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487 lines
16 KiB
487 lines
16 KiB
/*
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*/
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/*
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*
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*
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*
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*
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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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package java.util.concurrent;
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import java.util.concurrent.locks.LockSupport;
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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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* 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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/** 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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* 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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@SuppressWarnings("unchecked")
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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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return (V)x;
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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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* 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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* 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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public boolean isCancelled() {
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return state >= CANCELLED;
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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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public boolean cancel(boolean mayInterruptIfRunning) {
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if (!(state == NEW &&
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UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
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mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
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return false;
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try { // in case call to interrupt throws exception
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if (mayInterruptIfRunning) {
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try {
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Thread t = runner;
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if (t != null)
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t.interrupt();
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} finally { // final state
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UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
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}
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}
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} finally {
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finishCompletion();
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}
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return true;
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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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* @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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* 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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* 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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* 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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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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* 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 {@code 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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* 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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// assert state == INTERRUPTED;
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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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* 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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* 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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done();
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callable = null; // to reduce footprint
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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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final long deadline = timed ? System.nanoTime() + nanos : 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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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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nanos = deadline - System.nanoTime();
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if (nanos <= 0L) {
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removeWaiter(q);
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return state;
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}
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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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* 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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// 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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