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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent.locks; |
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|
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import java.util.ArrayList; |
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import java.util.Collection; |
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import java.util.Date; |
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import java.util.concurrent.TimeUnit; |
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import java.util.concurrent.locks.AbstractQueuedSynchronizer.Node; |
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|
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/** |
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* A version of {@link AbstractQueuedSynchronizer} in |
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* which synchronization state is maintained as a {@code long}. |
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* This class has exactly the same structure, properties, and methods |
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* as {@code AbstractQueuedSynchronizer} with the exception |
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* that all state-related parameters and results are defined |
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* as {@code long} rather than {@code int}. This class |
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* may be useful when creating synchronizers such as |
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* multilevel locks and barriers that require |
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* 64 bits of state. |
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* |
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* <p>See {@link AbstractQueuedSynchronizer} for usage |
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* notes and examples. |
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* |
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* @since 1.6 |
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* @author Doug Lea |
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*/ |
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public abstract class AbstractQueuedLongSynchronizer |
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extends AbstractOwnableSynchronizer |
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implements java.io.Serializable { |
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|
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private static final long serialVersionUID = 7373984972572414692L; |
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|
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/* |
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* To keep sources in sync, the remainder of this source file is |
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* exactly cloned from AbstractQueuedSynchronizer, replacing class |
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* name and changing ints related with sync state to longs. Please |
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* keep it that way. |
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*/ |
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|
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/** |
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* Creates a new {@code AbstractQueuedLongSynchronizer} instance |
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* with initial synchronization state of zero. |
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*/ |
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protected AbstractQueuedLongSynchronizer() { } |
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|
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/** |
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* Head of the wait queue, lazily initialized. Except for |
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* initialization, it is modified only via method setHead. Note: |
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* If head exists, its waitStatus is guaranteed not to be |
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* CANCELLED. |
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*/ |
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private transient volatile Node head; |
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|
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/** |
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* Tail of the wait queue, lazily initialized. Modified only via |
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* method enq to add new wait node. |
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*/ |
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private transient volatile Node tail; |
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|
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/** |
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* The synchronization state. |
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*/ |
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private volatile long state; |
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|
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/** |
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* Returns the current value of synchronization state. |
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* This operation has memory semantics of a {@code volatile} read. |
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* @return current state value |
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*/ |
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protected final long getState() { |
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return state; |
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} |
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|
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/** |
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* Sets the value of synchronization state. |
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* This operation has memory semantics of a {@code volatile} write. |
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* @param newState the new state value |
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*/ |
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protected final void setState(long newState) { |
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// Use putLongVolatile instead of ordinary volatile store when |
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// using compareAndSwapLong, for sake of some 32bit systems. |
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U.putLongVolatile(this, STATE, newState); |
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} |
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|
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/** |
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* Atomically sets synchronization state to the given updated |
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* value if the current state value equals the expected value. |
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* This operation has memory semantics of a {@code volatile} read |
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* and write. |
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* |
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* @param expect the expected value |
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* @param update the new value |
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* @return {@code true} if successful. False return indicates that the actual |
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* value was not equal to the expected value. |
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*/ |
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protected final boolean compareAndSetState(long expect, long update) { |
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return U.compareAndSwapLong(this, STATE, expect, update); |
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} |
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|
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// Queuing utilities |
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|
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/** |
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* The number of nanoseconds for which it is faster to spin |
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* rather than to use timed park. A rough estimate suffices |
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* to improve responsiveness with very short timeouts. |
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*/ |
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static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L; |
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|
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/** |
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* Inserts node into queue, initializing if necessary. See picture above. |
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* @param node the node to insert |
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* @return node's predecessor |
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*/ |
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private Node enq(Node node) { |
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for (;;) { |
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Node oldTail = tail; |
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if (oldTail != null) { |
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U.putObject(node, Node.PREV, oldTail); |
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if (compareAndSetTail(oldTail, node)) { |
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oldTail.next = node; |
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return oldTail; |
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} |
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} else { |
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initializeSyncQueue(); |
