src/java.base/share/classes/java/lang/invoke/MethodHandleInfo.java

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package java.lang.invoke;

import java.lang.reflect.*;
import java.util.*;
import java.lang.invoke.MethodHandleNatives.Constants;
import java.lang.invoke.MethodHandles.Lookup;
import static java.lang.invoke.MethodHandleStatics.*;

/**
 * A symbolic reference obtained by cracking a direct method handle
 * into its constituent symbolic parts.
 * To crack a direct method handle, call {@link Lookup#revealDirect Lookup.revealDirect}.
 * <h2><a id="directmh"></a>Direct Method Handles</h2>
 * A <em>direct method handle</em> represents a method, constructor, or field without
 * any intervening argument bindings or other transformations.
 * The method, constructor, or field referred to by a direct method handle is called
 * its <em>underlying member</em>.
 * Direct method handles may be obtained in any of these ways:
 * <ul>
 * <li>By executing an {@code ldc} instruction on a {@code CONSTANT_MethodHandle} constant.
 *     (See the Java Virtual Machine Specification, sections {@jvms
 *     4.4.8} and {@jvms 5.4.3}.)
 * <li>By calling one of the <a href="MethodHandles.Lookup.html#lookups">Lookup Factory Methods</a>,
 *     such as {@link Lookup#findVirtual Lookup.findVirtual},
 *     to resolve a symbolic reference into a method handle.
 *     A symbolic reference consists of a class, name string, and type.
 * <li>By calling the factory method {@link Lookup#unreflect Lookup.unreflect}
 *     or {@link Lookup#unreflectSpecial Lookup.unreflectSpecial}
 *     to convert a {@link Method} into a method handle.
 * <li>By calling the factory method {@link Lookup#unreflectConstructor Lookup.unreflectConstructor}
 *     to convert a {@link Constructor} into a method handle.
 * <li>By calling the factory method {@link Lookup#unreflectGetter Lookup.unreflectGetter}
 *     or {@link Lookup#unreflectSetter Lookup.unreflectSetter}
 *     to convert a {@link Field} into a method handle.
 * </ul>
 *
 * <h2>Restrictions on Cracking</h2>
 * Given a suitable {@code Lookup} object, it is possible to crack any direct method handle
 * to recover a symbolic reference for the underlying method, constructor, or field.
 * Cracking must be done via a {@code Lookup} object equivalent to that which created
 * the target method handle, or which has enough access permissions to recreate
 * an equivalent method handle.
 * <p>
 * If the underlying method is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>,
 * the direct method handle will have been "bound" to a particular caller class, the
 * {@linkplain java.lang.invoke.MethodHandles.Lookup#lookupClass() lookup class}
 * of the lookup object used to create it.
 * Cracking this method handle with a different lookup class will fail
 * even if the underlying method is public (like {@code Class.forName}).
 * <p>
 * The requirement of lookup object matching provides a "fast fail" behavior
 * for programs which may otherwise trust erroneous revelation of a method
 * handle with symbolic information (or caller binding) from an unexpected scope.
 * Use {@link java.lang.invoke.MethodHandles#reflectAs} to override this limitation.
 *
 * <h2><a id="refkinds"></a>Reference kinds</h2>
 * The <a href="MethodHandles.Lookup.html#lookups">Lookup Factory Methods</a>
 * correspond to all major use cases for methods, constructors, and fields.
 * These use cases may be distinguished using small integers as follows:
 * <table class="striped">
 * <caption style="display:none">reference kinds</caption>
 * <thead>
 * <tr><th scope="col">reference kind</th><th scope="col">descriptive name</th><th scope="col">scope</th><th scope="col">member</th><th scope="col">behavior</th></tr>
 * </thead>
 * <tbody>
 * <tr>
 *     <th scope="row">{@code 1}</th><td>{@code REF_getField}</td><td>{@code class}</td>
 *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 2}</th><td>{@code REF_getStatic}</td><td>{@code class} or {@code interface}</td>
 *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 3}</th><td>{@code REF_putField}</td><td>{@code class}</td>
 *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 4}</th><td>{@code REF_putStatic}</td><td>{@code class}</td>
 *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 5}</th><td>{@code REF_invokeVirtual}</td><td>{@code class}</td>
 *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 6}</th><td>{@code REF_invokeStatic}</td><td>{@code class} or {@code interface}</td>
 *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 7}</th><td>{@code REF_invokeSpecial}</td><td>{@code class} or {@code interface}</td>
 *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 8}</th><td>{@code REF_newInvokeSpecial}</td><td>{@code class}</td>
 *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 * </tr>
 * <tr>
 *     <th scope="row">{@code 9}</th><td>{@code REF_invokeInterface}</td><td>{@code interface}</td>
 *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 * </tr>
 * </tbody>
 * </table>
 * @since 1.8
 */
public interface MethodHandleInfo {
    /**
     * A direct method handle reference kind,
     * as defined in the <a href="MethodHandleInfo.html#refkinds">table above</a>.
     */
    public static final int
        REF_getField                = Constants.REF_getField,
        REF_getStatic               = Constants.REF_getStatic,
        REF_putField                = Constants.REF_putField,
        REF_putStatic               = Constants.REF_putStatic,
        REF_invokeVirtual           = Constants.REF_invokeVirtual,
        REF_invokeStatic            = Constants.REF_invokeStatic,
        REF_invokeSpecial           = Constants.REF_invokeSpecial,
        REF_newInvokeSpecial        = Constants.REF_newInvokeSpecial,
        REF_invokeInterface         = Constants.REF_invokeInterface;

