1 |
tim |
1.1 |
/* |
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dl |
1.2 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
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dl |
1.23 |
* Expert Group and released to the public domain, as explained at |
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jsr166 |
1.59 |
* http://creativecommons.org/publicdomain/zero/1.0/ |
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tim |
1.1 |
*/ |
6 |
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package java.util.concurrent; |
8 |
dl |
1.62 |
import java.util.concurrent.locks.LockSupport; |
9 |
dl |
1.13 |
|
10 |
tim |
1.1 |
/** |
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dl |
1.8 |
* 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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dl |
1.4 |
* retrieve the result of the computation. The result can only be |
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jsr166 |
1.64 |
* 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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dl |
1.8 |
* the computation has completed, the computation cannot be restarted |
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jsr166 |
1.64 |
* or cancelled (unless the computation is invoked using |
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* {@link #runAndReset}). |
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tim |
1.1 |
* |
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jsr166 |
1.64 |
* <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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tim |
1.1 |
* |
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dl |
1.14 |
* <p>In addition to serving as a standalone class, this class provides |
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jsr166 |
1.64 |
* {@code protected} functionality that may be useful when creating |
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dl |
1.14 |
* customized task classes. |
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* |
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tim |
1.1 |
* @since 1.5 |
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dl |
1.4 |
* @author Doug Lea |
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jsr166 |
1.64 |
* @param <V> The result type returned by this FutureTask's {@code get} methods |
33 |
tim |
1.1 |
*/ |
34 |
peierls |
1.39 |
public class FutureTask<V> implements RunnableFuture<V> { |
35 |
dl |
1.62 |
/* |
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* Revision notes: This differs from previous versions of this |
37 |
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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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jsr166 |
1.73 |
* The run state of this task, initially NEW. The run state |
49 |
dl |
1.78 |
* 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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jsr166 |
1.69 |
* |
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* Possible state transitions: |
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jsr166 |
1.73 |
* 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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dl |
1.62 |
*/ |
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private volatile int state; |
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jsr166 |
1.73 |
private static final int NEW = 0; |
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jsr166 |
1.70 |
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; |
70 |
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private static final int INTERRUPTED = 6; |
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dl |
1.62 |
|
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dl |
1.78 |
/** The underlying callable; nulled out after running */ |
73 |
dl |
1.77 |
private Callable<V> callable; |
74 |
dl |
1.62 |
/** 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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jsr166 |
1.64 |
* Returns result or throws exception for completed task. |
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* |
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dl |
1.62 |
* @param s completed state value |
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*/ |
86 |
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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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jsr166 |
1.69 |
if (s >= CANCELLED) |
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dl |
1.62 |
throw new CancellationException(); |
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throw new ExecutionException((Throwable)x); |
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} |
94 |
dl |
1.11 |
|
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tim |
1.1 |
/** |
96 |
jsr166 |
1.64 |
* Creates a {@code FutureTask} that will, upon running, execute the |