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} |
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} |
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} |
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|
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/** |
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* Creates and enqueues node for current thread and given mode. |
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* |
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* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared |
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* @return the new node |
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*/ |
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private Node addWaiter(Node mode) { |
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Node node = new Node(mode); |
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|
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for (;;) { |
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Node oldTail = tail; |
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if (oldTail != null) { |
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U.putObject(node, Node.PREV, oldTail); |
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if (compareAndSetTail(oldTail, node)) { |
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oldTail.next = node; |
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return node; |
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} |
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} else { |
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initializeSyncQueue(); |
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} |
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} |
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} |
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|
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/** |
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* Sets head of queue to be node, thus dequeuing. Called only by |
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* acquire methods. Also nulls out unused fields for sake of GC |
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* and to suppress unnecessary signals and traversals. |
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* |
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* @param node the node |
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*/ |
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private void setHead(Node node) { |
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head = node; |
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node.thread = null; |
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node.prev = null; |
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} |
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|
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/** |
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* Wakes up node's successor, if one exists. |
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* |
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* @param node the node |
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*/ |
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private void unparkSuccessor(Node node) { |
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/* |
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* If status is negative (i.e., possibly needing signal) try |
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* to clear in anticipation of signalling. It is OK if this |
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* fails or if status is changed by waiting thread. |
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*/ |
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int ws = node.waitStatus; |
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if (ws < 0) |
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node.compareAndSetWaitStatus(ws, 0); |
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|
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/* |
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* Thread to unpark is held in successor, which is normally |
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* just the next node. But if cancelled or apparently null, |
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* traverse backwards from tail to find the actual |
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* non-cancelled successor. |
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*/ |
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Node s = node.next; |
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if (s == null || s.waitStatus > 0) { |
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s = null; |
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for (Node p = tail; p != node && p != null; p = p.prev) |
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if (p.waitStatus <= 0) |
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s = p; |
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} |
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if (s != null) |
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LockSupport.unpark(s.thread); |
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} |
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|
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/** |
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* Release action for shared mode -- signals successor and ensures |
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* propagation. (Note: For exclusive mode, release just amounts |
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* to calling unparkSuccessor of head if it needs signal.) |
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*/ |
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private void doReleaseShared() { |
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/* |
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* Ensure that a release propagates, even if there are other |
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* in-progress acquires/releases. This proceeds in the usual |
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* way of trying to unparkSuccessor of head if it needs |
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* signal. But if it does not, status is set to PROPAGATE to |
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* ensure that upon release, propagation continues. |
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* Additionally, we must loop in case a new node is added |
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* while we are doing this. Also, unlike other uses of |
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* unparkSuccessor, we need to know if CAS to reset status |
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* fails, if so rechecking. |
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*/ |
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for (;;) { |
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Node h = head; |
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if (h != null && h != tail) { |
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int ws = h.waitStatus; |
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if (ws == Node.SIGNAL) { |
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if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0)) |
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continue; // loop to recheck cases |
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unparkSuccessor(h); |
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} |
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else if (ws == 0 && |
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!h.compareAndSetWaitStatus(0, Node.PROPAGATE)) |
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continue; // loop on failed CAS |
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} |
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if (h == head) // loop if head changed |
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break; |
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} |
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} |
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|
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/** |
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* Sets head of queue, and checks if successor may be waiting |
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* in shared mode, if so propagating if either propagate > 0 or |
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* PROPAGATE status was set. |
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* |
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* @param node the node |
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* @param propagate the return value from a tryAcquireShared |
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*/ |
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private void setHeadAndPropagate(Node node, long propagate) { |
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Node h = head; // Record old head for check below |
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setHead(node); |
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/* |
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* Try to signal next queued node if: |
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* Propagation was indicated by caller, |
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* or was recorded (as h.waitStatus either before |
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* or after setHead) by a previous operation |
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* (note: this uses sign-check of waitStatus because |
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* PROPAGATE status may transition to SIGNAL.) |
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* and |
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* The next node is waiting in shared mode, |
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* or we don't know, because it appears null |
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* |
