16_01_Reference_引用对象体系

一、Reference——强、软、若、虚、终结器引用的共同父类

public abstract class Reference<T> {
    private T referent;         /* Treated specially by GC */
    volatile ReferenceQueue<? super T> queue;
    @SuppressWarnings("rawtypes")
    volatile Reference next;
    private transient Reference<?> discovered;
    private static class ReferenceHandler extends Thread {
        private static void ensureClassInitialized(Class<?> clazz) {
            try {
                Class.forName(clazz.getName(), true, clazz.getClassLoader());
            } catch (ClassNotFoundException e) {
                throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
            }
        }
        static {
            ensureClassInitialized(Cleaner.class);
        }
        ReferenceHandler(ThreadGroup g, String name) {
            super(g, null, name, 0, false);
        }
        public void run() {
            while (true) {
                processPendingReferences();
            }
        }
    }
    /*
     * Atomically get and clear (set to null) the VM's pending-Reference list.
     */
    private static native Reference<?> getAndClearReferencePendingList();
    /*
     * Test whether the VM's pending-Reference list contains any entries.
     */
    private static native boolean hasReferencePendingList();
    /*
     * Wait until the VM's pending-Reference list may be non-null.
     */
    private static native void waitForReferencePendingList();
    /*
     * Enqueue a Reference taken from the pending list.  Calling this method
     * takes us from the Reference<?> domain of the pending list elements to
     * having a Reference<T> with a correspondingly typed queue.
     */
    private void enqueueFromPending() {
        var q = queue;
        if (q != ReferenceQueue.NULL) q.enqueue(this);
    }
    private static final Object processPendingLock = new Object();
    private static boolean processPendingActive = false;
    private static void processPendingReferences() {
        // Only the singleton reference processing thread calls
        // waitForReferencePendingList() and getAndClearReferencePendingList().
        // These are separate operations to avoid a race with other threads
        // that are calling waitForReferenceProcessing().
        waitForReferencePendingList();
        Reference<?> pendingList;
        synchronized (processPendingLock) {
            pendingList = getAndClearReferencePendingList();
            processPendingActive = true;
        }
        while (pendingList != null) {
            Reference<?> ref = pendingList;
            pendingList = ref.discovered;
            ref.discovered = null;
            if (ref instanceof Cleaner) {
                ((Cleaner)ref).clean();
                // Notify any waiters that progress has been made.
                // This improves latency for nio.Bits waiters, which
                // are the only important ones.
                synchronized (processPendingLock) {
                    processPendingLock.notifyAll();
                }
            } else {
                ref.enqueueFromPending();
            }
        }
        // Notify any waiters of completion of current round.
        synchronized (processPendingLock) {
            processPendingActive = false;
            processPendingLock.notifyAll();
        }
    }
    // Wait for progress in reference processing.
    //
    // Returns true after waiting (for notification from the reference
    // processing thread) if either (1) the VM has any pending
    // references, or (2) the reference processing thread is
    // processing references. Otherwise, returns false immediately.
    private static boolean waitForReferenceProcessing()
        throws InterruptedException
    {
        synchronized (processPendingLock) {
            if (processPendingActive || hasReferencePendingList()) {
                // Wait for progress, not necessarily completion.
                processPendingLock.wait();
                return true;
            } else {
                return false;
            }
        }
    }
    static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
        Thread handler = new ReferenceHandler(tg, "Reference Handler");
        /* If there were a special system-only priority greater than
         * MAX_PRIORITY, it would be used here
         */
        handler.setPriority(Thread.MAX_PRIORITY);
        handler.setDaemon(true);
        handler.start();
        // provide access in SharedSecrets
        SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
            @Override
            public boolean waitForReferenceProcessing()
                throws InterruptedException
            {
                return Reference.waitForReferenceProcessing();
            }
            @Override
            public void runFinalization() {
                Finalizer.runFinalization();
            }
        });
    }
    /* -- Referent accessor and setters -- */
    /**
     * Returns this reference object's referent.  If this reference object has
     * been cleared, either by the program or by the garbage collector, then
     * this method returns {@code null}.
     *
     * @apiNote
     * This method returns a strong reference to the referent. This may cause
     * the garbage collector to treat it as strongly reachable until some later
     * collection cycle.  The {@link #refersTo(Object) refersTo} method can be
     * used to avoid such strengthening when testing whether some object is
     * the referent of a reference object; that is, use {@code ref.refersTo(obj)}
     * rather than {@code ref.get() == obj}.
