jdk/dynalink/CallSiteDescriptor.java

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/*
   Copyright 2009-2013 Attila Szegedi

   Licensed under both the Apache License, Version 2.0 (the "Apache License")
   and the BSD License (the "BSD License"), with licensee being free to
   choose either of the two at their discretion.

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   License or the BSD License.

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       You may obtain a copy of the Apache License at

           http://www.apache.org/licenses/LICENSE-2.0

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       distributed under the License is distributed on an "AS IS" BASIS,
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package jdk.dynalink;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodHandles.Lookup;
import java.lang.invoke.MethodType;
import java.util.Objects;
import java.util.function.Supplier;

/**
 * Call site descriptors contain all the information necessary for linking a
 * call site. This information is normally passed as parameters to bootstrap
 * methods and consists of the {@code MethodHandles.Lookup} object on the caller
 * class in which the call site occurs, the dynamic operation at the call
 * site, and the method type of the call site. {@code CallSiteDescriptor}
 * objects are used in Dynalink to capture and store these parameters for
 * subsequent use by the {@link DynamicLinker}.
 * <p>
 * The constructors of built-in {@link RelinkableCallSite} implementations all
 * take a call site descriptor.
 * <p>
 * Call site descriptors must be immutable. You can use this class as-is or you
 * can subclass it, especially if you need to add further information to the
 * descriptors (typically, values passed in additional parameters to the
 * bootstrap method. Since the descriptors must be immutable, you can set up a
 * cache for equivalent descriptors to have the call sites share them.
 * <p>
 * The class extends {@link SecureLookupSupplier} for security-checked access to
 * the {@code MethodHandles.Lookup} object it carries. This lookup should be used
 * to find method handles to set as targets of the call site described by this
 * descriptor.
 */
public class CallSiteDescriptor extends SecureLookupSupplier {
    private final Operation operation;
    private final MethodType methodType;

    /**
     * Creates a new call site descriptor.
     * @param lookup the lookup object describing the class the call site belongs to.
     * When creating descriptors from a {@link java.lang.invoke} bootstrap method,
     * it should be the lookup passed to the bootstrap.
     * @param operation the dynamic operation at the call site.
     * @param methodType the method type of the call site. When creating
     * descriptors from a {@link java.lang.invoke} bootstrap method, it should be
     * the method type passed to the bootstrap.
     */
    public CallSiteDescriptor(final Lookup lookup, final Operation operation, final MethodType methodType) {
        super(lookup);
        this.operation = Objects.requireNonNull(operation, "name");
        this.methodType = Objects.requireNonNull(methodType, "methodType");
    }

    /**
     * Returns the operation at the call site.
     * @return the operation at the call site.
     */
    public final Operation getOperation() {
        return operation;
    }

    /**
     * The type of the method at the call site.
     *
     * @return type of the method at the call site.
     */
    public final MethodType getMethodType() {
        return methodType;
    }

    /**
     * Finds or creates a call site descriptor that only differs in its
     * method type from this descriptor.
     * Invokes {@link #changeMethodTypeInternal(MethodType)}.
     *
     * @param newMethodType the new method type
     * @return a call site descriptor with changed method type.
     * @throws NullPointerException if {@code newMethodType} is null.
     */
    public final CallSiteDescriptor changeMethodType(final MethodType newMethodType) {
        final CallSiteDescriptor changed = changeMethodTypeInternal(newMethodType);

        if (getClass() != CallSiteDescriptor.class) {
            assertChangeInvariants(changed, "changeMethodTypeInternal");
            alwaysAssert(operation == changed.operation, () -> "changeMethodTypeInternal must not change the descriptor's operation");
            alwaysAssert(newMethodType == changed.methodType, () -> "changeMethodTypeInternal didn't set the correct new method type");
        }
        return changed;
    }

    /**
     * Finds or creates a call site descriptor that only differs in its
     * method type from this descriptor. Subclasses must override this method
     * to return an object of their exact class. If an overridden method changes
     * something other than the method type in the descriptor (its class, lookup,
     * or operation), or returns null, an {@code AssertionError} will be thrown
     * from {@link #changeMethodType(MethodType)}.
     *
     * @param newMethodType the new method type
     * @return a call site descriptor with the changed method type.
     */
    protected CallSiteDescriptor changeMethodTypeInternal(final MethodType newMethodType) {
        return new CallSiteDescriptor(getLookupPrivileged(), operation, newMethodType);
    }

