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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; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* A cancellable asynchronous computation. This class provides a base |
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* implementation of {@link Future}, with methods to start and cancel |
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* a computation, query to see if the computation is complete, and |
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* retrieve the result of the computation. The result can only be |
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* retrieved when the computation has completed; the {@code get} |
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* methods will block if the computation has not yet completed. Once |
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* the computation has completed, the computation cannot be restarted |
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* or cancelled (unless the computation is invoked using |
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* {@link #runAndReset}). |
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* |
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* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or |
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* {@link Runnable} object. Because {@code FutureTask} implements |
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* {@code Runnable}, a {@code FutureTask} can be submitted to an |
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* {@link Executor} for execution. |
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* |
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* <p>In addition to serving as a standalone class, this class provides |
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* {@code protected} functionality that may be useful when creating |
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* customized task classes. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <V> The result type returned by this FutureTask's {@code get} methods |
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*/ |
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public class FutureTask<V> implements RunnableFuture<V> { |
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/* |
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* Revision notes: This differs from previous versions of this |
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* class that relied on AbstractQueuedSynchronizer, mainly to |
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* avoid surprising users about retaining interrupt status during |
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* cancellation races. Sync control in the current design relies |
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* on a "state" field updated via CAS to track completion, along |
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* with a simple Treiber stack to hold waiting threads. |
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* |
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* Style note: As usual, we bypass overhead of using |
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* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics. |
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*/ |
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|
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/** |
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* The run state of this task, initially NEW. The run state |
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* transitions to a terminal state only in methods set, |
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* setException, and cancel. During completion, state may take on |
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* transient values of COMPLETING (while outcome is being set) or |
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* INTERRUPTING (only while interrupting the runner to satisfy a |
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* cancel(true)). Transitions from these intermediate to final |
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* states use cheaper ordered/lazy writes because values are unique |
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* and cannot be further modified. |
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* |
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* Possible state transitions: |
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* NEW -> COMPLETING -> NORMAL |
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* NEW -> COMPLETING -> EXCEPTIONAL |
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* NEW -> CANCELLED |
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* NEW -> INTERRUPTING -> INTERRUPTED |
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*/ |
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private volatile int state; |
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private static final int NEW = 0; |
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private static final int COMPLETING = 1; |
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private static final int NORMAL = 2; |
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private static final int EXCEPTIONAL = 3; |
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private static final int CANCELLED = 4; |
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private static final int INTERRUPTING = 5; |
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private static final int INTERRUPTED = 6; |
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|
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/** The underlying callable; nulled out after running */ |
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private Callable<V> callable; |
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/** The result to return or exception to throw from get() */ |
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private Object outcome; // non-volatile, protected by state reads/writes |
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/** The thread running the callable; CASed during run() */ |
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private volatile Thread runner; |
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/** Treiber stack of waiting threads */ |
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private volatile WaitNode waiters; |
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|
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/** |
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* Returns result or throws exception for completed task. |
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* |
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* @param s completed state value |
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*/ |
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private V report(int s) throws ExecutionException { |
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Object x = outcome; |
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if (s == NORMAL) { |
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@SuppressWarnings("unchecked") V v = (V)x; |
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return v; |
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} |
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if (s >= CANCELLED) |
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throw new CancellationException(); |
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throw new ExecutionException((Throwable)x); |
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} |
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|
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/** |
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* Creates a {@code FutureTask} that will, upon running, execute the |
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* given {@code Callable}. |
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* |
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* @param callable the callable task |
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* @throws NullPointerException if the callable is null |
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*/ |
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public FutureTask(Callable<V> callable) { |
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if (callable == null) |
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throw new NullPointerException(); |
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this.callable = callable; |
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this.state = NEW; // ensure visibility of callable |
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} |
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|
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/** |
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* Creates a {@code FutureTask} that will, upon running, execute the |
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* given {@code Runnable}, and arrange that {@code get} will return the |
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* given result on successful completion. |
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* |
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* @param runnable the runnable task |
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* @param result the result to return on successful completion. If |
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* you don't need a particular result, consider using |
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* constructions of the form: |
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* {@code Future<?> f = new FutureTask<Void>(runnable, null)} |
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* @throws NullPointerException if the runnable is null |
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*/ |
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public FutureTask(Runnable runnable, V result) { |
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this.callable = Executors.callable(runnable, result); |
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this.state = NEW; // ensure visibility of callable |
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} |
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|
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public boolean isCancelled() { |
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return state >= CANCELLED; |
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} |
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|
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public boolean isDone() { |
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return state != NEW; |
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} |
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|
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public boolean cancel(boolean mayInterruptIfRunning) { |
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if (state != NEW) |
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return false; |
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if (mayInterruptIfRunning) { |
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if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING)) |
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return false; |
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Thread t = runner; |
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if (t != null) |
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t.interrupt(); |
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UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state |
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} |
