflt` and `armv6-vfp-hflt`. Note that soft-float ABIs
are no longer properly supported by the JDK.

#### Building for RISC-V

The RISC-V community provides a basic [GNU compiler toolchain](
https://github.com/riscv-collab/riscv-gnu-toolchain), but the [external
libraries](#external-library-requirements) required by the JDK complicate the
building process. The placeholder `<toolchain-installed-path>` shown below is
the path where you want to install the toolchain.

* Install the RISC-V GNU compiler toolchain:

  ```
  git clone --recursive https://github.com/riscv-collab/riscv-gnu-toolchain
  cd riscv-gnu-toolchain
  ./configure --prefix=<toolchain-installed-path>
  make linux
  export PATH=<toolchain-installed-path>/bin:$PATH
  ```

* Cross-compile all the required libraries:

  ```
  # An example for libffi
  git clone https://github.com/libffi/libffi
  cd libffi
  ./configure --host=riscv64-unknown-linux-gnu --prefix=<toolchain-installed-path>/sysroot/usr
  make
  make install
  ```

* Configure and build the JDK:

  ```
  bash configure \
    --with-boot-jdk=$BOOT_JDK \
    --openjdk-target=riscv64-linux-gnu \
    --with-sysroot=<toolchain-installed-path>/sysroot \
    --with-toolchain-path=<toolchain-installed-path>/bin \
    --with-extra-path=<toolchain-installed-path>/bin
  make images
  ```

#### Building for musl

Just like it's possible to cross-compile for a different CPU, it's possible to
cross-compile for `musl` libc on a glibc-based *build* system. A devkit
suitable for most target CPU architectures can be obtained from
[musl.cc](https://musl.cc). After installing the required packages in the
sysroot, configure the build with `--openjdk-target`:

```
sh ./configure --with-jvm-variants=server \
--with-boot-jdk=$BOOT_JDK \
--with-build-jdk=$BUILD_JDK \
--openjdk-target=x86_64-unknown-linux-musl \
--with-devkit=$DEVKIT \
--with-sysroot=$SYSROOT
```

and run `make` normally.

#### Building for Windows AArch64
The Visual Studio Build Tools can be used for building the JDK without a full
Visual Studio installation. To set up the Visual Studio 2022 Build Tools on a
Windows AArch64 machine for a native build, launch the installer as follows
in a Windows command prompt:

```
vs_buildtools.exe --quiet --wait --norestart --nocache ^
--installPath "%ProgramFiles(x86)%\Microsoft Visual Studio\2022\BuildTools" ^
--add Microsoft.VisualStudio.Component.VC.CoreBuildTools ^
--add Microsoft.VisualStudio.Component.VC.Tools.ARM64 ^
--add Microsoft.VisualStudio.Component.Windows11SDK.22621
```

To generate Windows AArch64 builds using Cygwin on a Windows x64 machine,
you must set the proper target platform by adding
`--openjdk-target=aarch64-unknown-cygwin` to your configure command line.

## Build Performance

Building the JDK requires a lot of horsepower. Some of the build tools can be
adjusted to utilize more or less of resources such as parallel threads and
memory. The `configure` script analyzes your system and selects reasonable
values for such options based on your hardware. If you encounter resource
problems, such as out of memory conditions, you can modify the detected values
with:

* `--with-num-cores` -- number of cores in the build system, e.g.
  `--with-num-cores=8`.

* `--with-memory-size` -- memory (in MB) available in the build system, e.g.
  `--with-memory-size=1024`

You can also specify directly the number of build jobs to use with
`--with-jobs=N` to `configure`, or `JOBS=N` to `make`. Do not use the `-j` flag
to `make`. In most cases it will be ignored by the makefiles, but it can cause
problems for some make targets.

It might also be necessary to specify the JVM arguments passed to the Boot JDK,
using e.g. `--with-boot-jdk-jvmargs="-Xmx8G"`. Doing so will override the
default JVM arguments passed to the Boot JDK.

At the end of a successful execution of `configure`, you will get a performance
summary, indicating how well the build will perform. Here you will also get
performance hints. If you want to build fast, pay attention to those!

If you want to tweak build performance, run with `make LOG=info` to get a build
time summary at the end of the build process.

