src/java.base/windows/classes/sun/net/dns/ResolverConfigurationImpl.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
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package sun.net.dns;

import jdk.internal.misc.InnocuousThread;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

/*
 * An implementation of sun.net.ResolverConfiguration for Windows.
 */

public final class ResolverConfigurationImpl
    extends ResolverConfiguration
{
    // Lock held whilst loading configuration or checking
    private static final Object lock = new Object();

    // Resolver options
    private final Options opts;

    // Addresses have changed. We default to true to make sure we
    // resolve the first time it is requested.
    private static boolean changed = true;

    // Time of last refresh.
    private static long lastRefresh;

    // Cache timeout (120 seconds) - should be converted into property
    // or configured as preference in the future.
    private static final long TIMEOUT_NANOS = TimeUnit.SECONDS.toNanos(120);

    // DNS suffix list and name servers populated by native method
    private static String os_searchlist;
    private static String os_nameservers;

    // Cached lists
    private static ArrayList<String> searchlist;
    private static ArrayList<String> nameservers;

    // Parse string that consists of token delimited by comma
    // and return ArrayList. Refer to ResolverConfigurationImpl.c and
    // strappend to see how the string is created.
    private ArrayList<String> stringToList(String str) {
        // String is delimited by comma.
        String[] tokens = str.split(",");
        ArrayList<String> l = new ArrayList<>(tokens.length);
        for (String s : tokens) {
            if (!s.isEmpty() && !l.contains(s)) {
                l.add(s);
            }
        }
        l.trimToSize();
        return l;
    }

    // Parse string that consists of token delimited by comma
    // and return ArrayList.  Refer to ResolverConfigurationImpl.c and
    // strappend to see how the string is created.
    // In addition to splitting the string, converts IPv6 addresses to
    // BSD-style.
    private ArrayList<String> addressesToList(String str) {
        // String is delimited by comma
        String[] tokens = str.split(",");
        ArrayList<String> l = new ArrayList<>(tokens.length);

        for (String s : tokens) {
            if (!s.isEmpty()) {
                if (s.indexOf(':') >= 0 && s.charAt(0) != '[') {
                    // Not BSD style
                    s = '[' + s + ']';
                }
                if (!s.isEmpty() && !l.contains(s)) {
                    l.add(s);
                }
            }
        }
        l.trimToSize();
        return l;
    }

    // Load DNS configuration from OS

    private void loadConfig() {
        assert Thread.holdsLock(lock);

        // A change in the network address of the machine usually indicates
        // a change in DNS configuration too so we always refresh the config
        // after such a change.
        if (changed) {
            changed = false;
        } else {
            // Otherwise we refresh if TIMEOUT_NANOS has passed since last
            // load.
            long currTime = System.nanoTime();
            // lastRefresh will always have been set once because we start with
            // changed = true.
            if ((currTime - lastRefresh) < TIMEOUT_NANOS) {
                return;
            }
        }

        // Native code that uses Windows API to find out the DNS server
        // addresses and search suffixes. It builds a comma-delimited string
        // of nameservers and domain suffixes and sets them to the static
        // os_nameservers and os_searchlist. We then split these into Java
        // Lists here.
        loadDNSconfig0();

        // Record the time of update and refresh the lists of addresses /
        // domain suffixes.
        lastRefresh = System.nanoTime();
        searchlist = stringToList(os_searchlist);
        nameservers = addressesToList(os_nameservers);
        os_searchlist = null;                       // can be GC'ed
        os_nameservers = null;
    }

    ResolverConfigurationImpl() {
        opts = new OptionsImpl();
    }

    @SuppressWarnings("unchecked") // clone()
    public List<String> searchlist() {
        synchronized (lock) {
            loadConfig();

            // List is mutable so return a shallow copy
            return (List<String>)searchlist.clone();
        }
    }

    @SuppressWarnings("unchecked") // clone()
    public List<String> nameservers() {
        synchronized (lock) {
            loadConfig();

            // List is mutable so return a shallow copy
            return (List<String>)nameservers.clone();
         }
    }

    public Options options() {
        return opts;
    }

    // --- Address Change Listener

    static class AddressChangeListener implements Runnable {
        @Override
        public void run() {
            for (;;) {
                // wait for configuration to change
                if (notifyAddrChange0() != 0)
                    return;
                synchronized (lock) {
                    changed = true;
                }
            }
        }
    }


    // --- Native methods --

    static native void init0();

    static native void loadDNSconfig0();

    static native int notifyAddrChange0();

    static {
        jdk.internal.loader.BootLoader.loadLibrary("net");
        init0();

        // start the address listener thread
        String name = "Jndi-Dns-address-change-listener";
        Thread addrChangeListener = InnocuousThread.newSystemThread(name,
                new AddressChangeListener());
        addrChangeListener.setDaemon(true);
        addrChangeListener.start();
    }
}

/**
 * Implementation of {@link ResolverConfiguration.Options}
 */
class OptionsImpl extends ResolverConfiguration.Options {
}