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else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED)) |
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return false; |
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finishCompletion(); |
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return true; |
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} |
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|
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/** |
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* @throws CancellationException {@inheritDoc} |
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*/ |
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public V get() throws InterruptedException, ExecutionException { |
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int s = state; |
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if (s <= COMPLETING) |
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s = awaitDone(false, 0L); |
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return report(s); |
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} |
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|
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/** |
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* @throws CancellationException {@inheritDoc} |
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*/ |
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public V get(long timeout, TimeUnit unit) |
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throws InterruptedException, ExecutionException, TimeoutException { |
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if (unit == null) |
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throw new NullPointerException(); |
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int s = state; |
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if (s <= COMPLETING && |
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(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) |
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throw new TimeoutException(); |
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return report(s); |
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} |
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|
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/** |
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* Protected method invoked when this task transitions to state |
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* {@code isDone} (whether normally or via cancellation). The |
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* default implementation does nothing. Subclasses may override |
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* this method to invoke completion callbacks or perform |
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* bookkeeping. Note that you can query status inside the |
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* implementation of this method to determine whether this task |
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* has been cancelled. |
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*/ |
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protected void done() { } |
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|
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/** |
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* Sets the result of this future to the given value unless |
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* this future has already been set or has been cancelled. |
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* |
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* <p>This method is invoked internally by the {@link #run} method |
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* upon successful completion of the computation. |
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* |
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* @param v the value |
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*/ |
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protected void set(V v) { |
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if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { |
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outcome = v; |
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UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state |
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finishCompletion(); |
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} |
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} |
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|
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/** |
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* Causes this future to report an {@link ExecutionException} |
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* with the given throwable as its cause, unless this future has |
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* already been set or has been cancelled. |
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* |
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* <p>This method is invoked internally by the {@link #run} method |
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* upon failure of the computation. |
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* |
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* @param t the cause of failure |
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*/ |
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protected void setException(Throwable t) { |
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if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { |
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outcome = t; |
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UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state |
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finishCompletion(); |
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} |
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} |
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|
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public void run() { |
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if (state != NEW || |
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!UNSAFE.compareAndSwapObject(this, runnerOffset, |
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null, Thread.currentThread())) |
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return; |
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try { |
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Callable<V> c = callable; |
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if (c != null && state == NEW) { |
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V result; |
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boolean ran; |
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try { |
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result = c.call(); |
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ran = true; |
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} catch (Throwable ex) { |
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result = null; |
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ran = false; |
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setException(ex); |
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} |
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if (ran) |
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set(result); |
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} |
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} finally { |
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// runner must be non-null until state is settled to |
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// prevent concurrent calls to run() |
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runner = null; |
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// state must be re-read after nulling runner to prevent |
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// leaked interrupts |
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int s = state; |
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if (s >= INTERRUPTING) |
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handlePossibleCancellationInterrupt(s); |
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} |
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} |
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|
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/** |
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* Executes the computation without setting its result, and then |
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* resets this future to initial state, failing to do so if the |
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* computation encounters an exception or is cancelled. This is |
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* designed for use with tasks that intrinsically execute more |
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* than once. |
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* |
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* @return true if successfully run and reset |
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*/ |
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protected boolean runAndReset() { |
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if (state != NEW || |
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!UNSAFE.compareAndSwapObject(this, runnerOffset, |
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null, Thread.currentThread())) |
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return false; |
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boolean ran = false; |
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int s = state; |
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try { |
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Callable<V> c = callable; |
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if (c != null && s == NEW) { |
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try { |
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c.call(); // don't set result |
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ran = true; |
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} catch (Throwable ex) { |
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setException(ex); |
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} |
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} |
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} finally { |
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// runner must be non-null until state is settled to |
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// prevent concurrent calls to run() |
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runner = null; |
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// state must be re-read after nulling runner to prevent |
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// leaked interrupts |
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s = state; |
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if (s >= INTERRUPTING) |
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handlePossibleCancellationInterrupt(s); |
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} |
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return ran && s == NEW; |
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} |
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|
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/** |
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* Ensures that any interrupt from a possible cancel(true) is only |
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* delivered to a task while in run or runAndReset. |
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*/ |
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private void handlePossibleCancellationInterrupt(int s) { |