    /**
     * Returns the reference kind of the cracked method handle, which in turn
     * determines whether the method handle's underlying member was a constructor, method, or field.
     * See the <a href="MethodHandleInfo.html#refkinds">table above</a> for definitions.
     * @return the integer code for the kind of reference used to access the underlying member
     */
    public int getReferenceKind();

    /**
     * Returns the class in which the cracked method handle's underlying member was defined.
     * @return the declaring class of the underlying member
     */
    public Class<?> getDeclaringClass();

    /**
     * Returns the name of the cracked method handle's underlying member.
     * This is {@value java.lang.constant.ConstantDescs#INIT_NAME}
     * if the underlying member was a constructor,
     * else it is a simple method name or field name.
     * @return the simple name of the underlying member
     */
    public String getName();

    /**
     * Returns the nominal type of the cracked symbolic reference, expressed as a method type.
     * If the reference is to a constructor, the return type will be {@code void}.
     * If it is to a non-static method, the method type will not mention the {@code this} parameter.
     * If it is to a field and the requested access is to read the field,
     * the method type will have no parameters and return the field type.
     * If it is to a field and the requested access is to write the field,
     * the method type will have one parameter of the field type and return {@code void}.
     * <p>
     * Note that original direct method handle may include a leading {@code this} parameter,
     * or (in the case of a constructor) will replace the {@code void} return type
     * with the constructed class.
     * The nominal type does not include any {@code this} parameter,
     * and (in the case of a constructor) will return {@code void}.
     * @return the type of the underlying member, expressed as a method type
     */
    public MethodType getMethodType();

    // Utility methods.
    // NOTE: class/name/type and reference kind constitute a symbolic reference
    // member and modifiers are an add-on, derived from Core Reflection (or the equivalent)

    /**
     * Reflects the underlying member as a method, constructor, or field object.
     * If the underlying member is public, it is reflected as if by
     * {@code getMethod}, {@code getConstructor}, or {@code getField}.
     * Otherwise, it is reflected as if by
     * {@code getDeclaredMethod}, {@code getDeclaredConstructor}, or {@code getDeclaredField}.
     * The underlying member must be accessible to the given lookup object.
     * @param <T> the desired type of the result, either {@link Member} or a subtype
     * @param expected a class object representing the desired result type {@code T}
     * @param lookup the lookup object that created this MethodHandleInfo, or one with equivalent access privileges
     * @return a reference to the method, constructor, or field object
     * @throws    ClassCastException if the member is not of the expected type
     * @throws    NullPointerException if either argument is {@code null}
     * @throws    IllegalArgumentException if the underlying member is not accessible to the given lookup object
     */
    public <T extends Member> T reflectAs(Class<T> expected, Lookup lookup);