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* given {@code Callable}. |
98 |
tim |
1.1 |
* |
99 |
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* @param callable the callable task |
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jsr166 |
1.79 |
* @throws NullPointerException if the callable is null |
101 |
tim |
1.1 |
*/ |
102 |
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public FutureTask(Callable<V> callable) { |
103 |
dl |
1.9 |
if (callable == null) |
104 |
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throw new NullPointerException(); |
105 |
dl |
1.62 |
this.callable = callable; |
106 |
jsr166 |
1.85 |
this.state = NEW; // ensure visibility of callable |
107 |
tim |
1.1 |
} |
108 |
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109 |
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/** |
110 |
jsr166 |
1.64 |
* Creates a {@code FutureTask} that will, upon running, execute the |
111 |
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* given {@code Runnable}, and arrange that {@code get} will return the |
112 |
tim |
1.1 |
* given result on successful completion. |
113 |
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* |
114 |
jsr166 |
1.54 |
* @param runnable the runnable task |
115 |
tim |
1.1 |
* @param result the result to return on successful completion. If |
116 |
dl |
1.9 |
* you don't need a particular result, consider using |
117 |
dl |
1.16 |
* constructions of the form: |
118 |
jsr166 |
1.58 |
* {@code Future<?> f = new FutureTask<Void>(runnable, null)} |
119 |
jsr166 |
1.79 |
* @throws NullPointerException if the runnable is null |
120 |
tim |
1.1 |
*/ |
121 |
dl |
1.15 |
public FutureTask(Runnable runnable, V result) { |
122 |
dl |
1.62 |
this.callable = Executors.callable(runnable, result); |
123 |
jsr166 |
1.85 |
this.state = NEW; // ensure visibility of callable |
124 |
dl |
1.20 |
} |
125 |
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126 |
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public boolean isCancelled() { |
127 |
jsr166 |
1.69 |
return state >= CANCELLED; |
128 |
dl |
1.20 |
} |
129 |
jsr166 |
1.35 |
|
130 |
dl |
1.20 |
public boolean isDone() { |
131 |
jsr166 |
1.73 |
return state != NEW; |
132 |
dl |
1.13 |
} |
133 |
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134 |
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public boolean cancel(boolean mayInterruptIfRunning) { |
135 |
dl |
1.78 |
if (state != NEW) |
136 |
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return false; |
137 |
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if (mayInterruptIfRunning) { |
138 |
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if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING)) |
139 |
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return false; |
140 |
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Thread t = runner; |
141 |
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if (t != null) |
142 |
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t.interrupt(); |
143 |
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UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state |
144 |
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} |
145 |
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else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED)) |
146 |
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return false; |
147 |
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finishCompletion(); |
148 |
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return true; |
149 |
dl |
1.13 |
} |
150 |
jsr166 |
1.35 |
|
151 |
jsr166 |
1.43 |
/** |
152 |
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* @throws CancellationException {@inheritDoc} |
153 |
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*/ |
154 |
dl |
1.2 |
public V get() throws InterruptedException, ExecutionException { |
155 |
jsr166 |
1.64 |
int s = state; |
156 |
jsr166 |
1.83 |
return report((s <= COMPLETING) ? awaitDone(false, 0L) : s); |
157 |
tim |
1.1 |
} |
158 |
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159 |
jsr166 |
1.43 |
/** |
160 |
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* @throws CancellationException {@inheritDoc} |
161 |
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*/ |
162 |
dl |
1.2 |
public V get(long timeout, TimeUnit unit) |
163 |
tim |
1.1 |
throws InterruptedException, ExecutionException, TimeoutException { |
164 |
jsr166 |
1.82 |
if (unit == null) |
165 |
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throw new NullPointerException(); |
166 |
jsr166 |
1.64 |
int s = state; |
167 |
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if (s <= COMPLETING && |
168 |
jsr166 |
1.82 |
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) |
169 |
dl |
1.62 |
throw new TimeoutException(); |
170 |
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return report(s); |
171 |
tim |
1.1 |
} |
172 |
