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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. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.concurrent.atomic.*; |
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|
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/** |
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* Base class for cancellable actions running in the Executor |
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* framework. In addition to serving as a standalone class, this |
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* provides <tt>protected</tt> functionality that may be useful when |
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* creating customized task classes. |
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*/ |
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|
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public class CancellableTask implements Cancellable, Runnable { |
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/** |
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* Holds the run-state, taking on values: |
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* null = not yet started, |
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* [some thread ref] = running, |
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* DONE = completed normally, |
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* CANCELLED = cancelled (may or may not have ever run). |
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* Transitions use atomic updates. |
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*/ |
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private volatile Object runner; |
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|
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/* |
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* Special value for "runner" indicating task is completed |
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*/ |
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private static final Object DONE = new Object(); |
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|
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/* |
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* Special value for "runner" indicating task is cancelled |
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*/ |
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private static final Object CANCELLED = new Object(); |
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|
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private static AtomicReferenceFieldUpdater<CancellableTask, Object> |
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runnerUpdater = new AtomicReferenceFieldUpdater<CancellableTask, Object>(new CancellableTask[0], new Object[0], "runner"); |
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|
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/** |
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* The runnable underlying this task |
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*/ |
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private volatile Runnable runnable; |
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|
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/** |
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* Creates a new CancellableTask invoking the given |
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* runnable when executed. |
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*/ |
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public CancellableTask(Runnable r) { |
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this.runnable = r; |
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} |
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|
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/** |
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* Creates a new CancellableTask without a runnable action, which |
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* must be set using <tt>setRunnable</tt> before use. This is |
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* intended for use in subclasses that must complete superclass |
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* construction beofre establishing the runnable action. |
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*/ |
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protected CancellableTask() { |
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} |
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|
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|
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public boolean cancel(boolean mayInterruptIfRunning) { |
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Object r = runner; |
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if (r == DONE || r == CANCELLED) |
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return false; |
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|
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if (mayInterruptIfRunning && |
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r != null && |
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r instanceof Thread && |
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r != Thread.currentThread() && |
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runnerUpdater.compareAndSet(this, r, CANCELLED)) |
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|
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((Thread)r).interrupt(); |
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else |
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runnerUpdater.set(this, CANCELLED); |
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return true; |
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} |
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|
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public boolean isCancelled() { |
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return runner == CANCELLED; |
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} |
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|
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public boolean isDone() { |
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Object r = runner; |
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return r == DONE || r == CANCELLED; |
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} |
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|
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/** |
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* Return the Runnable forming the basis of this task. |
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*/ |
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protected Runnable getRunnable() { |
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return runnable; |
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} |
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|
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/** |
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* Set the Runnable forming the basis of this task. |
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*/ |
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protected void setRunnable(Runnable r) { |
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runnable = r; |
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} |
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|
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/** |
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* Set the state of this task to Cancelled |
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*/ |
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protected void setCancelled() { |
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runnerUpdater.set(this, CANCELLED); |
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} |
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|
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/** |
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* Set the state of this task to Done, unless already |
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* in a Cancelled state, in which Cancelled status is preserved. |
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* |
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*/ |
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protected void setDone() { |
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for (;;) { |
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Object r = runner; |
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if (r == DONE || r == CANCELLED) |
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return; |
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if (runnerUpdater.compareAndSet(this, r, DONE)) |
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return; |
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} |
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} |
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|
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/** |
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* Attempt to set the state of this task to Running, succeeding |
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* only if the state is currently NOT Done, Running, or Cancelled. |
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* @return true if successful |
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*/ |
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protected boolean setRunning() { |
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return runnerUpdater.compareAndSet(this, null, Thread.currentThread()); |
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} |
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|
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public void run() { |
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if (setRunning()) { |
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try { |
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runnable.run(); |
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} |
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finally { |
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setDone(); |
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} |
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} |
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} |
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|
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/** |
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* Implementation of Future methods under the control of a current |
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* CancellableTask. This is split into an inner class to permit |
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* Future support to be mixed-in with other flavors of tasks. |
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*/ |
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protected class InnerCancellableFuture<V> implements Future<V>, Runnable { |
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private final Callable<V> callable; |
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private final ReentrantLock lock = new ReentrantLock(); |
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private final Condition accessible = lock.newCondition(); |
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private V result; |
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private Throwable exception; |
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protected InnerCancellableFuture(Callable<V> callable) { |
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this.callable = callable; |
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} |
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|
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public boolean isDone() { |
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return CancellableTask.this.isDone(); |
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} |
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|
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public void run() { |
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try { |
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set(callable.call()); |
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} |
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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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|
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public V get() throws InterruptedException, ExecutionException { |
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lock.lock(); |
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try { |
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while (!isDone()) |
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accessible.await(); |
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if (isCancelled()) |
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throw new CancellationException(); |
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else if (exception != null) |
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throw new ExecutionException(exception); |
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else |
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return result; |
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} |
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finally { |
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lock.unlock(); |
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} |
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} |
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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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lock.lock(); |
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try { |
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if (!isDone()) { |
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long nanos = unit.toNanos(timeout); |
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do { |
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if (nanos <= 0) |
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throw new TimeoutException(); |
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nanos = accessible.awaitNanos(nanos); |
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} while (!isDone()); |
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} |
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if (isCancelled()) |
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throw new CancellationException(); |
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else if (exception != null) |
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throw new ExecutionException(exception); |
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else |
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return result; |
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} |
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finally { |
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lock.unlock(); |
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} |
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} |
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|
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protected void set(V v) { |
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lock.lock(); |
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try { |
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result = v; |
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setDone(); |
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accessible.signalAll(); |
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} |
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finally { |
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lock.unlock(); |
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} |
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} |
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|
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protected void setException(Throwable t) { |
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lock.lock(); |
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try { |
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exception = t; |
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setDone(); |
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accessible.signalAll(); |
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} |
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finally { |
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lock.unlock(); |
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} |
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} |
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} |
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|
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} |