### Disk Speed

If you are using network shares, e.g. via NFS, for your source code, make sure
the build directory is situated on local disk (e.g. by `ln -s
/localdisk/jdk-build $JDK-SHARE/build`). The performance penalty is extremely
high for building on a network share; close to unusable.

Also, make sure that your build tools (including Boot JDK and toolchain) is
located on a local disk and not a network share.

As has been stressed elsewhere, do use SSD for source code and build directory,
as well as (if possible) the build tools.

### Virus Checking

The use of virus checking software, especially on Windows, can *significantly*
slow down building of the JDK. If possible, turn off such software, or exclude
the directory containing the JDK source code from on-the-fly checking.

### Ccache

The JDK build supports building with ccache when using gcc or clang. Using
ccache can radically speed up compilation of native code if you often rebuild
the same sources. Your mileage may vary however, so we recommend evaluating it
for yourself. To enable it, make sure it's on the path and configure with
`--enable-ccache`.

### Precompiled Headers

By default, the Hotspot build uses pre-compiled headers (PCH) on the toolchains
were it is properly supported (clang, gcc, and Visual Studio). Normally, this
speeds up the build process, but in some circumstances, it can actually slow
things down.

You can experiment by disabling pre-compiled headers using
`--disable-precompiled-headers`.

### Icecc / Icecream

[icecc/icecream](https://github.com/icecc/icecream) is a simple way to setup a
distributed compiler network. If you have multiple machines available for
building the JDK, you can drastically cut individual build times by utilizing
it.

To use, setup an icecc network, and install icecc on the build machine. Then
run `configure` using `--enable-icecc`.

### Using the javac Server

To speed up compilation of Java code, especially during incremental
compilations, the javac server is automatically enabled in the configuration
step by default. To explicitly enable or disable the javac server, use either
`--enable-javac-server` or `--disable-javac-server`.

### Building the Right Target

Selecting the proper target to build can have dramatic impact on build time.
For normal usage, `jdk` or the default target is just fine. You only need to
build `images` for shipping, or if your tests require it.

See also [Using Fine-Grained Make Targets](#using-fine-grained-make-targets) on
how to build an even smaller subset of the product.

## Troubleshooting

If your build fails, it can sometimes be difficult to pinpoint the problem or
find a proper solution.

### Locating the Source of the Error

When a build fails, it can be hard to pinpoint the actual cause of the error.
In a typical build process, different parts of the product build in parallel,
with the output interlaced.

#### Build Failure Summary

To help you, the build system will print a failure summary at the end. It looks
like this:

```
ERROR: Build failed for target 'hotspot' in configuration 'linux-x64' (exit code 2)

=== Output from failing command(s) repeated here ===
* For target hotspot_variant-server_libjvm_objs_psMemoryPool.o:
/src/jdk/hotspot/src/share/vm/services/psMemoryPool.cpp:1:1: error: 'failhere' does not name a type
   ... (rest of output omitted)

* All command lines available in /src/jdk/build/linux-x64/make-support/failure-logs.
=== End of repeated output ===

=== Make failed targets repeated here ===
lib/CompileJvm.gmk:207: recipe for target '/src/jdk/build/linux-x64/hotspot/variant-server/libjvm/objs/psMemoryPool.o' failed
make/Main.gmk:263: recipe for target 'hotspot-server-libs' failed
=== End of repeated output ===

HELP: Try searching the build log for the name of the first failed target.
HELP: Run 'make doctor' to diagnose build problems.
```

Let's break it down! First, the selected configuration, and the top-level
target you entered on the command line that caused the failure is printed.

Then, between the `Output from failing command(s) repeated here` and `End of
repeated output` the first lines of output (stdout and stderr) from the actual
failing command is repeated. In most cases, this is the error message that
caused the build to fail. If multiple commands were failing (this can happen in
a parallel build), output from all failed commands will be printed here.

The path to the `failure-logs` directory is printed. In this file you will find
a `<target>.log` file that contains the output from this command in its
entirety, and also a `<target>.cmd`, which contain the complete command line
used for running this command. You can re-run the failing command by executing
`. <path to failure-logs>/<target>.cmd` in your shell.