259 |
* The conservatism in both of these checks may cause |
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* unnecessary wake-ups, but only when there are multiple |
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* racing acquires/releases, so most need signals now or soon |
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* anyway. |
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*/ |
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if (propagate > 0 || h == null || h.waitStatus < 0 || |
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(h = head) == null || h.waitStatus < 0) { |
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Node s = node.next; |
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if (s == null || s.isShared()) |
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doReleaseShared(); |
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} |
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} |
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|
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// Utilities for various versions of acquire |
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|
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/** |
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* Cancels an ongoing attempt to acquire. |
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* |
277 |
* @param node the node |
278 |
*/ |
279 |
private void cancelAcquire(Node node) { |
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// Ignore if node doesn't exist |
281 |
if (node == null) |
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return; |
283 |
|
284 |
node.thread = null; |
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|
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// Skip cancelled predecessors |
287 |
Node pred = node.prev; |
288 |
while (pred.waitStatus > 0) |
289 |
node.prev = pred = pred.prev; |
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|
291 |
// predNext is the apparent node to unsplice. CASes below will |
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// fail if not, in which case, we lost race vs another cancel |
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// or signal, so no further action is necessary, although with |
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// a possibility that a cancelled node may transiently remain |
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// reachable. |
296 |
Node predNext = pred.next; |
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|
298 |
// Can use unconditional write instead of CAS here. |
299 |
// After this atomic step, other Nodes can skip past us. |
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// Before, we are free of interference from other threads. |
301 |
node.waitStatus = Node.CANCELLED; |
302 |
|
303 |
// If we are the tail, remove ourselves. |
304 |
if (node == tail && compareAndSetTail(node, pred)) { |
305 |
pred.compareAndSetNext(predNext, null); |
306 |
} else { |
307 |
// If successor needs signal, try to set pred's next-link |
308 |
// so it will get one. Otherwise wake it up to propagate. |
309 |
int ws; |
310 |
if (pred != head && |
311 |
((ws = pred.waitStatus) == Node.SIGNAL || |
312 |
(ws <= 0 && pred.compareAndSetWaitStatus(ws, Node.SIGNAL))) && |
313 |
pred.thread != null) { |
314 |
Node next = node.next; |
315 |
if (next != null && next.waitStatus <= 0) |
316 |
pred.compareAndSetNext(predNext, next); |
317 |
} else { |
318 |
unparkSuccessor(node); |
319 |
} |
320 |
|
321 |
node.next = node; // help GC |
322 |
} |
323 |
} |
324 |
|
325 |
/** |
326 |
* Checks and updates status for a node that failed to acquire. |
327 |
* Returns true if thread should block. This is the main signal |
328 |
* control in all acquire loops. Requires that pred == node.prev. |
329 |
* |
330 |
* @param pred node's predecessor holding status |
331 |
* @param node the node |
332 |
* @return {@code true} if thread should block |
333 |
*/ |
334 |
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { |
335 |
int ws = pred.waitStatus; |
336 |
if (ws == Node.SIGNAL) |
337 |
/* |
338 |
* This node has already set status asking a release |
339 |
* to signal it, so it can safely park. |
340 |
*/ |
341 |
return true; |
342 |
if (ws > 0) { |
343 |
/* |
344 |
* Predecessor was cancelled. Skip over predecessors and |
345 |
* indicate retry. |
346 |
*/ |
347 |
do { |
348 |
node.prev = pred = pred.prev; |
349 |
} while (pred.waitStatus > 0); |
350 |
pred.next = node; |
351 |
} else { |
352 |
/* |
353 |
* waitStatus must be 0 or PROPAGATE. Indicate that we |
354 |
* need a signal, but don't park yet. Caller will need to |
355 |
* retry to make sure it cannot acquire before parking. |
356 |
*/ |
357 |
pred.compareAndSetWaitStatus(ws, Node.SIGNAL); |
358 |
} |
359 |
return false; |
360 |
} |
361 |
|
362 |
/** |
363 |
* Convenience method to interrupt current thread. |
364 |
*/ |
365 |
static void selfInterrupt() { |
366 |
Thread.currentThread().interrupt(); |
367 |
} |
368 |
|
369 |
/** |
370 |
* Convenience method to park and then check if interrupted. |
371 |
* |
372 |
* @return {@code true} if interrupted |
373 |
*/ |
374 |
private final boolean parkAndCheckInterrupt() { |
375 |
LockSupport.park(this); |
376 |
return Thread.interrupted(); |
377 |
} |
378 |
|
379 |
/* |
380 |
* Various flavors of acquire, varying in exclusive/shared and |
381 |
* control modes. Each is mostly the same, but annoyingly |
382 |
* different. Only a little bit of factoring is possible due to |
383 |
* interactions of exception mechanics (including ensuring that we |
384 |
* cancel if tryAcquire throws exception) and other control, at |
385 |
* least not without hurting performance too much. |
386 |
*/ |
387 |
|
388 |
/** |
389 |
* Acquires in exclusive uninterruptible mode for thread already in |
390 |
* queue. Used by condition wait methods as well as acquire. |
391 |
* |
392 |
* @param node the node |
393 |
* @param arg the acquire argument |
394 |
* @return {@code true} if interrupted while waiting |
395 |
*/ |
396 |
final boolean acquireQueued(final Node node, long arg) { |
397 |
boolean interrupted = false; |
398 |
try { |
399 |
for (;;) { |
400 |
final Node p = node.predecessor(); |
401 |
if (p == head && tryAcquire(arg)) { |
402 |
setHead(node); |
403 |
p.next = null; // help GC |
404 |
return interrupted; |
405 |
} |
406 |
if (shouldParkAfterFailedAcquire(p, node)) |
407 |
interrupted |= parkAndCheckInterrupt(); |
408 |
} |
409 |
} catch (Throwable t) { |
410 |
cancelAcquire(node); |
411 |
if (interrupted) |
412 |
selfInterrupt(); |
413 |
throw t; |
414 |
} |
415 |
} |
416 |
|
417 |
/** |
418 |
* Acquires in exclusive interruptible mode. |
419 |
* @param arg the acquire argument |
420 |
*/ |
421 |
private void doAcquireInterruptibly(long arg) |
422 |
throws InterruptedException { |
423 |
final Node node = addWaiter(Node.EXCLUSIVE); |
424 |
try { |
425 |
for (;;) { |
426 |
final Node p = node.predecessor(); |
427 |
if (p == head && tryAcquire(arg)) { |
428 |
setHead(node); |
429 |
p.next = null; // help GC |
430 |
return; |
431 |
} |
432 |
if (shouldParkAfterFailedAcquire(p, node) && |
433 |
parkAndCheckInterrupt()) |
434 |
throw new InterruptedException(); |
435 |
} |
436 |
} catch (Throwable t) { |
437 |
cancelAcquire(node); |
438 |
throw t; |
439 |
} |
440 |
} |
441 |
|
442 |
/** |
443 |
* Acquires in exclusive timed mode. |
444 |
* |
445 |
* @param arg the acquire argument |
446 |
* @param nanosTimeout max wait time |
447 |
* @return {@code true} if acquired |
448 |
*/ |
449 |
private boolean doAcquireNanos(long arg, long nanosTimeout) |
450 |
throws InterruptedException { |
451 |
if (nanosTimeout <= 0L) |
452 |
return false; |
453 |
final long deadline = System.nanoTime() + nanosTimeout; |
454 |
final Node node = addWaiter(Node.EXCLUSIVE); |
455 |
try { |
456 |
for (;;) { |
457 |
final Node p = node.predecessor(); |
458 |
if (p == head && tryAcquire(arg)) { |
459 |
setHead(node); |
460 |
p.next = null; // help GC |
461 |
return true; |
462 |
} |
463 |
nanosTimeout = deadline - System.nanoTime(); |
464 |
if (nanosTimeout <= 0L) { |
465 |
cancelAcquire(node); |
466 |
return false; |
467 |
} |
468 |
if (shouldParkAfterFailedAcquire(p, node) && |
469 |
nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
470 |
LockSupport.parkNanos(this, nanosTimeout); |
471 |
if (Thread.interrupted()) |
472 |
throw new InterruptedException(); |
473 |
} |
474 |
} catch (Throwable t) { |
475 |
cancelAcquire(node); |
476 |
throw t; |
477 |
} |
478 |
} |
479 |
|
480 |
/** |
481 |
* Acquires in shared uninterruptible mode. |
482 |
* @param arg the acquire argument |
483 |
*/ |
484 |
private void doAcquireShared(long arg) { |
485 |
final Node node = addWaiter(Node.SHARED); |
486 |
boolean interrupted = false; |
487 |
try { |
488 |
for (;;) { |
489 |
final Node p = node.predecessor(); |
490 |
if (p == head) { |
491 |
long r = tryAcquireShared(arg); |
492 |
if (r >= 0) { |
493 |
setHeadAndPropagate(node, r); |
494 |
p.next = null; // help GC |
495 |
return; |
496 |
} |
497 |
} |
498 |
if (shouldParkAfterFailedAcquire(p, node)) |
499 |
interrupted |= parkAndCheckInterrupt(); |
500 |
} |
501 |
} catch (Throwable t) { |
502 |
cancelAcquire(node); |
503 |
throw t; |
504 |
} finally { |
505 |
if (interrupted) |
506 |
selfInterrupt(); |
507 |
} |
508 |
} |
509 |
|
510 |
/** |
511 |
* Acquires in shared interruptible mode. |
512 |
* @param arg the acquire argument |
513 |
*/ |
514 |
private void doAcquireSharedInterruptibly(long arg) |
515 |
throws InterruptedException { |
516 |
final Node node = addWaiter(Node.SHARED); |
517 |
try { |
518 |
for (;;) { |
519 |
final Node p = node.predecessor(); |
520 |
if (p == head) { |
521 |
long r = tryAcquireShared(arg); |
522 |
if (r >= 0) { |
523 |
setHeadAndPropagate(node, r); |
524 |
p.next = null; // help GC |
525 |
return; |
526 |
} |
527 |
} |
528 |
if (shouldParkAfterFailedAcquire(p, node) && |
529 |
parkAndCheckInterrupt()) |
530 |
throw new InterruptedException(); |
531 |
} |
532 |
} catch (Throwable t) { |
533 |
cancelAcquire(node); |
534 |
throw t; |
535 |
} |
536 |
} |
537 |
|
538 |
/** |
539 |
* Acquires in shared timed mode. |
540 |
* |
541 |
* @param arg the acquire argument |
542 |
* @param nanosTimeout max wait time |
543 |
* @return {@code true} if acquired |
544 |
*/ |
545 |
