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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/licenses/publicdomain |
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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.*; |
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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 <tt>get</tt> |
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* method 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. |
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* |
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* <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or |
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* {@link java.lang.Runnable} object. Because <tt>FutureTask</tt> |
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* implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be |
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* submitted to an {@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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* <tt>protected</tt> 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 <tt>get</tt> method |
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*/ |
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public class FutureTask<V> implements Future<V>, Runnable { |
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/** Synchronization control for FutureTask */ |
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private final Sync sync; |
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|
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/** |
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* Constructs a <tt>FutureTask</tt> that will upon running, execute the |
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* given <tt>Callable</tt>. |
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* |
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* @param callable the callable task |
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* @throws NullPointerException if 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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sync = new Sync(callable); |
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} |
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|
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/** |
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* Constructs a <tt>FutureTask</tt> that will upon running, execute the |
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* given <tt>Runnable</tt>, and arrange that <tt>get</tt> 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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* <tt>Future<?> f = new FutureTask<Object>(runnable, null)</tt> |
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* @throws NullPointerException if runnable is null |
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*/ |
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public FutureTask(Runnable runnable, V result) { |
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sync = new Sync(Executors.callable(runnable, result)); |
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} |
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|
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public boolean isCancelled() { |
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return sync.innerIsCancelled(); |
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} |
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|
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public boolean isDone() { |
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return sync.innerIsDone(); |
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} |
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|
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public boolean cancel(boolean mayInterruptIfRunning) { |
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return sync.innerCancel(mayInterruptIfRunning); |
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} |
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|
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public V get() throws InterruptedException, ExecutionException { |
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return sync.innerGet(); |
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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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return sync.innerGet(unit.toNanos(timeout)); |
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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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* <tt>isDone</tt> (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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* @param v the value |
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*/ |
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protected void set(V v) { |
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sync.innerSet(v); |
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} |
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|
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/** |
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* Causes this future to report an <tt>ExecutionException</tt> |
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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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* @param t the cause of failure. |
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*/ |
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protected void setException(Throwable t) { |
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sync.innerSetException(t); |
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} |
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|
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/** |
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* Sets this Future to the result of computation unless |
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* it has been cancelled. |
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*/ |
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public void run() { |
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sync.innerRun(); |
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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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* @return true if successfully run and reset |
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*/ |
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protected boolean runAndReset() { |
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return sync.innerRunAndReset(); |
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} |
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|
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/** |
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* Synchronization control for FutureTask. Note that this must be |
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* a non-static inner class in order to invoke the protected |
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* <tt>done</tt> method. For clarity, all inner class support |
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* methods are same as outer, prefixed with "inner". |
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* |
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* Uses AQS sync state to represent run status |
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*/ |
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private final class Sync extends AbstractQueuedSynchronizer { |
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/** State value representing that task is running */ |
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private static final int RUNNING = 1; |
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/** State value representing that task ran */ |
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private static final int RAN = 2; |
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/** State value representing that task was cancelled */ |
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private static final int CANCELLED = 4; |
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|
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/** The underlying callable */ |
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private final Callable<V> callable; |
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/** The result to return from get() */ |
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private V result; |
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/** The exception to throw from get() */ |
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private Throwable exception; |
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|
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/** |
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* The thread running task. When nulled after set/cancel, this |
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* indicates that the results are accessible. Must be |
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* volatile, to serve as write barrier on completion. |
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*/ |
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private volatile Thread runner; |
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|
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Sync(Callable<V> callable) { |
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this.callable = callable; |
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} |
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|
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private boolean ranOrCancelled(int state) { |
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return (state & (RAN | CANCELLED)) != 0; |
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} |
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|
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/** |
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* Implements AQS base acquire to succeed if ran or cancelled |
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*/ |
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protected int tryAcquireShared(int ignore) { |
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return innerIsDone()? 1 : -1; |
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} |
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|
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/** |
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* Implements AQS base release to always signal after setting |
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* final done status by nulling runner thread. |
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*/ |
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protected boolean tryReleaseShared(int ignore) { |
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runner = null; |
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return true; |
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} |
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|
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boolean innerIsCancelled() { |
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return getState() == CANCELLED; |
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} |
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|
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boolean innerIsDone() { |
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return ranOrCancelled(getState()) && runner == null; |
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} |
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|
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V innerGet() throws InterruptedException, ExecutionException { |
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acquireSharedInterruptibly(0); |
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if (getState() == CANCELLED) |
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throw new CancellationException(); |
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if (exception != null) |
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throw new ExecutionException(exception); |
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return result; |
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} |
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|
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V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException { |
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if (!tryAcquireSharedNanos(0, nanosTimeout)) |
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throw new TimeoutException(); |
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if (getState() == CANCELLED) |
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throw new CancellationException(); |
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if (exception != null) |
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throw new ExecutionException(exception); |
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return result; |
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} |
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|
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void innerSet(V v) { |
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int s = getState(); |
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if (ranOrCancelled(s) || !compareAndSetState(s, RAN)) |
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return; |
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result = v; |
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releaseShared(0); |
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done(); |
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} |
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|
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void innerSetException(Throwable t) { |
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int s = getState(); |
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if (ranOrCancelled(s) || !compareAndSetState(s, RAN)) |
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return; |
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exception = t; |
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result = null; |
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releaseShared(0); |
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done(); |
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} |
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|
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boolean innerCancel(boolean mayInterruptIfRunning) { |
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int s = getState(); |
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if (ranOrCancelled(s) || !compareAndSetState(s, CANCELLED)) |
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return false; |
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if (mayInterruptIfRunning) { |
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Thread r = runner; |
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if (r != null) |
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r.interrupt(); |
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} |
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releaseShared(0); |
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done(); |
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return true; |
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} |
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|
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void innerRun() { |
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if (!compareAndSetState(0, RUNNING)) |
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return; |
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try { |
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runner = Thread.currentThread(); |
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innerSet(callable.call()); |
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} catch(Throwable ex) { |
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innerSetException(ex); |
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} |
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} |
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|
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boolean innerRunAndReset() { |
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if (!compareAndSetState(0, RUNNING)) |
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return false; |
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try { |
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runner = Thread.currentThread(); |
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callable.call(); // don't set result |
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runner = null; |
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return compareAndSetState(RUNNING, 0); |
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} catch(Throwable ex) { |
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innerSetException(ex); |
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return false; |
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} |
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} |
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} |
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} |