Another way to trace the failure is to follow the chain of make targets, from
top-level targets to individual file targets. Between `Make failed targets
repeated here` and `End of repeated output` the output from make showing this
chain is repeated. The first failed recipe will typically contain the full path
to the file in question that failed to compile. Following lines will show a
trace of make targets why we ended up trying to compile that file.

Finally, some hints are given on how to locate the error in the complete log.
In this example, we would try searching the log file for "`psMemoryPool.o`".
Another way to quickly locate make errors in the log is to search for "`]
Error`" or "`***`".

Note that the build failure summary will only help you if the issue was a
compilation failure or similar. If the problem is more esoteric, or is due to
errors in the build machinery, you will likely get empty output logs, and `No
indication of failed target found` instead of the make target chain.

#### Checking the Build Log File

The output (stdout and stderr) from the latest build is always stored in
`$BUILD/build.log`. The previous build log is stored as `build.log.old`. This
means that it is not necessary to redirect the build output yourself if you
want to process it.

You can increase the verbosity of the log file, by the `LOG` control variable
to `make`. If you want to see the command lines used in compilations, use
`LOG=cmdlines`. To increase the general verbosity, use `LOG=info`, `LOG=debug`
or `LOG=trace`. Both of these can be combined with `cmdlines`, e.g.
`LOG=info,cmdlines`. The `debug` log level will show most shell commands
executed by make, and `trace` will show all. Beware that both these log levels
will produce a massive build log!

### Fixing Unexpected Build Failures

Most of the time, the build will fail due to incorrect changes in the source
code.

Sometimes the build can fail with no apparent changes that have caused the
failure. If this is the first time you are building the JDK on this particular
computer, and the build fails, the problem is likely with your build
environment. But even if you have previously built the JDK with success, and it
now fails, your build environment might have changed (perhaps due to OS
upgrades or similar). But most likely, such failures are due to problems with
the incremental rebuild.

#### Running "make doctor"

The build system comes with a built-in problem diagnosing system. If you
encounter unexpected build failures, you are highly encouraged to run `make
doctor`. The build system will check for common sources of build problems and
suggest suitable actions to take to fix those problems.

These checks are not done during normal build time since they are either too
expensive performance-wise to perform, or since they are not conclusive and
just an indication about a potential problem.

The output from `make doctor` can look like this:

```
"make doctor" will help you analyze your build environment. It can highlight
certain well-known problems, but it can never find all possible errors.

* Verifying that configure has picked up git...

* Checking for warnings from configure...
 ---
The following warnings were produced. Repeated here for convenience:
WARNING: pandoc is version 3.1.9, not the recommended version 2.19.2
 ---
! Inspect the warnings, fix any problems, and re-run configure

* Checking for left-over core files...
Found these potential core files. They might interfere with the build process:
 ---
src/hotspot/core.1337
 ---
! Remove left-over core files

* Checking for untracked files with illegal names...

* If all else fails, try removing the entire build directory and re-creating
the same configuration using:
 ---
configure_command_line=$(make print-configuration)
make dist-clean
bash configure $configure_command_line
 ---

* The build README (doc/building.md) is a great source of information,
especially the chapter "Fixing Unexpected Build Failures". Check it out!

* If you still need assistance please contact build-dev@openjdk.org.
```

#### Problems with the Build Environment

Make sure your configuration is correct. Re-run `configure`, and look for any
warnings. Warnings that appear in the middle of the `configure` output is also
repeated at the end, after the summary. The entire log is stored in
`$BUILD/configure.log`.

Verify that the summary at the end looks correct. Are you indeed using the Boot
JDK and native toolchain that you expect?

By default, the JDK has a strict approach where warnings from the compiler is
considered errors which fail the build. For very new or very old compiler
versions, this can trigger new classes of warnings, which thus fails the build.
Run `configure` with `--disable-warnings-as-errors` to turn of this behavior.
(The warnings will still show, but not make the build fail.)

#### Problems with Incremental Rebuilds

Incremental rebuilds mean that when you modify part of the product, only the
affected parts get rebuilt. While this works great in most cases, and
significantly speed up the development process, from time to time complex
interdependencies will result in an incorrect build result. This is the most
common cause for unexpected build problems.

Here are a suggested list of things to try if you are having unexpected build
problems. Each step requires more time than the one before, so try them in
order. Most issues will be solved at step 1 or 2.