private boolean doAcquireSharedNanos(long arg, long nanosTimeout) |
546 |
throws InterruptedException { |
547 |
if (nanosTimeout <= 0L) |
548 |
return false; |
549 |
final long deadline = System.nanoTime() + nanosTimeout; |
550 |
final Node node = addWaiter(Node.SHARED); |
551 |
try { |
552 |
for (;;) { |
553 |
final Node p = node.predecessor(); |
554 |
if (p == head) { |
555 |
long r = tryAcquireShared(arg); |
556 |
if (r >= 0) { |
557 |
setHeadAndPropagate(node, r); |
558 |
p.next = null; // help GC |
559 |
return true; |
560 |
} |
561 |
} |
562 |
nanosTimeout = deadline - System.nanoTime(); |
563 |
if (nanosTimeout <= 0L) { |
564 |
cancelAcquire(node); |
565 |
return false; |
566 |
} |
567 |
if (shouldParkAfterFailedAcquire(p, node) && |
568 |
nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
569 |
LockSupport.parkNanos(this, nanosTimeout); |
570 |
if (Thread.interrupted()) |
571 |
throw new InterruptedException(); |
572 |
} |
573 |
} catch (Throwable t) { |
574 |
cancelAcquire(node); |
575 |
throw t; |
576 |
} |
577 |
} |
578 |
|
579 |
// Main exported methods |
580 |
|
581 |
/** |
582 |
* Attempts to acquire in exclusive mode. This method should query |
583 |
* if the state of the object permits it to be acquired in the |
584 |
* exclusive mode, and if so to acquire it. |
585 |
* |
586 |
* <p>This method is always invoked by the thread performing |
587 |
* acquire. If this method reports failure, the acquire method |
588 |
* may queue the thread, if it is not already queued, until it is |
589 |
* signalled by a release from some other thread. This can be used |
590 |
* to implement method {@link Lock#tryLock()}. |
591 |
* |
592 |
* <p>The default |
593 |
* implementation throws {@link UnsupportedOperationException}. |
594 |
* |
595 |
* @param arg the acquire argument. This value is always the one |
596 |
* passed to an acquire method, or is the value saved on entry |
597 |
* to a condition wait. The value is otherwise uninterpreted |
598 |
* and can represent anything you like. |
599 |
* @return {@code true} if successful. Upon success, this object has |
600 |
* been acquired. |
601 |
* @throws IllegalMonitorStateException if acquiring would place this |
602 |
* synchronizer in an illegal state. This exception must be |
603 |
* thrown in a consistent fashion for synchronization to work |
604 |
* correctly. |
605 |
* @throws UnsupportedOperationException if exclusive mode is not supported |
606 |
*/ |
607 |
protected boolean tryAcquire(long arg) { |
608 |
throw new UnsupportedOperationException(); |
609 |
} |
610 |
|
611 |
/** |
612 |
* Attempts to set the state to reflect a release in exclusive |
613 |
* mode. |
614 |
* |
615 |
* <p>This method is always invoked by the thread performing release. |
616 |
* |
617 |
* <p>The default implementation throws |
618 |
* {@link UnsupportedOperationException}. |
619 |
* |
620 |
* @param arg the release argument. This value is always the one |
621 |
* passed to a release method, or the current state value upon |
622 |
* entry to a condition wait. The value is otherwise |
623 |
* uninterpreted and can represent anything you like. |
624 |
* @return {@code true} if this object is now in a fully released |
625 |
* state, so that any waiting threads may attempt to acquire; |
626 |
* and {@code false} otherwise. |
627 |
* @throws IllegalMonitorStateException if releasing would place this |
628 |
* synchronizer in an illegal state. This exception must be |
629 |
* thrown in a consistent fashion for synchronization to work |
630 |
* correctly. |
631 |
* @throws UnsupportedOperationException if exclusive mode is not supported |
632 |
*/ |
633 |
protected boolean tryRelease(long arg) { |
634 |
throw new UnsupportedOperationException(); |
635 |
} |
636 |
|
637 |
/** |
638 |
* Attempts to acquire in shared mode. This method should query if |
639 |
* the state of the object permits it to be acquired in the shared |
640 |
* mode, and if so to acquire it. |
641 |
* |
642 |
* <p>This method is always invoked by the thread performing |
643 |
* acquire. If this method reports failure, the acquire method |
644 |
* may queue the thread, if it is not already queued, until it is |
645 |
* signalled by a release from some other thread. |
646 |
* |
647 |
* <p>The default implementation throws {@link |
648 |
* UnsupportedOperationException}. |
649 |
* |
650 |
* @param arg the acquire argument. This value is always the one |
651 |
* passed to an acquire method, or is the value saved on entry |
652 |
* to a condition wait. The value is otherwise uninterpreted |
653 |
* and can represent anything you like. |
654 |
* @return a negative value on failure; zero if acquisition in shared |
655 |
* mode succeeded but no subsequent shared-mode acquire can |
656 |
* succeed; and a positive value if acquisition in shared |
657 |
* mode succeeded and subsequent shared-mode acquires might |
658 |
* also succeed, in which case a subsequent waiting thread |
659 |
* must check availability. (Support for three different |
660 |
* return values enables this method to be used in contexts |
661 |
* where acquires only sometimes act exclusively.) Upon |
662 |
* success, this object has been acquired. |
663 |
* @throws IllegalMonitorStateException if acquiring would place this |
664 |
* synchronizer in an illegal state. This exception must be |
665 |
* thrown in a consistent fashion for synchronization to work |
666 |
* correctly. |
667 |
* @throws UnsupportedOperationException if shared mode is not supported |
668 |
*/ |
669 |
protected long tryAcquireShared(long arg) { |
670 |
throw new UnsupportedOperationException(); |
671 |
} |
672 |
|
673 |
/** |
674 |
* Attempts to set the state to reflect a release in shared mode. |
675 |
* |
676 |
* <p>This method is always invoked by the thread performing release. |
677 |
* |
678 |
* <p>The default implementation throws |
679 |
* {@link UnsupportedOperationException}. |
680 |
* |
681 |
* @param arg the release argument. This value is always the one |
682 |
* passed to a release method, or the current state value upon |
683 |
* entry to a condition wait. The value is otherwise |
684 |
* uninterpreted and can represent anything you like. |
685 |
* @return {@code true} if this release of shared mode may permit a |
686 |
* waiting acquire (shared or exclusive) to succeed; and |
687 |
* {@code false} otherwise |
688 |
* @throws IllegalMonitorStateException if releasing would place this |
689 |
* synchronizer in an illegal state. This exception must be |
690 |
* thrown in a consistent fashion for synchronization to work |
691 |
* correctly. |
692 |
* @throws UnsupportedOperationException if shared mode is not supported |
693 |
*/ |
694 |
protected boolean tryReleaseShared(long arg) { |
695 |
throw new UnsupportedOperationException(); |
696 |
} |
697 |
|
698 |
/** |
699 |
* Returns {@code true} if synchronization is held exclusively with |
700 |
* respect to the current (calling) thread. This method is invoked |
701 |
* upon each call to a {@link ConditionObject} method. |
702 |
* |
703 |
* <p>The default implementation throws {@link |
704 |
* UnsupportedOperationException}. This method is invoked |
705 |
* internally only within {@link ConditionObject} methods, so need |
706 |
* not be defined if conditions are not used. |
707 |
* |
708 |
* @return {@code true} if synchronization is held exclusively; |
709 |
* {@code false} otherwise |
710 |
* @throws UnsupportedOperationException if conditions are not supported |
711 |
*/ |
712 |
protected boolean isHeldExclusively() { |
713 |
throw new UnsupportedOperationException(); |
714 |
} |
715 |
|
716 |
/** |
717 |
* Acquires in exclusive mode, ignoring interrupts. Implemented |
718 |
* by invoking at least once {@link #tryAcquire}, |
719 |
* returning on success. Otherwise the thread is queued, possibly |
720 |
* repeatedly blocking and unblocking, invoking {@link |
721 |
* #tryAcquire} until success. This method can be used |
722 |
* to implement method {@link Lock#lock}. |
723 |
* |
724 |
* @param arg the acquire argument. This value is conveyed to |
725 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
726 |
* can represent anything you like. |
727 |
*/ |
728 |
public final void acquire(long arg) { |
729 |
if (!tryAcquire(arg) && |
730 |
acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) |
731 |
selfInterrupt(); |
732 |
} |
733 |
|
734 |
/** |
735 |
* Acquires in exclusive mode, aborting if interrupted. |
736 |
* Implemented by first checking interrupt status, then invoking |
737 |
* at least once {@link #tryAcquire}, returning on |
738 |
* success. Otherwise the thread is queued, possibly repeatedly |
739 |
* blocking and unblocking, invoking {@link #tryAcquire} |
740 |
* until success or the thread is interrupted. This method can be |
741 |
* used to implement method {@link Lock#lockInterruptibly}. |
742 |
* |
743 |
* @param arg the acquire argument. This value is conveyed to |
744 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
745 |
* can represent anything you like. |
746 |
* @throws InterruptedException if the current thread is interrupted |
747 |
*/ |
748 |
public final void acquireInterruptibly(long arg) |
749 |
throws InterruptedException { |
750 |
if (Thread.interrupted()) |
751 |
throw new InterruptedException(); |
752 |
if (!tryAcquire(arg)) |
753 |
doAcquireInterruptibly(arg); |
754 |
} |
755 |
|
756 |
/** |
757 |
* Attempts to acquire in exclusive mode, aborting if interrupted, |
758 |
* and failing if the given timeout elapses. Implemented by first |
759 |
* checking interrupt status, then invoking at least once {@link |
760 |
* #tryAcquire}, returning on success. Otherwise, the thread is |
761 |
* queued, possibly repeatedly blocking and unblocking, invoking |
762 |
* {@link #tryAcquire} until success or the thread is interrupted |
763 |
* or the timeout elapses. This method can be used to implement |
764 |
* method {@link Lock#tryLock(long, TimeUnit)}. |
765 |
* |
766 |
* @param arg the acquire argument. This value is conveyed to |
767 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
768 |
* can represent anything you like. |
769 |
* @param nanosTimeout the maximum number of nanoseconds to wait |
770 |
* @return {@code true} if acquired; {@code false} if timed out |
771 |
* @throws InterruptedException if the current thread is interrupted |
772 |
*/ |
773 |
