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

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package java.lang.invoke;

import jdk.internal.vm.annotation.ForceInline;
import jdk.internal.vm.annotation.Stable;

import java.util.Optional;

import static java.lang.invoke.MethodHandleStatics.UNSAFE;
import static java.lang.invoke.MethodHandleStatics.uncaughtException;

/**
 * A lazy initializing var handle. It lazily initializes the referenced class before
 * any invocation of the target var handle to prevent reading uninitialized static
 * field values.
 */
final class LazyInitializingVarHandle extends VarHandle {

    // Implementation notes:
    // We put a barrier on both target() (for VH form impl direct invocation)
    // and on getMethodHandle() (for indirect VH invocation, toMethodHandle)
    private final VarHandle target;
    private final Class<?> refc;
    private @Stable boolean initialized;

    LazyInitializingVarHandle(VarHandle target, Class<?> refc) {
        super(target.vform, target.exact);
        this.target = target;
        this.refc = refc;
    }

    @Override
    MethodType accessModeTypeUncached(AccessType at) {
        return target.accessModeType(at.ordinal());
    }

    @Override
    @ForceInline
    VarHandle asDirect() {
        return target;
    }

    @Override
    @ForceInline
    VarHandle target() {
        ensureInitialized();
        return target;
    }

    @Override
    public VarHandle withInvokeExactBehavior() {
        if (!initialized && hasInvokeExactBehavior())
            return this;
        var exactTarget = target.withInvokeExactBehavior();
        return initialized ? exactTarget : new LazyInitializingVarHandle(exactTarget, refc);
    }

    @Override
    public VarHandle withInvokeBehavior() {
        if (!initialized && !hasInvokeExactBehavior())
            return this;
        var nonExactTarget = target.withInvokeBehavior();
        return initialized ? nonExactTarget : new LazyInitializingVarHandle(nonExactTarget, refc);
    }

    @Override
    public Optional<VarHandleDesc> describeConstable() {
        return target.describeConstable();
    }

    @Override
    public MethodHandle getMethodHandleUncached(int accessMode) {
        var mh = target.getMethodHandle(accessMode);
        if (this.initialized)
            return mh;

        return MethodHandles.collectArguments(mh, 0, ensureInitializedMh()).bindTo(this);
    }

    @ForceInline
    private void ensureInitialized() {
        if (this.initialized)
            return;

        initialize();
    }

    private void initialize() {
        UNSAFE.ensureClassInitialized(refc);
        this.initialized = true;

        this.methodHandleTable = target.methodHandleTable;
    }

    private static @Stable MethodHandle MH_ensureInitialized;

    private static MethodHandle ensureInitializedMh() {
        var mh = MH_ensureInitialized;
        if (mh != null)
            return mh;

        try {
            return MH_ensureInitialized = MethodHandles.lookup().findVirtual(
                    LazyInitializingVarHandle.class,
                    "ensureInitialized",
                    MethodType.methodType(void.class));
        } catch (Throwable ex) {
            throw uncaughtException(ex);
        }
    }
}