 1. Make sure your repository is up-to-date

    Run `git pull origin master` to make sure you have the latest changes.

 2. Clean build results

    The simplest way to fix incremental rebuild issues is to run `make clean`.
    This will remove all build results, but not the configuration or any build
    system support artifacts. In most cases, this will solve build errors
    resulting from incremental build mismatches.

 3. Completely clean the build directory.

    If this does not work, the next step is to run `make dist-clean`, or
    removing the build output directory (`$BUILD`). This will clean all
    generated output, including your configuration. You will need to re-run
    `configure` after this step. A good idea is to run `make
    print-configuration` before running `make dist-clean`, as this will print
    your current `configure` command line. Here's a way to do this:

    ```
    make print-configuration > current-configuration
    make dist-clean
    bash configure $(cat current-configuration)
    make
    ```

 4. Re-clone the Git repository

    Sometimes the Git repository gets in a state that causes the product to be
    un-buildable. In such a case, the simplest solution is often the
    "sledgehammer approach": delete the entire repository, and re-clone it. If
    you have local changes, save them first to a different location using `git
    format-patch`.

### Specific Build Issues

#### Clock Skew

If you get an error message like this:

```
File 'xxx' has modification time in the future.
Clock skew detected. Your build may be incomplete.
```

then the clock on your build machine is out of sync with the timestamps on the
source files. Other errors, apparently unrelated but in fact caused by the
clock skew, can occur along with the clock skew warnings. These secondary
errors may tend to obscure the fact that the true root cause of the problem is
an out-of-sync clock.

If you see these warnings, reset the clock on the build machine, run `make
clean` and restart the build.

#### Out of Memory Errors

On Windows, you might get error messages like this:

```
fatal error - couldn't allocate heap
cannot create ... Permission denied
spawn failed
```

This can be a sign of a Cygwin problem. See the information about solving
problems in the [Cygwin](#cygwin) section. Rebooting the computer might help
temporarily.

#### Spaces in Path

On Windows, when configuring, `fixpath.sh` may report that some directory names
have spaces. Usually, it assumes those directories have [short
paths](https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/fsutil-8dot3name).
You can run `fsutil file setshortname` in `cmd` on directories to assign
arbitrary short paths so `configure` can access them. If the result says "Access
denied", it may be that there are processes running in that directory; in this
case, you can reboot Windows in safe mode and run the command on those directories
again.

The only directories required to have short paths are `Microsoft Visual Studio`
and `Windows Kits`; the rest of the "contains space" warnings from `configure`,
such as `IntelliJ IDEA`, can be ignored. You can choose any short name; once it
is set, `configure`'s tools like `cygpath` can convert the directory with spaces
to your chosen short name and pass it to the build system.

### Getting Help

If none of the suggestions in this document helps you, or if you find what you
believe is a bug in the build system, please contact the Build Group by sending
a mail to [build-dev@openjdk.org](mailto:build-dev@openjdk.org). Please include
the relevant parts of the configure and/or build log.

If you need general help or advice about developing for the JDK, you can also
contact the Adoption Group. See the section on [Contributing to OpenJDK](
#contributing-to-the-jdk) for more information.

## Reproducible Builds

Build reproducibility is the property of getting exactly the same bits out when
building, every time, independent on who builds the product, or where. This is
for many reasons a harder goal than it initially appears, but it is an
important goal, for security reasons and others. Please see [Reproducible
Builds]( https://reproducible-builds.org) for more information about the
background and reasons for reproducible builds.

Currently, it is not possible to build the JDK fully reproducibly, but getting
there is an ongoing effort.

An absolute prerequisite for building reproducible is to specify a fixed build
time, since time stamps are embedded in many file formats. This is done by
setting the `SOURCE_DATE_EPOCH` environment variable, which is an [industry
standard]( https://reproducible-builds.org/docs/source-date-epoch/), that many
tools, such as gcc, recognize, and use in place of the current time when
generating output.

To generate reproducible builds, you must set `SOURCE_DATE_EPOCH` before
running `configure`. The value in `SOURCE_DATE_EPOCH` will be stored in the
configuration, and used by `make`. Setting `SOURCE_DATE_EPOCH` before running
`make` will have no effect on the build.

