src/java.base/share/classes/jdk/internal/ref/PhantomCleanable.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * accompanied this code).
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package jdk.internal.ref;

import java.lang.ref.Cleaner;
import java.lang.ref.Reference;
import java.lang.ref.PhantomReference;
import java.util.Objects;

/**
 * PhantomCleanable subclasses efficiently encapsulate cleanup state and
 * the cleaning action.
 * Subclasses implement the abstract {@link #performCleanup()}  method
 * to provide the cleaning action.
 * When constructed, the object reference and the {@link Cleaner.Cleanable Cleanable}
 * are registered with the {@link Cleaner}.
 * The Cleaner invokes {@link Cleaner.Cleanable#clean() clean} after the
 * referent becomes phantom reachable.
 */
public abstract class PhantomCleanable<T> extends PhantomReference<T>
        implements Cleaner.Cleanable {

    /**
     * The list of PhantomCleanable; synchronizes insert and remove.
     */
    private final CleanerImpl.CleanableList list;

    /**
     * Node for this PhantomCleanable in the list.
     * Synchronized by the same lock as the list itself.
     */
    CleanerImpl.CleanableList.Node node;

    /**
     * Index of this PhantomCleanable in the list node.
     * Synchronized by the same lock as the list itself.
     */
    int index;

    /**
     * Constructs new {@code PhantomCleanable} with
     * {@code non-null referent} and {@code non-null cleaner}.
     * The {@code cleaner} is not retained; it is only used to
     * register the newly constructed {@link Cleaner.Cleanable Cleanable}.
     *
     * @param referent the referent to track
     * @param cleaner  the {@code Cleaner} to register with
     */
    @SuppressWarnings("this-escape")
    public PhantomCleanable(T referent, Cleaner cleaner) {
        super(Objects.requireNonNull(referent), CleanerImpl.getCleanerImpl(cleaner).queue);
        index = -1;
        list = CleanerImpl.getCleanerImpl(cleaner).activeList;
        list.insert(this);

        // Check that list insertion populated the backlinks.
        assert node != null;
        assert index >= 0;

        // Ensure referent and cleaner remain accessible
        Reference.reachabilityFence(referent);
        Reference.reachabilityFence(cleaner);
    }

    /**
     * Unregister this PhantomCleanable and invoke {@link #performCleanup()},
     * ensuring at-most-once semantics.
     */
    @Override
    public final void clean() {
        if (list.remove(this)) {
            super.clear();
            performCleanup();
        }
    }

    /**
     * Unregister this PhantomCleanable and clear the reference.
     * Due to inherent concurrency, {@link #performCleanup()} may still be invoked.
     */
    @Override
    public void clear() {
        if (list.remove(this)) {
            super.clear();
        }
    }

    /**
     * The {@code performCleanup} abstract method is overridden
     * to implement the cleaning logic.
     * The {@code performCleanup} method should not be called except
     * by the {@link #clean} method which ensures at most once semantics.
     */
    protected abstract void performCleanup();

    /**
     * This method always throws {@link UnsupportedOperationException}.
     * Enqueuing details of {@link Cleaner.Cleanable}
     * are a private implementation detail.
     *
     * @throws UnsupportedOperationException always
     */
    @SuppressWarnings("deprecation")
    @Override
    public final boolean isEnqueued() {
        throw new UnsupportedOperationException("isEnqueued");
    }

    /**
     * This method always throws {@link UnsupportedOperationException}.
     * Enqueuing details of {@link Cleaner.Cleanable}
     * are a private implementation detail.
     *
     * @throws UnsupportedOperationException always
     */
    @Override
    public final boolean enqueue() {
        throw new UnsupportedOperationException("enqueue");
    }
}