    /**
     * Finds or creates a call site descriptor that only differs in its
     * operation from this descriptor.
     * Invokes {@link #changeOperationInternal(Operation)}.
     *
     * @param newOperation the new operation
     * @return a call site descriptor with the changed operation.
     * @throws NullPointerException if {@code newOperation} is null.
     * @throws SecurityException if the descriptor's lookup isn't the
     * {@link MethodHandles#publicLookup()}, and a security manager is present,
     * and a check for {@code RuntimePermission("dynalink.getLookup")} fails.
     * This is necessary as changing the operation in the call site descriptor
     * allows fabrication of descriptors for arbitrary operations with the lookup.
     */
    public final CallSiteDescriptor changeOperation(final Operation newOperation) {
        getLookup(); // force security check
        final CallSiteDescriptor changed = changeOperationInternal(newOperation);

        if (getClass() != CallSiteDescriptor.class) {
            assertChangeInvariants(changed, "changeOperationInternal");
            alwaysAssert(methodType == changed.methodType, () -> "changeOperationInternal must not change the descriptor's method type");
            alwaysAssert(newOperation == changed.operation, () -> "changeOperationInternal didn't set the correct new operation");
        }
        return changed;
    }

    /**
     * Finds or creates a call site descriptor that only differs in its
     * operation from this descriptor. Subclasses must override this method
     * to return an object of their exact class. If an overridden method changes
     * something other than the operation in the descriptor (its class, lookup,
     * or method type), or returns null, an {@code AssertionError} will be thrown
     * from {@link #changeOperation(Operation)}.
     *
     * @param newOperation the new operation
     * @return a call site descriptor with the changed operation.
     */
    protected CallSiteDescriptor changeOperationInternal(final Operation newOperation) {
        return new CallSiteDescriptor(getLookupPrivileged(), newOperation, methodType);
    }

    /**
     * Returns true if this call site descriptor is equal to the passed object.
     * It is considered equal if the other object is of the exact same class,
     * their operations and method types are equal, and their lookups have the
     * same {@link java.lang.invoke.MethodHandles.Lookup#lookupClass()} and
     * {@link java.lang.invoke.MethodHandles.Lookup#lookupModes()}.
     */
    @Override
    public boolean equals(final Object obj) {
        if (obj == this) {
            return true;
        } else if (obj == null) {
            return false;
        } else if (obj.getClass() != getClass()) {
            return false;
        }
        final CallSiteDescriptor other = (CallSiteDescriptor)obj;
        return operation.equals(other.operation) &&
               methodType.equals(other.methodType) &&
               lookupsEqual(getLookupPrivileged(), other.getLookupPrivileged());
    }

    /**
     * Compares two lookup objects for value-based equality. They are considered
     * equal if they have the same
     * {@link java.lang.invoke.MethodHandles.Lookup#lookupClass()} and
     * {@link java.lang.invoke.MethodHandles.Lookup#lookupModes()}.
     * @param l1 first lookup
     * @param l2 second lookup
     * @return true if the two lookups are equal, false otherwise.
     */
    private static boolean lookupsEqual(final Lookup l1, final Lookup l2) {
        return l1.lookupClass() == l2.lookupClass() && l1.lookupModes() == l2.lookupModes();
    }

    /**
     * Returns a value-based hash code of this call site descriptor computed
     * from its operation, method type, and lookup object's lookup class and
     * lookup modes.
     * @return value-based hash code for this call site descriptor.
     */
    @Override
    public int hashCode() {
        return operation.hashCode() + 31 * methodType.hashCode() + 31 * 31 * lookupHashCode(getLookupPrivileged());
    }

    /**
     * Returns a value-based hash code for the passed lookup object. It is
     * based on the lookup object's
     * {@link java.lang.invoke.MethodHandles.Lookup#lookupClass()} and
     * {@link java.lang.invoke.MethodHandles.Lookup#lookupModes()} values.
     * @param lookup the lookup object.
     * @return a hash code for the object..
     */
    private static int lookupHashCode(final Lookup lookup) {
        return lookup.lookupClass().hashCode() + 31 * lookup.lookupModes();
    }

    /**
     * Returns the string representation of this call site descriptor, of the
     * format {@code name(parameterTypes)returnType@lookup}.
     */
    @Override
    public String toString() {
        final String mt = methodType.toString();
        final String l = getLookupPrivileged().toString();
        final String o = operation.toString();
        final StringBuilder b = new StringBuilder(o.length() + mt.length() + 1 + l.length());
        return b.append(o).append(mt).append('@').append(l).toString();
    }

    private void assertChangeInvariants(final CallSiteDescriptor changed, final String caller) {
        alwaysAssert(changed != null, () -> caller + " must not return null.");
        alwaysAssert(getClass() == changed.getClass(), () -> caller + " must not change the descriptor's class");
        alwaysAssert(lookupsEqual(getLookupPrivileged(), changed.getLookupPrivileged()), () -> caller + " must not change the descriptor's lookup");
    }

    private static void alwaysAssert(final boolean cond, final Supplier<String> errorMessage) {
        if (!cond) {
            throw new AssertionError(errorMessage.get());
        }
    }
}