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173 |
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/** |
174 |
dl |
1.20 |
* Protected method invoked when this task transitions to state |
175 |
jsr166 |
1.64 |
* {@code isDone} (whether normally or via cancellation). The |
176 |
dl |
1.20 |
* default implementation does nothing. Subclasses may override |
177 |
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* this method to invoke completion callbacks or perform |
178 |
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* bookkeeping. Note that you can query status inside the |
179 |
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* implementation of this method to determine whether this task |
180 |
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* has been cancelled. |
181 |
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*/ |
182 |
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protected void done() { } |
183 |
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184 |
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/** |
185 |
jsr166 |
1.64 |
* Sets the result of this future to the given value unless |
186 |
dl |
1.29 |
* this future has already been set or has been cancelled. |
187 |
jsr166 |
1.64 |
* |
188 |
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* <p>This method is invoked internally by the {@link #run} method |
189 |
dl |
1.40 |
* upon successful completion of the computation. |
190 |
jsr166 |
1.64 |
* |
191 |
tim |
1.1 |
* @param v the value |
192 |
jsr166 |
1.35 |
*/ |
193 |
dl |
1.2 |
protected void set(V v) { |
194 |
dl |
1.78 |
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { |
195 |
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outcome = v; |
196 |
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UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state |
197 |
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finishCompletion(); |
198 |
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} |
199 |
tim |
1.1 |
} |
200 |
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201 |
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/** |
202 |
jsr166 |
1.64 |
* Causes this future to report an {@link ExecutionException} |
203 |
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* with the given throwable as its cause, unless this future has |
204 |
dl |
1.24 |
* already been set or has been cancelled. |
205 |
jsr166 |
1.64 |
* |
206 |
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* <p>This method is invoked internally by the {@link #run} method |
207 |
dl |
1.40 |
* upon failure of the computation. |
208 |
jsr166 |
1.64 |
* |
209 |
jsr166 |
1.41 |
* @param t the cause of failure |
210 |
jsr166 |
1.35 |
*/ |
211 |
dl |
1.2 |
protected void setException(Throwable t) { |
212 |
dl |
1.78 |
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { |
213 |
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outcome = t; |
214 |
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UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state |
215 |
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finishCompletion(); |
216 |
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} |
217 |
tim |
1.1 |
} |
218 |
jsr166 |
1.35 |
|
219 |
dl |
1.24 |
public void run() { |
220 |
jsr166 |
1.87 |
if (state != NEW || |
221 |
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!UNSAFE.compareAndSwapObject(this, runnerOffset, |
222 |
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null, Thread.currentThread())) |
223 |
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return; |
224 |
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try { |
225 |
dl |
1.77 |
Callable<V> c = callable; |
226 |
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if (c != null && state == NEW) { |
227 |
jsr166 |
1.87 |
V result; |
228 |
dl |
1.77 |
try { |
229 |
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result = c.call(); |
230 |
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} catch (Throwable ex) { |
231 |
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setException(ex); |
232 |
jsr166 |
1.87 |
return; |
233 |
dl |
1.77 |
} |
234 |
jsr166 |
1.87 |
set(result); |
235 |
dl |
1.62 |
} |
236 |
jsr166 |
1.87 |
} finally { |
237 |
jsr166 |
1.68 |
runner = null; |
238 |
jsr166 |
1.86 |
int s = state; |
239 |
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if (s >= INTERRUPTING) |
240 |
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handlePossibleCancellationInterrupt(s); |
241 |
dl |
1.62 |
} |
242 |
dl |
1.24 |
} |
243 |
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244 |
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/** |
245 |
dl |
1.30 |
* Executes the computation without setting its result, and then |
246 |
jsr166 |
1.64 |
* resets this future to initial state, failing to do so if the |
247 |
dl |
1.24 |
* computation encounters an exception or is cancelled. This is |
248 |