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// It is possible for our interrupter to stall before getting a |
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// chance to interrupt us. Let's spin-wait patiently. |
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if (s == INTERRUPTING) |
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while (state == INTERRUPTING) |
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Thread.yield(); // wait out pending interrupt |
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|
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// assert state == INTERRUPTED; |
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|
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// We want to clear any interrupt we may have received from |
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// cancel(true). However, it is permissible to use interrupts |
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// as an independent mechanism for a task to communicate with |
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// its caller, and there is no way to clear only the |
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// cancellation interrupt. |
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// |
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// Thread.interrupted(); |
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} |
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|
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/** |
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* Simple linked list nodes to record waiting threads in a Treiber |
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* stack. See other classes such as Phaser and SynchronousQueue |
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* for more detailed explanation. |
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*/ |
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static final class WaitNode { |
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volatile Thread thread; |
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volatile WaitNode next; |
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WaitNode() { thread = Thread.currentThread(); } |
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} |
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|
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/** |
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* Removes and signals all waiting threads, invokes done(), and |
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* nulls out callable. |
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*/ |
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private void finishCompletion() { |
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// assert state > COMPLETING; |
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for (WaitNode q; (q = waiters) != null;) { |
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if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) { |
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for (;;) { |
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Thread t = q.thread; |
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if (t != null) { |
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q.thread = null; |
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LockSupport.unpark(t); |
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} |
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WaitNode next = q.next; |
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if (next == null) |
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break; |
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q.next = null; // unlink to help gc |
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q = next; |
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} |
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break; |
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} |
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} |
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|
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done(); |
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|
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callable = null; // to reduce footprint |
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} |
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|
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/** |
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* Awaits completion or aborts on interrupt or timeout. |
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* |
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* @param timed true if use timed waits |
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* @param nanos time to wait, if timed |
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* @return state upon completion |
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*/ |
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private int awaitDone(boolean timed, long nanos) |
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throws InterruptedException { |
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long last = timed ? System.nanoTime() : 0L; |
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WaitNode q = null; |
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boolean queued = false; |
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for (;;) { |
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if (Thread.interrupted()) { |
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removeWaiter(q); |
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throw new InterruptedException(); |
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} |
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|
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int s = state; |
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if (s > COMPLETING) { |
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if (q != null) |
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q.thread = null; |
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return s; |
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} |
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else if (s == COMPLETING) // cannot time out yet |
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Thread.yield(); |
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else if (q == null) |
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q = new WaitNode(); |
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else if (!queued) |
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queued = UNSAFE.compareAndSwapObject(this, waitersOffset, |
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q.next = waiters, q); |
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else if (timed) { |
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long now = System.nanoTime(); |
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if ((nanos -= (now - last)) <= 0L) { |
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removeWaiter(q); |
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return state; |
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} |
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last = now; |
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LockSupport.parkNanos(this, nanos); |
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} |
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else |
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LockSupport.park(this); |
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} |
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} |
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|
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/** |
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* Tries to unlink a timed-out or interrupted wait node to avoid |
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* accumulating garbage. Internal nodes are simply unspliced |
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* without CAS since it is harmless if they are traversed anyway |
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* by releasers. To avoid effects of unsplicing from already |
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* removed nodes, the list is retraversed in case of an apparent |
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* race. This is slow when there are a lot of nodes, but we don't |
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* expect lists to be long enough to outweigh higher-overhead |
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* schemes. |
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*/ |
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private void removeWaiter(WaitNode node) { |
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if (node != null) { |
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node.thread = null; |
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retry: |
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for (;;) { // restart on removeWaiter race |
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for (WaitNode pred = null, q = waiters, s; q != null; q = s) { |
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s = q.next; |
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if (q.thread != null) |
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pred = q; |
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else if (pred != null) { |
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pred.next = s; |
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if (pred.thread == null) // check for race |
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continue retry; |
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} |
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else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, |
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q, s)) |
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continue retry; |
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} |
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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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// Unsafe mechanics |
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private static final sun.misc.Unsafe UNSAFE; |
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private static final long stateOffset; |
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private static final long runnerOffset; |
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private static final long waitersOffset; |
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static { |
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try { |
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UNSAFE = sun.misc.Unsafe.getUnsafe(); |
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Class<?> k = FutureTask.class; |
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stateOffset = UNSAFE.objectFieldOffset |
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(k.getDeclaredField("state")); |
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runnerOffset = UNSAFE.objectFieldOffset |
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(k.getDeclaredField("runner")); |
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waitersOffset = UNSAFE.objectFieldOffset |
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(k.getDeclaredField("waiters")); |
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} catch (Exception e) { |
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throw new Error(e); |
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} |
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} |
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|
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} |