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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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|
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import java.util.concurrent.atomic.*; |
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import java.util.concurrent.locks.*; |
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|
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/** |
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* Base class for {@link Cancellable} {@link java.lang.Runnable} |
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* actions within the {@link Executor} framework. In addition to |
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* serving as a standalone class, this provides <tt>protected</tt> |
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* functionality that may be useful when creating customized task |
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* classes. |
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* @since 1.5 |
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* @author Doug Lea |
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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 = |
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AtomicReferenceFieldUpdater.newUpdater( |
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CancellableTask.class, |
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Object.class, |
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"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 which invokes the given |
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* <tt>Runnable</tt> when executed. |
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* @param r the runnable action |
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* @throws NullPointerException if runnable is null |
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*/ |
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public CancellableTask(Runnable r) { |
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if (r == null) |
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throw new NullPointerException(); |
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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 before 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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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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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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|
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done(); |
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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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* Returns the Runnable forming the basis of this task. |
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* @return the runnable action |
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* @see #setRunnable |
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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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* Sets the Runnable forming the basis of this task. |
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* @param r the runnable action |
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* @see #getRunnable |
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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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* Sets 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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* Sets the state of this task to Done, unless already in a |
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* Cancelled state, in which case Cancelled status is preserved. |
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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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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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/** |
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* Attempts 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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/** |
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* Runs the runnable if not cancelled, maintaining run status. |
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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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} 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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* 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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* Resets the run state of this task to its initial state unless |
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* it has been cancelled. (Note that a cancelled task cannot be |
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* reset.) |
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* @return true if successful |
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*/ |
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protected boolean reset() { |
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for (;;) { |
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Object r = runner; |
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if (r == CANCELLED) |
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return false; |
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if (runnerUpdater.compareAndSet(this, r, null)) |
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return true; |
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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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* <tt>CancellableTask</tt>, which it relies on for methods |
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* <tt>isDone</tt>, <tt>isCancelled</tt> and <tt>cancel</tt>. This |
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* class is split into an inner class to permit Future support to |
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* be mixed-in with other flavors of tasks. Normally, such a |
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* class will delegate <tt>get</tt> methods to the |
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* <tt>InnerCancellableFuture</tt>, and internally arrange that |
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* <tt>set</tt> methods be invoked when computations are ready. |
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* |
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* <p><b>Sample Usage</b>. Here are fragments of an example subclass. |
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* <pre> |
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* class MyFutureTask<V> extends CancellableTask implements Future<V> { |
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* |
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* MyFutureTask(Callable<V> callable) { |
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* setRunnable(new InnerCancellableFuture<V>(callable)); |
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* } |
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* |
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* public V get() throws InterruptedException, ExecutionException { |
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* return ((InnerCancellableFuture<V>)getRunnable()).get(); |
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* } |
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* // (And similarly for timeout version.) |
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* |
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* void action() { // whatever action causes execution |
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* try { |
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* ((InnerCancellableFuture<V>)getRunnable()).set(compute()); |
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* } catch (Exception ex) { |
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* ((InnerCancellableFuture<V>)getRunnable()).setException(ex); |
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* } |
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* } |
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* } |
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*</pre> |
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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 ReentrantLock.ConditionObject accessible = lock.newCondition(); |
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private V result; |
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private Throwable exception; |
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|
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/** |
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* Creates an InnerCancellableFuture that will execute the |
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* given callable. |
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* @param callable the function to execute |
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*/ |
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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 cancel(boolean mayInterruptIfRunning) { |
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return CancellableTask.this.cancel(mayInterruptIfRunning); |
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} |
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|
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public boolean isCancelled() { |
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return CancellableTask.this.isCancelled(); |
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} |
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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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|
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/** |
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* Sets this Future to the results of <tt>callable.call</tt> |
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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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} 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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/** |
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* Waits if necessary for the call to <tt>callable.call</tt> to |
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* complete, and then retrieves its result. |
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* |
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* @return computed result |
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* @throws CancellationException if underlying computation was |
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* cancelled |
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* @throws ExecutionException if underlying computation threw an |
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* exception |
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* @throws InterruptedException if current thread was interrupted |
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* while waiting |
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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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} 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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* Waits if necessary for at most the given time for the call to |
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* <tt>callable.call</tt> to complete, and then retrieves its |
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* result. |
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* |
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* @param timeout the maximum time to wait |
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* @param unit the time unit of the timeout argument |
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* @return computed result |
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* @throws CancellationException if underlying computation was |
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* cancelled |
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* @throws ExecutionException if underlying computation threw an |
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* exception |
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* @throws InterruptedException if current thread was interrupted |
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* while waiting |
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* @throws TimeoutException if the wait timed out |
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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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} 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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* Sets the result of this Future to the given value. |
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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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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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} 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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* Causes this future to report an <tt>ExecutionException</tt> |
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* with the given throwable as its cause. |
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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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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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} 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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} |