You must also make sure your build does not rely on `configure`'s default ad
hoc version strings. Default ad hoc version strings `OPT` segment include user
name and source directory. You can either override just the `OPT` segment using
`--with-version-opt=<any fixed string>`, or you can specify the entire version
string using `--with-version-string=<your version>`.

This is a typical example of how to build the JDK in a reproducible way:

```
export SOURCE_DATE_EPOCH=946684800
bash configure --with-version-opt=adhoc
make
```

Note that regardless of whether you specify a source date for `configure` or
not, the JDK build system will set `SOURCE_DATE_EPOCH` for all build tools when
building. If `--with-source-date` has the value `current` (which is the default
unless `SOURCE_DATE_EPOCH` is found by in the environment by `configure`), the
source date value will be determined at configure time.

There are several aspects of reproducible builds that can be individually
adjusted by `configure` arguments. If any of these are given, they will
override the value derived from `SOURCE_DATE_EPOCH`. These arguments are:

* `--with-source-date`

 This option controls how the JDK build sets `SOURCE_DATE_EPOCH` when building.
 It can be set to a value describing a date, either an epoch based timestamp as
 an integer, or a valid ISO-8601 date.

 It can also be set to one of the special values `current`, `updated` or
 `version`. `current` means that the time of running `configure` will be used.
 `version` will use the nominal release date for the current JDK version.
 `updated`, which means that `SOURCE_DATE_EPOCH` will be set to the current
 time each time you are running `make`. All choices, except for `updated`, will
 set a fixed value for the source date timestamp.

 When `SOURCE_DATE_EPOCH` is set, the default value for `--with-source-date`
 will be the value given by `SOURCE_DATE_EPOCH`. Otherwise, the default value
 is `current`.

* `--with-hotspot-build-time`

 This option controls the build time string that will be included in the
 hotspot library (`libjvm.so` or `jvm.dll`). If the source date is fixed (e.g.
 by setting `SOURCE_DATE_EPOCH`), the default value for
 `--with-hotspot-build-time` will be an ISO 8601 representation of that time
 stamp. Otherwise the default value will be the current time when building
 hotspot.

* `--with-copyright-year`

 This option controls the copyright year in some generated text files. When the
 source date is fixed (e.g. by setting `SOURCE_DATE_EPOCH`), the default value
 for `--with-copyright-year` will be the year of that time stamp. Otherwise the
 default is the current year at the time of running configure. This can be
 overridden by `--with-copyright-year=<year>`.

* `--enable-reproducible-build`

 This option controls additional behavior needed to make the build
 reproducible. When the source date is fixed (e.g. by setting
 `SOURCE_DATE_EPOCH`), this flag will be turned on by default. Otherwise, the
 value is determined by heuristics. If it is explicitly turned off, the build
 might not be reproducible.

## Hints and Suggestions for Advanced Users

### Bash Completion

The `configure` and `make` commands try to play nice with bash command-line
completion (using `<tab>` or `<tab><tab>`). To use this functionality, make
sure you enable completion in your `~/.bashrc` (see instructions for bash in
your operating system).

Make completion will work out of the box, and will complete valid make targets.
For instance, typing `make jdk-i<tab>` will complete to `make jdk-image`.

The `configure` script can get completion for options, but for this to work you
need to help `bash` on the way. The standard way of running the script, `bash
configure`, will not be understood by bash completion. You need `configure` to
be the command to run. One way to achieve this is to add a simple helper script
to your path:

```
cat << EOT > /tmp/configure
#!/bin/bash
if [ \$(pwd) = \$(cd \$(dirname \$0); pwd) ] ; then
  echo >&2 "Abort: Trying to call configure helper recursively"
  exit 1
fi

bash \$PWD/configure "\$@"
EOT
chmod +x /tmp/configure
sudo mv /tmp/configure /usr/local/bin
```

Now `configure --en<tab>-dt<tab>` will result in `configure --enable-dtrace`.

### Using Multiple Configurations

You can have multiple configurations for a single source repository. When you
create a new configuration, run `configure --with-conf-name=<name>` to create a
configuration with the name `<name>`. Alternatively, you can create a directory
under `build` and run `configure` from there, e.g. `mkdir build/<name> && cd
build/<name> && bash ../../configure`.