public final boolean tryAcquireNanos(long arg, long nanosTimeout) |
774 |
throws InterruptedException { |
775 |
if (Thread.interrupted()) |
776 |
throw new InterruptedException(); |
777 |
return tryAcquire(arg) || |
778 |
doAcquireNanos(arg, nanosTimeout); |
779 |
} |
780 |
|
781 |
/** |
782 |
* Releases in exclusive mode. Implemented by unblocking one or |
783 |
* more threads if {@link #tryRelease} returns true. |
784 |
* This method can be used to implement method {@link Lock#unlock}. |
785 |
* |
786 |
* @param arg the release argument. This value is conveyed to |
787 |
* {@link #tryRelease} but is otherwise uninterpreted and |
788 |
* can represent anything you like. |
789 |
* @return the value returned from {@link #tryRelease} |
790 |
*/ |
791 |
public final boolean release(long arg) { |
792 |
if (tryRelease(arg)) { |
793 |
Node h = head; |
794 |
if (h != null && h.waitStatus != 0) |
795 |
unparkSuccessor(h); |
796 |
return true; |
797 |
} |
798 |
return false; |
799 |
} |
800 |
|
801 |
/** |
802 |
* Acquires in shared mode, ignoring interrupts. Implemented by |
803 |
* first invoking at least once {@link #tryAcquireShared}, |
804 |
* returning on success. Otherwise the thread is queued, possibly |
805 |
* repeatedly blocking and unblocking, invoking {@link |
806 |
* #tryAcquireShared} until success. |
807 |
* |
808 |
* @param arg the acquire argument. This value is conveyed to |
809 |
* {@link #tryAcquireShared} but is otherwise uninterpreted |
810 |
* and can represent anything you like. |
811 |
*/ |
812 |
public final void acquireShared(long arg) { |
813 |
if (tryAcquireShared(arg) < 0) |
814 |
doAcquireShared(arg); |
815 |
} |
816 |
|
817 |
/** |
818 |
* Acquires in shared mode, aborting if interrupted. Implemented |
819 |
* by first checking interrupt status, then invoking at least once |
820 |
* {@link #tryAcquireShared}, returning on success. Otherwise the |
821 |
* thread is queued, possibly repeatedly blocking and unblocking, |
822 |
* invoking {@link #tryAcquireShared} until success or the thread |
823 |
* is interrupted. |
824 |
* @param arg the acquire argument. |
825 |
* This value is conveyed to {@link #tryAcquireShared} but is |
826 |
* otherwise uninterpreted and can represent anything |
827 |
* you like. |
828 |
* @throws InterruptedException if the current thread is interrupted |
829 |
*/ |
830 |
public final void acquireSharedInterruptibly(long arg) |
831 |
throws InterruptedException { |
832 |
if (Thread.interrupted()) |
833 |
throw new InterruptedException(); |
834 |
if (tryAcquireShared(arg) < 0) |
835 |
doAcquireSharedInterruptibly(arg); |
836 |
} |
837 |
|
838 |
/** |
839 |
* Attempts to acquire in shared mode, aborting if interrupted, and |
840 |
* failing if the given timeout elapses. Implemented by first |
841 |
* checking interrupt status, then invoking at least once {@link |
842 |
* #tryAcquireShared}, returning on success. Otherwise, the |
843 |
* thread is queued, possibly repeatedly blocking and unblocking, |
844 |
* invoking {@link #tryAcquireShared} until success or the thread |
845 |
* is interrupted or the timeout elapses. |
846 |
* |
847 |
* @param arg the acquire argument. This value is conveyed to |
848 |
* {@link #tryAcquireShared} but is otherwise uninterpreted |
849 |
* and can represent anything you like. |
850 |
* @param nanosTimeout the maximum number of nanoseconds to wait |
851 |
* @return {@code true} if acquired; {@code false} if timed out |
852 |
* @throws InterruptedException if the current thread is interrupted |
853 |
*/ |
854 |
public final boolean tryAcquireSharedNanos(long arg, long nanosTimeout) |
855 |
throws InterruptedException { |
856 |
if (Thread.interrupted()) |
857 |
throw new InterruptedException(); |
858 |
return tryAcquireShared(arg) >= 0 || |
859 |
doAcquireSharedNanos(arg, nanosTimeout); |
860 |
} |
861 |
|
862 |
/** |
863 |
* Releases in shared mode. Implemented by unblocking one or more |
864 |
* threads if {@link #tryReleaseShared} returns true. |
865 |
* |
866 |
* @param arg the release argument. This value is conveyed to |
867 |
* {@link #tryReleaseShared} but is otherwise uninterpreted |
868 |
* and can represent anything you like. |
869 |
* @return the value returned from {@link #tryReleaseShared} |
870 |
*/ |
871 |
public final boolean releaseShared(long arg) { |
872 |
if (tryReleaseShared(arg)) { |
873 |
doReleaseShared(); |
874 |
return true; |
875 |
} |
876 |
return false; |
877 |
} |
878 |
|
879 |
// Queue inspection methods |
880 |
|
881 |
/** |
882 |
* Queries whether any threads are waiting to acquire. Note that |
883 |
* because cancellations due to interrupts and timeouts may occur |
884 |
* at any time, a {@code true} return does not guarantee that any |
885 |
* other thread will ever acquire. |
886 |
* |
887 |
* @return {@code true} if there may be other threads waiting to acquire |
888 |
*/ |
889 |
public final boolean hasQueuedThreads() { |
890 |
for (Node p = tail, h = head; p != h && p != null; p = p.prev) |
891 |
if (p.waitStatus <= 0) |
892 |
return true; |
893 |
return false; |
894 |
} |
895 |
|
896 |
/** |
897 |
* Queries whether any threads have ever contended to acquire this |
898 |
* synchronizer; that is, if an acquire method has ever blocked. |
899 |
* |
900 |
* <p>In this implementation, this operation returns in |
901 |
* constant time. |
902 |
* |
903 |
* @return {@code true} if there has ever been contention |
904 |
*/ |
905 |
public final boolean hasContended() { |
906 |
return head != null; |
907 |
} |
908 |
|
909 |
/** |
910 |
* Returns the first (longest-waiting) thread in the queue, or |
911 |
* {@code null} if no threads are currently queued. |
912 |
* |
913 |
* <p>In this implementation, this operation normally returns in |
914 |
* constant time, but may iterate upon contention if other threads are |
915 |
* concurrently modifying the queue. |
916 |
* |
917 |
* @return the first (longest-waiting) thread in the queue, or |
918 |
* {@code null} if no threads are currently queued |
919 |
*/ |
920 |
public final Thread getFirstQueuedThread() { |
921 |
// handle only fast path, else relay |
922 |
return (head == tail) ? null : fullGetFirstQueuedThread(); |
923 |
} |
924 |
|
925 |
/** |
926 |
* Version of getFirstQueuedThread called when fastpath fails. |
927 |
*/ |
928 |
private Thread fullGetFirstQueuedThread() { |
929 |
/* |
930 |
* The first node is normally head.next. Try to get its |
931 |
* thread field, ensuring consistent reads: If thread |
932 |
* field is nulled out or s.prev is no longer head, then |
933 |
* some other thread(s) concurrently performed setHead in |
934 |
* between some of our reads. We try this twice before |
935 |
* resorting to traversal. |
936 |
*/ |
937 |
Node h, s; |
938 |
Thread st; |
939 |
if (((h = head) != null && (s = h.next) != null && |
940 |
s.prev == head && (st = s.thread) != null) || |
941 |
((h = head) != null && (s = h.next) != null && |
942 |
s.prev == head && (st = s.thread) != null)) |
943 |
return st; |
944 |
|
945 |
/* |
946 |
* Head's next field might not have been set yet, or may have |
947 |
* been unset after setHead. So we must check to see if tail |
948 |
* is actually first node. If not, we continue on, safely |
949 |
* traversing from tail back to head to find first, |
950 |
* guaranteeing termination. |
951 |
*/ |
952 |
|
953 |
Thread firstThread = null; |
954 |
for (Node p = tail; p != null && p != head; p = p.prev) { |
955 |
Thread t = p.thread; |
956 |
if (t != null) |
957 |
firstThread = t; |
958 |
} |
959 |
return firstThread; |
960 |
} |
961 |
|
962 |
/** |
963 |
* Returns true if the given thread is currently queued. |
964 |
* |
965 |
* <p>This implementation traverses the queue to determine |
966 |
* presence of the given thread. |
967 |
* |
968 |
* @param thread the thread |
969 |
* @return {@code true} if the given thread is on the queue |
970 |
* @throws NullPointerException if the thread is null |
971 |
*/ |
972 |
public final boolean isQueued(Thread thread) { |
973 |
if (thread == null) |
974 |
throw new NullPointerException(); |
975 |
for (Node p = tail; p != null; p = p.prev) |
976 |
if (p.thread == thread) |
977 |
return true; |
978 |
return false; |
979 |
} |
980 |
|
981 |
/** |
982 |
* Returns {@code true} if the apparent first queued thread, if one |
983 |
* exists, is waiting in exclusive mode. If this method returns |
984 |
* {@code true}, and the current thread is attempting to acquire in |
985 |
* shared mode (that is, this method is invoked from {@link |
986 |
* #tryAcquireShared}) then it is guaranteed that the current thread |
987 |
* is not the first queued thread. Used only as a heuristic in |
988 |
* ReentrantReadWriteLock. |
989 |
*/ |
990 |
final boolean apparentlyFirstQueuedIsExclusive() { |
991 |
Node h, s; |
992 |
return (h = head) != null && |
993 |
(s = h.next) != null && |
994 |
!s.isShared() && |
995 |
s.thread != null; |
996 |
} |
997 |
|
998 |
/** |
999 |
* Queries whether any threads have been waiting to acquire longer |
1000 |
* than the current thread. |
1001 |
* |
1002 |
* <p>An invocation of this method is equivalent to (but may be |
1003 |
* more efficient than): |
1004 |
* <pre> {@code |
1005 |
* getFirstQueuedThread() != Thread.currentThread() |
1006 |
* && hasQueuedThreads()}</pre> |
1007 |
* |
1008 |
* <p>Note that because cancellations due to interrupts and |
1009 |
* timeouts may occur at any time, a {@code true} return does not |
1010 |
* guarantee that some other thread will acquire before the current |
1011 |
* thread. Likewise, it is possible for another thread to win a |
1012 |
* race to enqueue after this method has returned {@code false}, |
1013 |
* due to the queue being empty. |
1014 |
* |
1015 |
* <p>This method is designed to be used by a fair synchronizer to |
1016 |
* avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>. |
1017 |
* Such a synchronizer's {@link #tryAcquire} method should return |
1018 |
* {@code false}, and its {@link #tryAcquireShared} method should |
1019 |
* return a negative value, if this method returns {@code true} |
1020 |
* (unless this is a reentrant acquire). For example, the {@code |
1021 |
* tryAcquire} method for a fair, reentrant, exclusive mode |
1022 |
* synchronizer might look like this: |
1023 |
* |
1024 |
* <pre> {@code |
1025 |
* protected boolean tryAcquire(int arg) { |
1026 |
* if (isHeldExclusively()) { |
1027 |
* // A reentrant acquire; increment hold count |
