8187443: Forest Consolidation: Move files to unified layout

Reviewed-by: darcy, ihse

src/java.base/share/classes/java/lang/ref/ReferenceQueue.java

@@ -0,0 +1,203 @@
+/*
+ * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.lang.ref;
+
+import java.util.function.Consumer;
+import jdk.internal.misc.VM;
+
+/**
+ * Reference queues, to which registered reference objects are appended by the
+ * garbage collector after the appropriate reachability changes are detected.
+ *
+ * @author   Mark Reinhold
+ * @since    1.2
+ */
+
+public class ReferenceQueue<T> {
+
+    /**
+     * Constructs a new reference-object queue.
+     */
+    public ReferenceQueue() { }
+
+    private static class Null<S> extends ReferenceQueue<S> {
+        boolean enqueue(Reference<? extends S> r) {
+            return false;
+        }
+    }
+
+    static ReferenceQueue<Object> NULL = new Null<>();
+    static ReferenceQueue<Object> ENQUEUED = new Null<>();
+
+    private static class Lock { };
+    private Lock lock = new Lock();
+    private volatile Reference<? extends T> head;
+    private long queueLength = 0;
+
+    boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
+        synchronized (lock) {
+            // Check that since getting the lock this reference hasn't already been
+            // enqueued (and even then removed)
+            ReferenceQueue<?> queue = r.queue;
+            if ((queue == NULL) || (queue == ENQUEUED)) {
+                return false;
+            }
+            assert queue == this;
+            r.next = (head == null) ? r : head;
+            head = r;
+            queueLength++;
+            // Update r.queue *after* adding to list, to avoid race
+            // with concurrent enqueued checks and fast-path poll().
+            // Volatiles ensure ordering.
+            r.queue = ENQUEUED;
+            if (r instanceof FinalReference) {
+                VM.addFinalRefCount(1);
+            }
+            lock.notifyAll();
+            return true;
+        }
+    }
+
+    private Reference<? extends T> reallyPoll() {       /* Must hold lock */
+        Reference<? extends T> r = head;
+        if (r != null) {
+            r.queue = NULL;
+            // Update r.queue *before* removing from list, to avoid
+            // race with concurrent enqueued checks and fast-path
+            // poll().  Volatiles ensure ordering.
+            @SuppressWarnings("unchecked")
+            Reference<? extends T> rn = r.next;
+            head = (rn == r) ? null : rn;
+            r.next = r;
+            queueLength--;
+            if (r instanceof FinalReference) {
+                VM.addFinalRefCount(-1);
+            }
+            return r;
+        }
+        return null;
+    }
+
+    /**
+     * Polls this queue to see if a reference object is available.  If one is
+     * available without further delay then it is removed from the queue and
+     * returned.  Otherwise this method immediately returns {@code null}.
+     *
+     * @return  A reference object, if one was immediately available,
+     *          otherwise {@code null}
+     */
+    public Reference<? extends T> poll() {
+        if (head == null)
+            return null;
+        synchronized (lock) {
+            return reallyPoll();
+        }
+    }
+
+    /**
+     * Removes the next reference object in this queue, blocking until either
+     * one becomes available or the given timeout period expires.
+     *
+     * <p> This method does not offer real-time guarantees: It schedules the
+     * timeout as if by invoking the {@link Object#wait(long)} method.
+     *
+     * @param  timeout  If positive, block for up to {@code timeout}
+     *                  milliseconds while waiting for a reference to be
+     *                  added to this queue.  If zero, block indefinitely.
+     *
+     * @return  A reference object, if one was available within the specified
+     *          timeout period, otherwise {@code null}
+     *
+     * @throws  IllegalArgumentException
+     *          If the value of the timeout argument is negative
+     *
+     * @throws  InterruptedException
+     *          If the timeout wait is interrupted
+     */
+    public Reference<? extends T> remove(long timeout)
+        throws IllegalArgumentException, InterruptedException
+    {
+        if (timeout < 0) {
+            throw new IllegalArgumentException("Negative timeout value");
+        }
+        synchronized (lock) {
+            Reference<? extends T> r = reallyPoll();
+            if (r != null) return r;
+            long start = (timeout == 0) ? 0 : System.nanoTime();
+            for (;;) {
+                lock.wait(timeout);
+                r = reallyPoll();
+                if (r != null) return r;
+                if (timeout != 0) {
+                    long end = System.nanoTime();
+                    timeout -= (end - start) / 1000_000;
+                    if (timeout <= 0) return null;
+                    start = end;
+                }
+            }
+        }
+    }
+
+    /**
+     * Removes the next reference object in this queue, blocking until one
+     * becomes available.
+     *
+     * @return A reference object, blocking until one becomes available
+     * @throws  InterruptedException  If the wait is interrupted
+     */
+    public Reference<? extends T> remove() throws InterruptedException {
+        return remove(0);
+    }
+
+    /**
+     * Iterate queue and invoke given action with each Reference.
+     * Suitable for diagnostic purposes.
+     * WARNING: any use of this method should make sure to not
+     * retain the referents of iterated references (in case of
+     * FinalReference(s)) so that their life is not prolonged more
+     * than necessary.
+     */
+    void forEach(Consumer<? super Reference<? extends T>> action) {
+        for (Reference<? extends T> r = head; r != null;) {
+            action.accept(r);
+            @SuppressWarnings("unchecked")
+            Reference<? extends T> rn = r.next;
+            if (rn == r) {
+                if (r.queue == ENQUEUED) {
+                    // still enqueued -> we reached end of chain
+                    r = null;
+                } else {
+                    // already dequeued: r.queue == NULL; ->
+                    // restart from head when overtaken by queue poller(s)
+                    r = head;
+                }
+            } else {
+                // next in chain
+                r = rn;
+            }
+        }
+    }
+}