Then you can build that configuration using `make CONF=<selector>`, where
`<selector>` is interpreted as follows:

* If `<selector>` exacly matches the name of a configuration, this and only
  this configuration will be selected.
* If `<selector>` matches (i.e. is a substring of) the names of several
  configurations, then all these configurations will be selected.
* If `<selector>` is empty (i.e. `CONF=`), then all configurations will be
  selected.
* If `<selector>` begins with `!`, then all configurations **not** matching the
  string following `!` will be selected.

A more specialized version, `CONF_NAME=<name>` also exists, which will only
match if the given `<name>` exactly matches a single configuration.

Alternatively, you can execute `make` in the configuration directory, e.g. `cd
build/<name> && make`.

`make CONF_NAME=<name>` or

### Handling Reconfigurations

If you update the repository and part of the configure script has changed, the
build system will force you to re-run `configure`.

Most of the time, you will be fine by running `configure` again with the same
arguments as the last time, which can easily be performed by `make
reconfigure`. To simplify this, you can use the `CONF_CHECK` make control
variable, either as `make CONF_CHECK=auto`, or by setting an environment
variable. For instance, if you add `export CONF_CHECK=auto` to your `.bashrc`
file, `make` will always run `reconfigure` automatically whenever the configure
script has changed.

You can also use `CONF_CHECK=ignore` to skip the check for a needed configure
update. This might speed up the build, but comes at the risk of an incorrect
build result. This is only recommended if you know what you're doing.

From time to time, you will also need to modify the command line to `configure`
due to changes. Use `make print-configuration` to show the command line used
for your current configuration.

### Using Fine-Grained Make Targets

The default behavior for make is to create consistent and correct output, at
the expense of build speed, if necessary.

If you are prepared to take some risk of an incorrect build, and know enough of
the system to understand how things build and interact, you can speed up the
build process considerably by instructing make to only build a portion of the
product.

#### Building Individual Modules

The safe way to use fine-grained make targets is to use the module specific
make targets. All source code in the JDK is organized so it belongs to a
module, e.g. `java.base` or `jdk.jdwp.agent`. You can build only a specific
module, by giving it as make target: `make jdk.jdwp.agent`. If the specified
module depends on other modules (e.g. `java.base`), those modules will be built
first.

You can also specify a set of modules, just as you can always specify a set of
make targets: `make jdk.crypto.cryptoki jdk.crypto.ec jdk.crypto.mscapi`

#### Building Individual Module Phases

The build process for each module is divided into separate phases. Not all
modules need all phases. Which are needed depends on what kind of source code
and other artifact the module consists of. The phases are:

* `gensrc` (Generate source code to compile)
* `gendata` (Generate non-source code artifacts)
* `copy` (Copy resource artifacts)
* `java` (Compile Java code)
* `launchers` (Compile native executables)
* `libs` (Compile native libraries)

You can build only a single phase for a module by using the notation
`$MODULE-$PHASE`. For instance, to build the `gensrc` phase for `java.base`,
use `make java.base-gensrc`.

Note that some phases may depend on others, e.g. `java` depends on `gensrc` (if
present). Make will build all needed prerequisites before building the
requested phase.

#### Skipping the Dependency Check

When using an iterative development style with frequent quick rebuilds, the
dependency check made by make can take up a significant portion of the time
spent on the rebuild. In such cases, it can be useful to bypass the dependency
check in make.

**Note that if used incorrectly, this method can lead to a broken build!**

To achieve this, append `-only` to the build target. For instance, `make
jdk.jdwp.agent-java-only` will *only* build the `java` phase of the
`jdk.jdwp.agent` module. If the required dependencies are not present, the
build can fail. On the other hand, the execution time measures in milliseconds.

A useful pattern is to build the first time normally (e.g. `make
jdk.jdwp.agent`) and then on subsequent builds, use the `-only` make target.

#### Rebuilding Part of java.base (JDK\_FILTER)

In older versions of the JDK, inefficiencies when building `java.base` (by far
the largest module in the JDK) could be overcome by using the make control
variable `JDK_FILTER`. This is not needed anymore for performance reasons, but
the functionality is still present.