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* designed for use with tasks that intrinsically execute more |
249 |
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* than once. |
250 |
jsr166 |
1.64 |
* |
251 |
dl |
1.24 |
* @return true if successfully run and reset |
252 |
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*/ |
253 |
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protected boolean runAndReset() { |
254 |
jsr166 |
1.87 |
if (state != NEW || |
255 |
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!UNSAFE.compareAndSwapObject(this, runnerOffset, |
256 |
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null, Thread.currentThread())) |
257 |
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return false; |
258 |
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try { |
259 |
dl |
1.77 |
Callable<V> c = callable; |
260 |
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if (c != null && state == NEW) { |
261 |
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try { |
262 |
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c.call(); // don't set result |
263 |
jsr166 |
1.87 |
return state == NEW; |
264 |
dl |
1.77 |
} catch (Throwable ex) { |
265 |
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setException(ex); |
266 |
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} |
267 |
jsr166 |
1.68 |
} |
268 |
jsr166 |
1.87 |
return false; |
269 |
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} finally { |
270 |
jsr166 |
1.68 |
runner = null; |
271 |
jsr166 |
1.69 |
int s = state; |
272 |
jsr166 |
1.87 |
if (s >= INTERRUPTING) |
273 |
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handlePossibleCancellationInterrupt(s); |
274 |
dl |
1.62 |
} |
275 |
dl |
1.14 |
} |
276 |
dl |
1.3 |
|
277 |
dl |
1.14 |
/** |
278 |
jsr166 |
1.86 |
* Ensures that any interrupt from a possible cancel(true) does |
279 |
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* not leak into subsequent code. |
280 |
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*/ |
281 |
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private void handlePossibleCancellationInterrupt(int s) { |
282 |
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// It is possible for our interrupter to stall before getting a |
283 |
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// chance to interrupt us. Let's spin-wait patiently. |
284 |
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if (s == INTERRUPTING) { |
285 |
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while ((s = state) == INTERRUPTING) |
286 |
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Thread.yield(); // wait out pending interrupt |
287 |
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} |
288 |
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assert state == INTERRUPTED; |
289 |
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// Clear any interrupt we may have received. |
290 |
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Thread.interrupted(); // clear interrupt from cancel(true) |
291 |
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} |
292 |
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|
293 |
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/** |
294 |
dl |
1.62 |
* Simple linked list nodes to record waiting threads in a Treiber |
295 |
jsr166 |
1.64 |
* stack. See other classes such as Phaser and SynchronousQueue |
296 |
dl |
1.62 |
* for more detailed explanation. |
297 |
dl |
1.20 |
*/ |
298 |
dl |
1.62 |
static final class WaitNode { |
299 |
|
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volatile Thread thread; |
300 |
dl |
1.76 |
volatile WaitNode next; |
301 |
|
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WaitNode() { thread = Thread.currentThread(); } |
302 |
dl |
1.62 |
} |
303 |
dl |
1.42 |
|
304 |
dl |
1.62 |
/** |
305 |
jsr166 |
1.85 |
* Removes and signals all waiting threads, invokes done(), and |
306 |
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* nulls out callable. |
307 |
dl |
1.62 |
*/ |
308 |
dl |
1.78 |
private void finishCompletion() { |
309 |
jsr166 |
1.86 |
assert state > NEW; |
310 |
jsr166 |
1.81 |
for (WaitNode q; (q = waiters) != null;) { |
311 |
dl |
1.62 |
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) { |
312 |
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for (;;) { |
313 |
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Thread t = q.thread; |
314 |
|
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if (t != null) { |
315 |
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q.thread = null; |
316 |
|
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LockSupport.unpark(t); |
317 |
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} |
318 |
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WaitNode next = q.next; |
319 |
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if (next == null) |
320 |
dl |
1.78 |
break; |
321 |
dl |
1.62 |
q.next = null; // unlink to help gc |
322 |