1028 |
* return true; |
1029 |
* } else if (hasQueuedPredecessors()) { |
1030 |
* return false; |
1031 |
* } else { |
1032 |
* // try to acquire normally |
1033 |
* } |
1034 |
* }}</pre> |
1035 |
* |
1036 |
* @return {@code true} if there is a queued thread preceding the |
1037 |
* current thread, and {@code false} if the current thread |
1038 |
* is at the head of the queue or the queue is empty |
1039 |
* @since 1.7 |
1040 |
*/ |
1041 |
public final boolean hasQueuedPredecessors() { |
1042 |
Node h, s; |
1043 |
if ((h = head) != null) { |
1044 |
if ((s = h.next) == null || s.waitStatus > 0) { |
1045 |
s = null; // traverse in case of concurrent cancellation |
1046 |
for (Node p = tail; p != h && p != null; p = p.prev) { |
1047 |
if (p.waitStatus <= 0) |
1048 |
s = p; |
1049 |
} |
1050 |
} |
1051 |
if (s != null && s.thread != Thread.currentThread()) |
1052 |
return true; |
1053 |
} |
1054 |
return false; |
1055 |
} |
1056 |
|
1057 |
// Instrumentation and monitoring methods |
1058 |
|
1059 |
/** |
1060 |
* Returns an estimate of the number of threads waiting to |
1061 |
* acquire. The value is only an estimate because the number of |
1062 |
* threads may change dynamically while this method traverses |
1063 |
* internal data structures. This method is designed for use in |
1064 |
* monitoring system state, not for synchronization control. |
1065 |
* |
1066 |
* @return the estimated number of threads waiting to acquire |
1067 |
*/ |
1068 |
public final int getQueueLength() { |
1069 |
int n = 0; |
1070 |
for (Node p = tail; p != null; p = p.prev) { |
1071 |
if (p.thread != null) |
1072 |
++n; |
1073 |
} |
1074 |
return n; |
1075 |
} |
1076 |
|
1077 |
/** |
1078 |
* Returns a collection containing threads that may be waiting to |
1079 |
* acquire. Because the actual set of threads may change |
1080 |
* dynamically while constructing this result, the returned |
1081 |
* collection is only a best-effort estimate. The elements of the |
1082 |
* returned collection are in no particular order. This method is |
1083 |
* designed to facilitate construction of subclasses that provide |
1084 |
* more extensive monitoring facilities. |
1085 |
* |
1086 |
* @return the collection of threads |
1087 |
*/ |
1088 |
public final Collection<Thread> getQueuedThreads() { |
1089 |
ArrayList<Thread> list = new ArrayList<>(); |
1090 |
for (Node p = tail; p != null; p = p.prev) { |
1091 |
Thread t = p.thread; |
1092 |
if (t != null) |
1093 |
list.add(t); |
1094 |
} |
1095 |
return list; |
1096 |
} |
1097 |
|
1098 |
/** |
1099 |
* Returns a collection containing threads that may be waiting to |
1100 |
* acquire in exclusive mode. This has the same properties |
1101 |
* as {@link #getQueuedThreads} except that it only returns |
1102 |
* those threads waiting due to an exclusive acquire. |
1103 |
* |
1104 |
* @return the collection of threads |
1105 |
*/ |
1106 |
public final Collection<Thread> getExclusiveQueuedThreads() { |
1107 |
ArrayList<Thread> list = new ArrayList<>(); |
1108 |
for (Node p = tail; p != null; p = p.prev) { |
1109 |
if (!p.isShared()) { |
1110 |
Thread t = p.thread; |
1111 |
if (t != null) |
1112 |
list.add(t); |
1113 |
} |
1114 |
} |
1115 |
return list; |
1116 |
} |
1117 |
|
1118 |
/** |
1119 |
* Returns a collection containing threads that may be waiting to |
1120 |
* acquire in shared mode. This has the same properties |
1121 |
* as {@link #getQueuedThreads} except that it only returns |
1122 |
* those threads waiting due to a shared acquire. |
1123 |
* |
1124 |
* @return the collection of threads |
1125 |
*/ |
1126 |
public final Collection<Thread> getSharedQueuedThreads() { |
1127 |
ArrayList<Thread> list = new ArrayList<>(); |
1128 |
for (Node p = tail; p != null; p = p.prev) { |
1129 |
if (p.isShared()) { |
1130 |
Thread t = p.thread; |
1131 |
if (t != null) |
1132 |
list.add(t); |
1133 |
} |
1134 |
} |
1135 |
return list; |
1136 |
} |
1137 |
|
1138 |
/** |
1139 |
* Returns a string identifying this synchronizer, as well as its state. |
1140 |
* The state, in brackets, includes the String {@code "State ="} |
1141 |
* followed by the current value of {@link #getState}, and either |
1142 |
* {@code "nonempty"} or {@code "empty"} depending on whether the |
1143 |
* queue is empty. |
1144 |
* |
1145 |
* @return a string identifying this synchronizer, as well as its state |
1146 |
*/ |
1147 |
public String toString() { |
1148 |
return super.toString() |
1149 |
+ "[State = " + getState() + ", " |
1150 |
+ (hasQueuedThreads() ? "non" : "") + "empty queue]"; |
1151 |
} |
1152 |
|
1153 |
|
1154 |
// Internal support methods for Conditions |
1155 |
|
1156 |
/** |
1157 |
* Returns true if a node, always one that was initially placed on |
1158 |
* a condition queue, is now waiting to reacquire on sync queue. |
1159 |
* @param node the node |
1160 |
* @return true if is reacquiring |
1161 |
*/ |
1162 |
final boolean isOnSyncQueue(Node node) { |
1163 |
if (node.waitStatus == Node.CONDITION || node.prev == null) |
1164 |
return false; |
1165 |
if (node.next != null) // If has successor, it must be on queue |
1166 |
return true; |
1167 |
/* |
1168 |
* node.prev can be non-null, but not yet on queue because |
1169 |
* the CAS to place it on queue can fail. So we have to |
1170 |
* traverse from tail to make sure it actually made it. It |
1171 |
* will always be near the tail in calls to this method, and |
1172 |
* unless the CAS failed (which is unlikely), it will be |
1173 |
* there, so we hardly ever traverse much. |
1174 |
*/ |
1175 |
return findNodeFromTail(node); |
1176 |
} |
1177 |
|
1178 |
/** |
1179 |
* Returns true if node is on sync queue by searching backwards from tail. |
1180 |
* Called only when needed by isOnSyncQueue. |
1181 |
* @return true if present |
1182 |
*/ |
1183 |
private boolean findNodeFromTail(Node node) { |
1184 |
// We check for node first, since it's likely to be at or near tail. |
1185 |
// tail is known to be non-null, so we could re-order to "save" |
1186 |
// one null check, but we leave it this way to help the VM. |
1187 |
for (Node p = tail;;) { |
1188 |
if (p == node) |
1189 |
return true; |
1190 |
if (p == null) |
1191 |
return false; |
1192 |
p = p.prev; |
1193 |
} |
1194 |
} |
1195 |
|
1196 |
/** |
1197 |
* Transfers a node from a condition queue onto sync queue. |
1198 |
* Returns true if successful. |
1199 |
* @param node the node |
1200 |
* @return true if successfully transferred (else the node was |
1201 |
* cancelled before signal) |
1202 |
*/ |
1203 |
final boolean transferForSignal(Node node) { |
1204 |
/* |
1205 |
* If cannot change waitStatus, the node has been cancelled. |
1206 |
*/ |
1207 |
if (!node.compareAndSetWaitStatus(Node.CONDITION, 0)) |
1208 |
return false; |
1209 |
|
1210 |
/* |
1211 |
* Splice onto queue and try to set waitStatus of predecessor to |
1212 |
* indicate that thread is (probably) waiting. If cancelled or |
1213 |
* attempt to set waitStatus fails, wake up to resync (in which |
1214 |
* case the waitStatus can be transiently and harmlessly wrong). |
1215 |
*/ |
1216 |
Node p = enq(node); |
1217 |
int ws = p.waitStatus; |
1218 |
if (ws > 0 || !p.compareAndSetWaitStatus(ws, Node.SIGNAL)) |
1219 |
LockSupport.unpark(node.thread); |
1220 |
return true; |
1221 |
} |
1222 |
|
1223 |
/** |
1224 |
* Transfers node, if necessary, to sync queue after a cancelled wait. |
1225 |
* Returns true if thread was cancelled before being signalled. |
1226 |
* |
1227 |
* @param node the node |
1228 |
* @return true if cancelled before the node was signalled |
1229 |
*/ |
1230 |
final boolean transferAfterCancelledWait(Node node) { |
1231 |
if (node.compareAndSetWaitStatus(Node.CONDITION, 0)) { |
1232 |
enq(node); |
1233 |
return true; |
1234 |
} |
1235 |
/* |
1236 |
* If we lost out to a signal(), then we can't proceed |
1237 |
* until it finishes its enq(). Cancelling during an |
1238 |
* incomplete transfer is both rare and transient, so just |
1239 |
* spin. |
1240 |
*/ |
1241 |
while (!isOnSyncQueue(node)) |
1242 |
Thread.yield(); |
1243 |
return false; |
1244 |
} |
1245 |
|
1246 |
/** |
1247 |
* Invokes release with current state value; returns saved state. |
1248 |
* Cancels node and throws exception on failure. |
1249 |
* @param node the condition node for this wait |
1250 |
* @return previous sync state |
1251 |
*/ |
1252 |
final long fullyRelease(Node node) { |
1253 |
try { |
1254 |
long savedState = getState(); |
1255 |
if (release(savedState)) |
1256 |
return savedState; |
1257 |
throw new IllegalMonitorStateException(); |
1258 |
} catch (Throwable t) { |
1259 |
node.waitStatus = Node.CANCELLED; |
1260 |
throw t; |
1261 |
} |
1262 |
} |
1263 |
|
1264 |
// Instrumentation methods for conditions |
1265 |
|
1266 |
/** |
1267 |
* Queries whether the given ConditionObject |
1268 |
* uses this synchronizer as its lock. |
1269 |
* |
1270 |
* @param condition the condition |
1271 |
* @return {@code true} if owned |
1272 |
* @throws NullPointerException if the condition is null |
1273 |
*/ |
1274 |
public final boolean owns(ConditionObject condition) { |
1275 |
return condition.isOwnedBy(this); |
1276 |
} |
1277 |
|
1278 |
/** |
1279 |
* Queries whether any threads are waiting on the given condition |
1280 |
* associated with this synchronizer. Note that because timeouts |
1281 |
* and interrupts may occur at any time, a {@code true} return |
1282 |
* does not guarantee that a future {@code signal} will awaken |
1283 |
* any threads. This method is designed primarily for use in |
1284 |
* monitoring of the system state. |
1285 |
* |
1286 |
* @param condition the condition |
1287 |
* @return {@code true} if there are any waiting threads |
1288 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1289 |
* is not held |
1290 |
* @throws IllegalArgumentException if the given condition is |
1291 |
* not associated with this synchronizer |
1292 |
* @throws NullPointerException if the condition is null |
1293 |
*/ |
1294 |
public final boolean hasWaiters(ConditionObject condition) { |
1295 |
if (!owns(condition)) |
1296 |
throw new IllegalArgumentException("Not owner"); |
1297 |
return condition.hasWaiters(); |
1298 |
} |
1299 |
|
1300 |
/** |
1301 |
* Returns an estimate of the number of threads waiting on the |
1302 |
* given condition associated with this synchronizer. Note that |
1303 |
* because timeouts and interrupts may occur at any time, the |