To use this, set the make control variable `JDK_FILTER` to specify a pattern
that will be used to limit the set of files being recompiled. For instance,
`make java.base JDK_FILTER=javax/crypto` will limit the compilation to files in
the `javax.crypto` package.

## Understanding the Build System

This section will give you a more technical description on the details of the
build system.

### Configurations

The build system expects to find one or more configuration. These are
technically defined by the `spec.gmk` in a subdirectory to the `build`
subdirectory. The `spec.gmk` file is generated by `configure`, and contains in
principle the configuration (directly or by files included by `spec.gmk`).

You can, in fact, select a configuration to build by pointing to the `spec.gmk`
file with the `SPEC` make control variable, e.g. `make SPEC=$BUILD/spec.gmk`.
While this is not the recommended way to call `make` as a user, it is what is
used under the hood by the build system.

### Build Output Structure

The build output for a configuration will end up in `build/<configuration
name>`, which we refer to as `$BUILD` in this document. The `$BUILD` directory
contains the following important directories:

```
buildtools/
configure-support/
hotspot/
images/
jdk/
make-support/
support/
test-results/
test-support/
```

This is what they are used for:

* `images`: This is the directory were the output of the `*-image` make targets
  end up. For instance, `make jdk-image` ends up in `images/jdk`.

* `jdk`: This is the "exploded image". After `make jdk`, you will be able to
  launch the newly built JDK by running `$BUILD/jdk/bin/java`.

* `test-results`: This directory contains the results from running tests.

* `support`: This is an area for intermediate files needed during the build,
  e.g. generated source code, object files and class files. Some noteworthy
  directories in `support` is `gensrc`, which contains the generated source
  code, and the `modules_*` directories, which contains the files in a
  per-module hierarchy that will later be collapsed into the `jdk` directory of
  the exploded image.

* `buildtools`: This is an area for tools compiled for the build platform that
  are used during the rest of the build.

* `hotspot`: This is an area for intermediate files needed when building
  hotspot.

* `configure-support`, `make-support` and `test-support`: These directories
  contain files that are needed by the build system for `configure`, `make` and
  for running tests.

### Fixpath

Windows path typically look like `C:\User\foo`, while Unix paths look like
`/home/foo`. Tools with roots from Unix often experience issues related to this
mismatch when running on Windows.

In the JDK build, we always use Unix paths internally, and only just before
calling a tool that does not understand Unix paths do we convert them to
Windows paths.

This conversion is done by the `fixpath.sh` tool, which is a small wrapper that
modifies Unix-style paths to Windows-style paths. The fixpath tool is called
with the first argument as a sub-command describing the action it should take.
Available actions are `import`, `exec`, `print` and `verify`.

* `import` is called at configure time to convert a path given by the user and
  that might be in Windows format to Unix path, which is used internally.

* `exec` is called at build time. This will take the command line provided,
  complete with arguments, converting the paths in the command line, and then
  execute the command.

* `print` is called at build time, in the rare cases where a path might be
  needed in Windows format, but not as an argument to a command to execute.

* `verify` is called at configure time to check that a path is correctly
  specified and reachable by Windows.

The fixpath tool uses a somewhat complex heuristic to infer which part of the
command line arguments refer to paths, and converts those. In some
circumstances, these heuristics can fail.

If you are having strange build issues related to path conversion, you might
need to debug how fixpath treats your paths. Here are some ways to do this.

One way is to define the environment variable `DEBUG_FIXPATH`, e.g.
`DEBUG_FIXPATH=1 make jdk`. When set, any call to `fixpath exec` will result in
an output like this:

```
fixpath: debug: input: ls /mnt/c/windows
fixpath: debug: output: ls c:\windows
```

You can also call fixpath yourself manually with your paths to see how they are
translated. For this, use `print` and `import`. For example:

```
$ bash make/scripts/fixpath.sh print /mnt/c/windows
c:\windows
$ bash make/scripts/fixpath.sh import "c:\\windows"
/mnt/c/windows
```

Remember that backslash is used as an escape character in the shell, and needs
to be doubled when used in Windows paths.

### Native Debug Symbols

Native libraries and executables can have debug symbol (and other debug
information) associated with them. How this works is very much platform
dependent, but a common problem is that debug symbol information takes a lot of
disk space, but is rarely needed by the end user.