    /**
     * Returns the access modifiers of the underlying member.
     * @return the Java language modifiers for underlying member,
     *         or -1 if the member cannot be accessed
     * @see Modifier
     * @see #reflectAs
     */
    public int getModifiers();

    /**
     * Determines if the underlying member was a variable arity method or constructor.
     * Such members are represented by method handles that are varargs collectors.
     * @implSpec
     * This produces a result equivalent to:
     * <pre>{@code
     *     getReferenceKind() >= REF_invokeVirtual && Modifier.isTransient(getModifiers())
     * }</pre>
     *
     *
     * @return {@code true} if and only if the underlying member was declared with variable arity.
     */
    // spelling derived from java.lang.reflect.Executable, not MethodHandle.isVarargsCollector
    public default boolean isVarArgs()  {
        // fields are never varargs:
        if (MethodHandleNatives.refKindIsField((byte) getReferenceKind()))
            return false;
        // not in the public API: Modifier.VARARGS
        final int ACC_VARARGS = 0x00000080;  // from JVMS 4.6 (Table 4.20)
        assert(ACC_VARARGS == Modifier.TRANSIENT);
        return Modifier.isTransient(getModifiers());
    }

    /**
     * Returns the descriptive name of the given reference kind,
     * as defined in the <a href="MethodHandleInfo.html#refkinds">table above</a>.
     * The conventional prefix "REF_" is omitted.
     * @param referenceKind an integer code for a kind of reference used to access a class member
     * @return a mixed-case string such as {@code "getField"}
     * @throws    IllegalArgumentException if the argument is not a valid
     *            <a href="MethodHandleInfo.html#refkinds">reference kind number</a>
     */
    public static String referenceKindToString(int referenceKind) {
        if (!MethodHandleNatives.refKindIsValid(referenceKind))
            throw newIllegalArgumentException("invalid reference kind", referenceKind);
        return MethodHandleNatives.refKindName((byte)referenceKind);
    }

    /**
     * Returns a string representation for a {@code MethodHandleInfo},
     * given the four parts of its symbolic reference.
     * This is defined to be of the form {@code "RK C.N:MT"}, where {@code RK} is the
     * {@linkplain #referenceKindToString reference kind string} for {@code kind},
     * {@code C} is the {@linkplain java.lang.Class#getName name} of {@code defc}
     * {@code N} is the {@code name}, and
     * {@code MT} is the {@code type}.
     * These four values may be obtained from the
     * {@linkplain #getReferenceKind reference kind},
     * {@linkplain #getDeclaringClass declaring class},
     * {@linkplain #getName member name},
     * and {@linkplain #getMethodType method type}
     * of a {@code MethodHandleInfo} object.
     *
     * @implSpec
     * This produces a result equivalent to:
     * <pre>{@code
     *     String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type)
     * }</pre>
     *
     * @param kind the {@linkplain #getReferenceKind reference kind} part of the symbolic reference
     * @param defc the {@linkplain #getDeclaringClass declaring class} part of the symbolic reference
     * @param name the {@linkplain #getName member name} part of the symbolic reference
     * @param type the {@linkplain #getMethodType method type} part of the symbolic reference
     * @return a string of the form {@code "RK C.N:MT"}
     * @throws    IllegalArgumentException if the first argument is not a valid
     *            <a href="MethodHandleInfo.html#refkinds">reference kind number</a>
     * @throws    NullPointerException if any reference argument is {@code null}
     */
    public static String toString(int kind, Class<?> defc, String name, MethodType type) {
        Objects.requireNonNull(name); Objects.requireNonNull(type);
        return String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type);
    }
}