1304 |
* estimate serves only as an upper bound on the actual number of |
1305 |
* waiters. This method is designed for use in monitoring system |
1306 |
* state, not for synchronization control. |
1307 |
* |
1308 |
* @param condition the condition |
1309 |
* @return the estimated number of waiting threads |
1310 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1311 |
* is not held |
1312 |
* @throws IllegalArgumentException if the given condition is |
1313 |
* not associated with this synchronizer |
1314 |
* @throws NullPointerException if the condition is null |
1315 |
*/ |
1316 |
public final int getWaitQueueLength(ConditionObject condition) { |
1317 |
if (!owns(condition)) |
1318 |
throw new IllegalArgumentException("Not owner"); |
1319 |
return condition.getWaitQueueLength(); |
1320 |
} |
1321 |
|
1322 |
/** |
1323 |
* Returns a collection containing those threads that may be |
1324 |
* waiting on the given condition associated with this |
1325 |
* synchronizer. Because the actual set of threads may change |
1326 |
* dynamically while constructing this result, the returned |
1327 |
* collection is only a best-effort estimate. The elements of the |
1328 |
* returned collection are in no particular order. |
1329 |
* |
1330 |
* @param condition the condition |
1331 |
* @return the collection of threads |
1332 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1333 |
* is not held |
1334 |
* @throws IllegalArgumentException if the given condition is |
1335 |
* not associated with this synchronizer |
1336 |
* @throws NullPointerException if the condition is null |
1337 |
*/ |
1338 |
public final Collection<Thread> getWaitingThreads(ConditionObject condition) { |
1339 |
if (!owns(condition)) |
1340 |
throw new IllegalArgumentException("Not owner"); |
1341 |
return condition.getWaitingThreads(); |
1342 |
} |
1343 |
|
1344 |
/** |
1345 |
* Condition implementation for a {@link AbstractQueuedLongSynchronizer} |
1346 |
* serving as the basis of a {@link Lock} implementation. |
1347 |
* |
1348 |
* <p>Method documentation for this class describes mechanics, |
1349 |
* not behavioral specifications from the point of view of Lock |
1350 |
* and Condition users. Exported versions of this class will in |
1351 |
* general need to be accompanied by documentation describing |
1352 |
* condition semantics that rely on those of the associated |
1353 |
* {@code AbstractQueuedLongSynchronizer}. |
1354 |
* |
1355 |
* <p>This class is Serializable, but all fields are transient, |
1356 |
* so deserialized conditions have no waiters. |
1357 |
* |
1358 |
* @since 1.6 |
1359 |
*/ |
1360 |
public class ConditionObject implements Condition, java.io.Serializable { |
1361 |
private static final long serialVersionUID = 1173984872572414699L; |
1362 |
/** First node of condition queue. */ |
1363 |
private transient Node firstWaiter; |
1364 |
/** Last node of condition queue. */ |
1365 |
private transient Node lastWaiter; |
1366 |
|
1367 |
/** |
1368 |
* Creates a new {@code ConditionObject} instance. |
1369 |
*/ |
1370 |
public ConditionObject() { } |
1371 |
|
1372 |
// Internal methods |
1373 |
|
1374 |
/** |
1375 |
* Adds a new waiter to wait queue. |
1376 |
* @return its new wait node |
1377 |
*/ |
1378 |
private Node addConditionWaiter() { |
1379 |
if (!isHeldExclusively()) |
1380 |
throw new IllegalMonitorStateException(); |
1381 |
Node t = lastWaiter; |
1382 |
// If lastWaiter is cancelled, clean out. |
1383 |
if (t != null && t.waitStatus != Node.CONDITION) { |
1384 |
unlinkCancelledWaiters(); |
1385 |
t = lastWaiter; |
1386 |
} |
1387 |
|
1388 |
Node node = new Node(Node.CONDITION); |
1389 |
|
1390 |
if (t == null) |
1391 |
firstWaiter = node; |
1392 |
else |
1393 |
t.nextWaiter = node; |
1394 |
lastWaiter = node; |
1395 |
return node; |
1396 |
} |
1397 |
|
1398 |
/** |
1399 |
* Removes and transfers nodes until hit non-cancelled one or |
1400 |
* null. Split out from signal in part to encourage compilers |
1401 |
* to inline the case of no waiters. |
1402 |
* @param first (non-null) the first node on condition queue |
1403 |
*/ |
1404 |
private void doSignal(Node first) { |
1405 |
do { |
1406 |
if ( (firstWaiter = first.nextWaiter) == null) |
1407 |
lastWaiter = null; |
1408 |
first.nextWaiter = null; |
1409 |
} while (!transferForSignal(first) && |
1410 |
(first = firstWaiter) != null); |
1411 |
} |
1412 |
|
1413 |
/** |
1414 |
* Removes and transfers all nodes. |
1415 |
* @param first (non-null) the first node on condition queue |
1416 |
*/ |
1417 |
private void doSignalAll(Node first) { |
1418 |
lastWaiter = firstWaiter = null; |
1419 |
do { |
1420 |
Node next = first.nextWaiter; |
1421 |
first.nextWaiter = null; |
1422 |
transferForSignal(first); |
1423 |
first = next; |
1424 |
} while (first != null); |
1425 |
} |
1426 |
|
1427 |
/** |
1428 |
* Unlinks cancelled waiter nodes from condition queue. |
1429 |
* Called only while holding lock. This is called when |
1430 |
* cancellation occurred during condition wait, and upon |
1431 |
* insertion of a new waiter when lastWaiter is seen to have |
1432 |
* been cancelled. This method is needed to avoid garbage |
1433 |
* retention in the absence of signals. So even though it may |
1434 |
* require a full traversal, it comes into play only when |
1435 |
* timeouts or cancellations occur in the absence of |
1436 |
* signals. It traverses all nodes rather than stopping at a |
1437 |
* particular target to unlink all pointers to garbage nodes |
1438 |
* without requiring many re-traversals during cancellation |
1439 |
* storms. |
1440 |
*/ |
1441 |
private void unlinkCancelledWaiters() { |
1442 |
Node t = firstWaiter; |
1443 |
Node trail = null; |
1444 |
while (t != null) { |
1445 |
Node next = t.nextWaiter; |
1446 |
if (t.waitStatus != Node.CONDITION) { |
1447 |
t.nextWaiter = null; |
1448 |
if (trail == null) |
1449 |
firstWaiter = next; |
1450 |
else |
1451 |
trail.nextWaiter = next; |
1452 |
if (next == null) |
1453 |
lastWaiter = trail; |
1454 |
} |
1455 |
else |
1456 |
trail = t; |
1457 |
t = next; |
1458 |
} |
1459 |
} |
1460 |
|
1461 |
// public methods |
1462 |
|
1463 |
/** |
1464 |
* Moves the longest-waiting thread, if one exists, from the |
1465 |
* wait queue for this condition to the wait queue for the |
1466 |
* owning lock. |
1467 |
* |
1468 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1469 |
* returns {@code false} |
1470 |
*/ |
1471 |
public final void signal() { |
1472 |
if (!isHeldExclusively()) |
1473 |
throw new IllegalMonitorStateException(); |
1474 |
Node first = firstWaiter; |
1475 |
if (first != null) |
1476 |
doSignal(first); |
1477 |
} |
1478 |
|
1479 |
/** |
1480 |
* Moves all threads from the wait queue for this condition to |
1481 |
* the wait queue for the owning lock. |
1482 |
* |
1483 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1484 |
* returns {@code false} |
1485 |
*/ |
1486 |
public final void signalAll() { |
1487 |
if (!isHeldExclusively()) |
1488 |
throw new IllegalMonitorStateException(); |
1489 |
Node first = firstWaiter; |
1490 |
if (first != null) |
1491 |
doSignalAll(first); |
1492 |
} |
1493 |
|
1494 |
/** |
1495 |
* Implements uninterruptible condition wait. |
1496 |
* <ol> |
1497 |
* <li>Save lock state returned by {@link #getState}. |
1498 |
* <li>Invoke {@link #release} with saved state as argument, |
1499 |
* throwing IllegalMonitorStateException if it fails. |
1500 |
* <li>Block until signalled. |
1501 |
* <li>Reacquire by invoking specialized version of |
1502 |
* {@link #acquire} with saved state as argument. |
1503 |
* </ol> |
1504 |
*/ |
1505 |
public final void awaitUninterruptibly() { |
1506 |
Node node = addConditionWaiter(); |
1507 |
long savedState = fullyRelease(node); |
1508 |
boolean interrupted = false; |
1509 |
while (!isOnSyncQueue(node)) { |
1510 |
LockSupport.park(this); |
1511 |
if (Thread.interrupted()) |
1512 |
interrupted = true; |
1513 |
} |
1514 |
if (acquireQueued(node, savedState) || interrupted) |
1515 |
selfInterrupt(); |
1516 |
} |
1517 |
|
1518 |
/* |
1519 |
* For interruptible waits, we need to track whether to throw |
1520 |
* InterruptedException, if interrupted while blocked on |
1521 |
* condition, versus reinterrupt current thread, if |
1522 |
* interrupted while blocked waiting to re-acquire. |
1523 |
*/ |
1524 |
|
1525 |
/** Mode meaning to reinterrupt on exit from wait */ |
1526 |
private static final int REINTERRUPT = 1; |
1527 |
/** Mode meaning to throw InterruptedException on exit from wait */ |
1528 |
private static final int THROW_IE = -1; |
1529 |
|
1530 |
/** |
1531 |
* Checks for interrupt, returning THROW_IE if interrupted |
1532 |
* before signalled, REINTERRUPT if after signalled, or |
1533 |
* 0 if not interrupted. |
1534 |
*/ |
1535 |
private int checkInterruptWhileWaiting(Node node) { |
1536 |
return Thread.interrupted() ? |
1537 |
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : |
1538 |
0; |
1539 |
} |
1540 |
|
1541 |
/** |
1542 |
* Throws InterruptedException, reinterrupts current thread, or |
1543 |
* does nothing, depending on mode. |
1544 |
*/ |
1545 |
private void reportInterruptAfterWait(int interruptMode) |
1546 |
throws InterruptedException { |
1547 |
if (interruptMode == THROW_IE) |
1548 |
throw new InterruptedException(); |
1549 |
else if (interruptMode == REINTERRUPT) |
1550 |
selfInterrupt(); |
1551 |
} |
1552 |
|
1553 |
/** |
1554 |
* Implements interruptible condition wait. |
1555 |
* <ol> |
1556 |
* <li>If current thread is interrupted, throw InterruptedException. |
1557 |
* <li>Save lock state returned by {@link #getState}. |
1558 |
* <li>Invoke {@link #release} with saved state as argument, |
1559 |
* throwing IllegalMonitorStateException if it fails. |
1560 |
* <li>Block until signalled or interrupted. |
1561 |
* <li>Reacquire by invoking specialized version of |
1562 |
* {@link #acquire} with saved state as argument. |
1563 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
1564 |
* </ol> |
1565 |
*/ |
1566 |
public final void await() throws InterruptedException { |
1567 |
if (Thread.interrupted()) |
1568 |
throw new InterruptedException(); |
1569 |
Node node = addConditionWaiter(); |
1570 |
long savedState = fullyRelease(node); |
1571 |
int interruptMode = 0; |
1572 |
while (!isOnSyncQueue(node)) { |
1573 |
LockSupport.park(this); |
1574 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
1575 |
break; |