     *
     * @return   The object to which this reference refers, or
     *           {@code null} if this reference object has been cleared
     * @see #refersTo
     */
    @IntrinsicCandidate
    public T get() {
        return this.referent;
    }
    /**
     * Tests if the referent of this reference object is {@code obj}.
     * Using a {@code null} {@code obj} returns {@code true} if the
     * reference object has been cleared.
     *
     * @param  obj the object to compare with this reference object's referent
     * @return {@code true} if {@code obj} is the referent of this reference object
     * @since 16
     */
    public final boolean refersTo(T obj) {
        return refersToImpl(obj);
    }
    /* Implementation of refersTo(), overridden for phantom references.
     * This method exists only to avoid making refersTo0() virtual. Making
     * refersTo0() virtual has the undesirable effect of C2 often preferring
     * to call the native implementation over the intrinsic.
     */
    boolean refersToImpl(T obj) {
        return refersTo0(obj);
    }
    @IntrinsicCandidate
    private native boolean refersTo0(Object o);
    /**
     * Clears this reference object.  Invoking this method will not cause this
     * object to be enqueued.
     *
     * <p> This method is invoked only by Java code; when the garbage collector
     * clears references it does so directly, without invoking this method.
     */
    public void clear() {
        clear0();
    }
    private native void clear0();
    /* -- Operations on inactive FinalReferences -- */
    /* These functions are only used by FinalReference, and must only be
     * called after the reference becomes inactive. While active, a
     * FinalReference is considered weak but the referent is not normally
     * accessed. Once a FinalReference becomes inactive it is considered a
     * strong reference. These functions are used to bypass the
     * corresponding weak implementations, directly accessing the referent
     * field with strong semantics.
     */
    /**
     * Load referent with strong semantics.
     */
    T getFromInactiveFinalReference() {
        assert this instanceof FinalReference;
        assert next != null; // I.e. FinalReference is inactive
        return this.referent;
    }
    /**
     * Clear referent with strong semantics.
     */
    void clearInactiveFinalReference() {
        assert this instanceof FinalReference;
        assert next != null; // I.e. FinalReference is inactive
        this.referent = null;
    }
    /* -- Queue operations -- */
    /**
     * Tests if this reference object is in its associated queue, if any.
     * This method returns {@code true} only if all of the following conditions
     * are met:
     * <ul>
     * <li>this reference object was registered with a queue when it was created; and
     * <li>the garbage collector has added this reference object to the queue
     *     or {@link #enqueue()} is called; and
     * <li>this reference object is not yet removed from the queue.
     * </ul>
     * Otherwise, this method returns {@code false}.
     * This method may return {@code false} if this reference object has been cleared
     * but not enqueued due to the race condition.
     *
     * @deprecated
     * This method was originally specified to test if a reference object has
     * been cleared and enqueued but was never implemented to do this test.
     * This method could be misused due to the inherent race condition
     * or without an associated {@code ReferenceQueue}.
     * An application relying on this method to release critical resources
     * could cause serious performance issue.
     * An application should use {@link ReferenceQueue} to reliably determine
     * what reference objects that have been enqueued or
     * {@link #refersTo(Object) refersTo(null)} to determine if this reference
     * object has been cleared.
     *
     * @return   {@code true} if and only if this reference object is
     *           in its associated queue (if any).
     */
    @Deprecated(since="16")
    public boolean isEnqueued() {
        return (this.queue == ReferenceQueue.ENQUEUED);
    }
    /**
     * Clears this reference object and adds it to the queue with which
     * it is registered, if any.
     *
     * <p> This method is invoked only by Java code; when the garbage collector
     * enqueues references it does so directly, without invoking this method.
     *
     * @return   {@code true} if this reference object was successfully
     *           enqueued; {@code false} if it was already enqueued or if
     *           it was not registered with a queue when it was created
     */
    public boolean enqueue() {
        clear0();               // Intentionally clear0() rather than clear()
        return this.queue.enqueue(this);
    }
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
    /* -- Constructors -- */
    Reference(T referent) {
        this(referent, null);
    }
    Reference(T referent, ReferenceQueue<? super T> queue) {
        this.referent = referent;
        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
    }
    @ForceInline
    public static void reachabilityFence(Object ref) {
    }
}