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q = next; |
323 |
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} |
324 |
dl |
1.78 |
break; |
325 |
dl |
1.62 |
} |
326 |
dl |
1.24 |
} |
327 |
jsr166 |
1.85 |
|
328 |
dl |
1.78 |
done(); |
329 |
jsr166 |
1.85 |
|
330 |
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callable = null; // to reduce footprint |
331 |
dl |
1.62 |
} |
332 |
dl |
1.24 |
|
333 |
dl |
1.62 |
/** |
334 |
jsr166 |
1.64 |
* Awaits completion or aborts on interrupt or timeout. |
335 |
|
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* |
336 |
dl |
1.62 |
* @param timed true if use timed waits |
337 |
jsr166 |
1.64 |
* @param nanos time to wait, if timed |
338 |
dl |
1.62 |
* @return state upon completion |
339 |
|
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*/ |
340 |
|
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private int awaitDone(boolean timed, long nanos) |
341 |
|
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throws InterruptedException { |
342 |
jsr166 |
1.63 |
long last = timed ? System.nanoTime() : 0L; |
343 |
dl |
1.62 |
WaitNode q = null; |
344 |
|
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boolean queued = false; |
345 |
jsr166 |
1.64 |
for (;;) { |
346 |
dl |
1.62 |
if (Thread.interrupted()) { |
347 |
|
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removeWaiter(q); |
348 |
|
|
throw new InterruptedException(); |
349 |
|
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} |
350 |
jsr166 |
1.64 |
|
351 |
|
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int s = state; |
352 |
|
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if (s > COMPLETING) { |
353 |
dl |
1.62 |
if (q != null) |
354 |
|
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q.thread = null; |
355 |
|
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return s; |
356 |
|
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} |
357 |
|
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else if (q == null) |
358 |
|
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q = new WaitNode(); |
359 |
|
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else if (!queued) |
360 |
|
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queued = UNSAFE.compareAndSwapObject(this, waitersOffset, |
361 |
|
|
q.next = waiters, q); |
362 |
|
|
else if (timed) { |
363 |
|
|
long now = System.nanoTime(); |
364 |
|
|
if ((nanos -= (now - last)) <= 0L) { |
365 |
|
|
removeWaiter(q); |
366 |
|
|
return state; |
367 |
dl |
1.50 |
} |
368 |
dl |
1.62 |
last = now; |
369 |
|
|
LockSupport.parkNanos(this, nanos); |
370 |
dl |
1.50 |
} |
371 |
dl |
1.62 |
else |
372 |
|
|
LockSupport.park(this); |
373 |
dl |
1.24 |
} |
374 |
dl |
1.62 |
} |
375 |
dl |
1.24 |
|
376 |
dl |
1.62 |
/** |
377 |
jsr166 |
1.64 |
* Tries to unlink a timed-out or interrupted wait node to avoid |
378 |
|
|
* accumulating garbage. Internal nodes are simply unspliced |
379 |
dl |
1.62 |
* without CAS since it is harmless if they are traversed anyway |
380 |
jsr166 |
1.81 |
* by releasers. To avoid effects of unsplicing from already |
381 |
|
|
* removed nodes, the list is retraversed in case of an apparent |
382 |
|
|
* race. This is slow when there are a lot of nodes, but we don't |
383 |
|
|
* expect lists to be long enough to outweigh higher-overhead |
384 |
|
|
* schemes. |
385 |
dl |
1.62 |
*/ |
386 |
|
|
private void removeWaiter(WaitNode node) { |
387 |
|
|
if (node != null) { |
388 |
|
|
node.thread = null; |
389 |
jsr166 |
1.81 |
retry: |
390 |
|
|
for (;;) { // restart on removeWaiter race |
391 |
|
|
for (WaitNode pred = null, q = waiters, s; q != null; q = s) { |
392 |
|
|
s = q.next; |
393 |
|
|
if (q.thread != null) |
394 |
|
|
pred = q; |
395 |
|
|
else if (pred != null) { |
396 |
|
|
pred.next = s; |
397 |
|
|
if (pred.thread == null) // check for race |
398 |
|
|
continue retry; |
399 |
|
|
} |
400 |
|
|
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, |
401 |
|
|
q, s)) |
402 |
|
|
continue retry; |
403 |
jsr166 |
1.55 |
} |
404 |
jsr166 |
1.81 |
break; |
405 |
jsr166 |
1.56 |
} |
406 |
dl |
1.14 |
} |
407 |
dl |
1.62 |
} |
408 |
dl |
1.14 |
|
409 |
dl |
1.62 |
// Unsafe mechanics |
410 |
|
|
private static final sun.misc.Unsafe UNSAFE; |
411 |
|
|
private static final long stateOffset; |
412 |
|
|
private static final long runnerOffset; |
413 |
|
|
private static final long waitersOffset; |
414 |
|
|
static { |
415 |
|
|
try { |
416 |
|
|
UNSAFE = sun.misc.Unsafe.getUnsafe(); |
417 |
|
|
Class<?> k = FutureTask.class; |
418 |
|
|
stateOffset = UNSAFE.objectFieldOffset |
419 |
|
|
(k.getDeclaredField("state")); |
420 |
|
|
runnerOffset = UNSAFE.objectFieldOffset |
421 |
|
|
(k.getDeclaredField("runner")); |
422 |
|
|
waitersOffset = UNSAFE.objectFieldOffset |
423 |
|
|
(k.getDeclaredField("waiters")); |
424 |
|
|
} catch (Exception e) { |
425 |
|
|
throw new Error(e); |
426 |
dl |
1.14 |
} |
427 |
dl |
1.15 |
} |
428 |
dl |
1.62 |
|
429 |
dl |
1.15 |
} |