The JDK supports different methods on how to handle debug symbols. The method
used is selected by `--with-native-debug-symbols`, and available methods are
`none`, `internal`, `external`, `zipped`.

* `none` means that no debug symbols will be generated during the build.

* `internal` means that debug symbols will be generated during the build, and
  they will be stored in the generated binary.

* `external` means that debug symbols will be generated during the build, and
  after the compilation, they will be moved into a separate `.debuginfo` file.
  (This was previously known as FDS, Full Debug Symbols).

* `zipped` is like `external`, but the .debuginfo file will also be zipped into
  a `.diz` file.

When building for distribution, `zipped` is a good solution. Binaries built
with `internal` is suitable for use by developers, since they facilitate
debugging, but should be stripped before distributed to end users.

### Autoconf Details

The `configure` script is using the autoconf framework, but it has grown to
deviate quite a lot from a traditional autoconf `configure` script.

The `configure` script in the top level directory of the JDK is just a thin
wrapper that calls `make/autoconf/configure`. This in turn will run `autoconf`
to create the runnable (generated) configure script, as
`.build/generated-configure.sh`. Apart from being responsible for the
generation of the runnable script, the `configure` script also provides
functionality that is not easily expressed in the normal Autoconf framework. As
part of this functionality, the generated script is called.

The build system will detect if the Autoconf source files have changed, and
will trigger a regeneration of the generated script if needed. You can also
manually request such an update by `bash configure autogen`.

### Developing the Build System Itself

This section contains a few remarks about how to develop for the build system
itself. It is not relevant if you are only making changes in the product source
code.

While technically using `make`, the make source files of the JDK does not
resemble most other Makefiles. Instead of listing specific targets and actions
(perhaps using patterns), the basic modus operandi is to call a high-level
function (or properly, macro) from the API in `make/common`. For instance, to
compile all classes in the `jdk.internal.foo` package in the `jdk.foo` module,
a call like this would be made:

```
$(eval $(call SetupJavaCompilation, BUILD_FOO_CLASSES, \
    SETUP := GENERATE_OLDBYTECODE, \
    SRC := $(TOPDIR)/src/jkd.foo/share/classes, \
    INCLUDES := jdk/internal/foo, \
    BIN := $(SUPPORT_OUTPUTDIR)/foo_classes, \
))
```

By encapsulating and expressing the high-level knowledge of *what* should be
done, rather than *how* it should be done (as is normal in Makefiles), we can
build a much more powerful and flexible build system.

Correct dependency tracking is paramount. Sloppy dependency tracking will lead
to improper parallelization, or worse, race conditions.

To test for/debug race conditions, try running `make JOBS=1` and `make
JOBS=100` and see if it makes any difference. (It shouldn't).

To compare the output of two different builds and see if, and how, they differ,
run `$BUILD1/compare.sh -o $BUILD2`, where `$BUILD1` and `$BUILD2` are the two
builds you want to compare.

To automatically build two consecutive versions and compare them, use
`COMPARE_BUILD`. The value of `COMPARE_BUILD` is a set of variable=value
assignments, like this:

```
make COMPARE_BUILD=CONF=--enable-new-hotspot-feature:MAKE=hotspot
```

See `make/InitSupport.gmk` for details on how to use `COMPARE_BUILD`.

To analyze build performance, run with `LOG=trace` and check
`$BUILD/build-trace-time.log`. Use `JOBS=1` to avoid parallelism.

Please check that you adhere to the [Code Conventions for the Build System](
https://openjdk.org/groups/build/doc/code-conventions.html) before submitting
patches.

## Contributing to the JDK

So, now you've built your JDK, and made your first patch, and want to
contribute it back to the OpenJDK Community.

First of all: Thank you! We gladly welcome your contribution. However, please
bear in mind that the JDK is a massive project, and we must ask you to follow
our rules and guidelines to be able to accept your contribution.

The official place to start is the [OpenJDK Developers’ Guide](
https://openjdk.org/guide/).

## Editing This Document

If you want to contribute changes to this document, edit `doc/building.md` and
then run `make update-build-docs` to generate the same changes in
`doc/building.html`.

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