1576 |
} |
1577 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
1578 |
interruptMode = REINTERRUPT; |
1579 |
if (node.nextWaiter != null) // clean up if cancelled |
1580 |
unlinkCancelledWaiters(); |
1581 |
if (interruptMode != 0) |
1582 |
reportInterruptAfterWait(interruptMode); |
1583 |
} |
1584 |
|
1585 |
/** |
1586 |
* Implements timed condition wait. |
1587 |
* <ol> |
1588 |
* <li>If current thread is interrupted, throw InterruptedException. |
1589 |
* <li>Save lock state returned by {@link #getState}. |
1590 |
* <li>Invoke {@link #release} with saved state as argument, |
1591 |
* throwing IllegalMonitorStateException if it fails. |
1592 |
* <li>Block until signalled, interrupted, or timed out. |
1593 |
* <li>Reacquire by invoking specialized version of |
1594 |
* {@link #acquire} with saved state as argument. |
1595 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
1596 |
* </ol> |
1597 |
*/ |
1598 |
public final long awaitNanos(long nanosTimeout) |
1599 |
throws InterruptedException { |
1600 |
if (Thread.interrupted()) |
1601 |
throw new InterruptedException(); |
1602 |
// We don't check for nanosTimeout <= 0L here, to allow |
1603 |
// awaitNanos(0) as a way to "yield the lock". |
1604 |
final long deadline = System.nanoTime() + nanosTimeout; |
1605 |
long initialNanos = nanosTimeout; |
1606 |
Node node = addConditionWaiter(); |
1607 |
long savedState = fullyRelease(node); |
1608 |
int interruptMode = 0; |
1609 |
while (!isOnSyncQueue(node)) { |
1610 |
if (nanosTimeout <= 0L) { |
1611 |
transferAfterCancelledWait(node); |
1612 |
break; |
1613 |
} |
1614 |
if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
1615 |
LockSupport.parkNanos(this, nanosTimeout); |
1616 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
1617 |
break; |
1618 |
nanosTimeout = deadline - System.nanoTime(); |
1619 |
} |
1620 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
1621 |
interruptMode = REINTERRUPT; |
1622 |
if (node.nextWaiter != null) |
1623 |
unlinkCancelledWaiters(); |
1624 |
if (interruptMode != 0) |
1625 |
reportInterruptAfterWait(interruptMode); |
1626 |
long remaining = deadline - System.nanoTime(); // avoid overflow |
1627 |
return (remaining <= initialNanos) ? remaining : Long.MIN_VALUE; |
1628 |
} |
1629 |
|
1630 |
/** |
1631 |
* Implements absolute timed condition wait. |
1632 |
* <ol> |
1633 |
* <li>If current thread is interrupted, throw InterruptedException. |
1634 |
* <li>Save lock state returned by {@link #getState}. |
1635 |
* <li>Invoke {@link #release} with saved state as argument, |
1636 |
* throwing IllegalMonitorStateException if it fails. |
1637 |
* <li>Block until signalled, interrupted, or timed out. |
1638 |
* <li>Reacquire by invoking specialized version of |
1639 |
* {@link #acquire} with saved state as argument. |
1640 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
1641 |
* <li>If timed out while blocked in step 4, return false, else true. |
1642 |
* </ol> |
1643 |
*/ |
1644 |
public final boolean awaitUntil(Date deadline) |
1645 |
throws InterruptedException { |
1646 |
long abstime = deadline.getTime(); |
1647 |
if (Thread.interrupted()) |
1648 |
throw new InterruptedException(); |
1649 |
Node node = addConditionWaiter(); |
1650 |
long savedState = fullyRelease(node); |
1651 |
boolean timedout = false; |
1652 |
int interruptMode = 0; |
1653 |
while (!isOnSyncQueue(node)) { |
1654 |
if (System.currentTimeMillis() >= abstime) { |
1655 |
timedout = transferAfterCancelledWait(node); |
1656 |
break; |
1657 |
} |
1658 |
LockSupport.parkUntil(this, abstime); |
1659 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
1660 |
break; |
1661 |
} |
1662 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
1663 |
interruptMode = REINTERRUPT; |
1664 |
if (node.nextWaiter != null) |
1665 |
unlinkCancelledWaiters(); |
1666 |
if (interruptMode != 0) |
1667 |
reportInterruptAfterWait(interruptMode); |
1668 |
return !timedout; |
1669 |
} |
1670 |
|
1671 |
/** |
1672 |
* Implements timed condition wait. |
1673 |
* <ol> |
1674 |
* <li>If current thread is interrupted, throw InterruptedException. |
1675 |
* <li>Save lock state returned by {@link #getState}. |
1676 |
* <li>Invoke {@link #release} with saved state as argument, |
1677 |
* throwing IllegalMonitorStateException if it fails. |
1678 |
* <li>Block until signalled, interrupted, or timed out. |
1679 |
* <li>Reacquire by invoking specialized version of |
1680 |
* {@link #acquire} with saved state as argument. |
1681 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
1682 |
* <li>If timed out while blocked in step 4, return false, else true. |
1683 |
* </ol> |
1684 |
*/ |
1685 |
public final boolean await(long time, TimeUnit unit) |
1686 |
throws InterruptedException { |
1687 |
long nanosTimeout = unit.toNanos(time); |
1688 |
if (Thread.interrupted()) |
1689 |
throw new InterruptedException(); |
1690 |
// We don't check for nanosTimeout <= 0L here, to allow |
1691 |
// await(0, unit) as a way to "yield the lock". |
1692 |
final long deadline = System.nanoTime() + nanosTimeout; |
1693 |
Node node = addConditionWaiter(); |
1694 |
long savedState = fullyRelease(node); |
1695 |
boolean timedout = false; |
1696 |
int interruptMode = 0; |
1697 |
while (!isOnSyncQueue(node)) { |
1698 |
if (nanosTimeout <= 0L) { |
1699 |
timedout = transferAfterCancelledWait(node); |
1700 |
break; |
1701 |
} |
1702 |
if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
1703 |
LockSupport.parkNanos(this, nanosTimeout); |
1704 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
1705 |
break; |
1706 |
nanosTimeout = deadline - System.nanoTime(); |
1707 |
} |
1708 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
1709 |
interruptMode = REINTERRUPT; |
1710 |
if (node.nextWaiter != null) |
1711 |
unlinkCancelledWaiters(); |
1712 |
if (interruptMode != 0) |
1713 |
reportInterruptAfterWait(interruptMode); |
1714 |
return !timedout; |
1715 |
} |
1716 |
|
1717 |
// support for instrumentation |
1718 |
|
1719 |
/** |
1720 |
* Returns true if this condition was created by the given |
1721 |
* synchronization object. |
1722 |
* |
1723 |
* @return {@code true} if owned |
1724 |
*/ |
1725 |
final boolean isOwnedBy(AbstractQueuedLongSynchronizer sync) { |
1726 |
return sync == AbstractQueuedLongSynchronizer.this; |
1727 |
} |
1728 |
|
1729 |
/** |
1730 |
* Queries whether any threads are waiting on this condition. |
1731 |
* Implements {@link AbstractQueuedLongSynchronizer#hasWaiters(ConditionObject)}. |
1732 |
* |
1733 |
* @return {@code true} if there are any waiting threads |
1734 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1735 |
* returns {@code false} |
1736 |
*/ |
1737 |
protected final boolean hasWaiters() { |
1738 |
if (!isHeldExclusively()) |
1739 |
throw new IllegalMonitorStateException(); |
1740 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
1741 |
if (w.waitStatus == Node.CONDITION) |
1742 |
return true; |
1743 |
} |
1744 |
return false; |
1745 |
} |
1746 |
|
1747 |
/** |
1748 |
* Returns an estimate of the number of threads waiting on |
1749 |
* this condition. |
1750 |
* Implements {@link AbstractQueuedLongSynchronizer#getWaitQueueLength(ConditionObject)}. |
1751 |
* |
1752 |
* @return the estimated number of waiting threads |
1753 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1754 |
* returns {@code false} |
1755 |
*/ |
1756 |
protected final int getWaitQueueLength() { |
1757 |
if (!isHeldExclusively()) |
1758 |
throw new IllegalMonitorStateException(); |
1759 |
int n = 0; |
1760 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
1761 |
if (w.waitStatus == Node.CONDITION) |
1762 |
++n; |
1763 |
} |
1764 |
return n; |
1765 |
} |
1766 |
|
1767 |
/** |
1768 |
* Returns a collection containing those threads that may be |
1769 |
* waiting on this Condition. |
1770 |
* Implements {@link AbstractQueuedLongSynchronizer#getWaitingThreads(ConditionObject)}. |
1771 |
* |
1772 |
* @return the collection of threads |
1773 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1774 |
* returns {@code false} |
1775 |
*/ |
1776 |
protected final Collection<Thread> getWaitingThreads() { |
1777 |
if (!isHeldExclusively()) |
1778 |
throw new IllegalMonitorStateException(); |
1779 |
ArrayList<Thread> list = new ArrayList<>(); |
1780 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
1781 |
if (w.waitStatus == Node.CONDITION) { |
1782 |
Thread t = w.thread; |
1783 |
if (t != null) |
1784 |
list.add(t); |
1785 |
} |
1786 |
} |
1787 |
return list; |
1788 |
} |
1789 |
} |
1790 |
|
1791 |
/** |
1792 |
* Setup to support compareAndSet. We need to natively implement |
1793 |
* this here: For the sake of permitting future enhancements, we |
1794 |
* cannot explicitly subclass AtomicLong, which would be |
1795 |
* efficient and useful otherwise. So, as the lesser of evils, we |
1796 |
* natively implement using hotspot intrinsics API. And while we |
1797 |
* are at it, we do the same for other CASable fields (which could |
1798 |
* otherwise be done with atomic field updaters). |
1799 |
*/ |
1800 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
1801 |
private static final long STATE; |
1802 |
private static final long HEAD; |
1803 |
private static final long TAIL; |
1804 |
|
1805 |
static { |
1806 |
try { |
1807 |
STATE = U.objectFieldOffset |
1808 |
(AbstractQueuedLongSynchronizer.class.getDeclaredField("state")); |
1809 |
HEAD = U.objectFieldOffset |
1810 |
(AbstractQueuedLongSynchronizer.class.getDeclaredField("head")); |
1811 |
TAIL = U.objectFieldOffset |
1812 |
(AbstractQueuedLongSynchronizer.class.getDeclaredField("tail")); |
1813 |
} catch (ReflectiveOperationException e) { |
1814 |
throw new Error(e); |
1815 |
} |
1816 |
|
1817 |
// Reduce the risk of rare disastrous classloading in first call to |
1818 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1819 |
Class<?> ensureLoaded = LockSupport.class; |
1820 |
} |
1821 |
|
1822 |
/** |
1823 |
* Initializes head and tail fields on first contention. |
1824 |
*/ |
1825 |
private final void initializeSyncQueue() { |
1826 |
Node h; |
1827 |
if (U.compareAndSwapObject(this, HEAD, null, (h = new Node()))) |
1828 |
tail = h; |
1829 |
} |
1830 |
|
1831 |
/** |
1832 |
* CASes tail field. |
1833 |
*/ |
1834 |
private final boolean compareAndSetTail(Node expect, Node update) { |
1835 |
return U.compareAndSwapObject(this, TAIL, expect, update); |
1836 |
} |
1837 |
} |