1 /*
  2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  3  *
  4  * This code is free software; you can redistribute it and/or modify it
  5  * under the terms of the GNU General Public License version 2 only, as
  6  * published by the Free Software Foundation.  Oracle designates this
  7  * particular file as subject to the "Classpath" exception as provided
  8  * by Oracle in the LICENSE file that accompanied this code.
  9  *
 10  * This code is distributed in the hope that it will be useful, but WITHOUT
 11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 13  * version 2 for more details (a copy is included in the LICENSE file that
 14  * accompanied this code).
 15  *
 16  * You should have received a copy of the GNU General Public License version
 17  * 2 along with this work; if not, write to the Free Software Foundation,
 18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 19  *
 20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 21  * or visit www.oracle.com if you need additional information or have any
 22  * questions.
 23  */
 24 
 25 /*
 26  * This file is available under and governed by the GNU General Public
 27  * License version 2 only, as published by the Free Software Foundation.
 28  * However, the following notice accompanied the original version of this
 29  * file:
 30  *
 31  * Written by Doug Lea with assistance from members of JCP JSR-166
 32  * Expert Group and released to the public domain, as explained at
 33  * http://creativecommons.org/publicdomain/zero/1.0/
 34  */
 35 
 36 package java.util.concurrent;
 37 
 38 import java.lang.invoke.MethodHandles;
 39 import java.lang.invoke.VarHandle;
 40 import java.util.concurrent.locks.LockSupport;
 41 
 42 /**
 43  * A cancellable asynchronous computation.  This class provides a base
 44  * implementation of {@link Future}, with methods to start and cancel
 45  * a computation, query to see if the computation is complete, and
 46  * retrieve the result of the computation.  The result can only be
 47  * retrieved when the computation has completed; the {@code get}
 48  * methods will block if the computation has not yet completed.  Once
 49  * the computation has completed, the computation cannot be restarted
 50  * or cancelled (unless the computation is invoked using
 51  * {@link #runAndReset}).
 52  *
 53  * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 54  * {@link Runnable} object.  Because {@code FutureTask} implements
 55  * {@code Runnable}, a {@code FutureTask} can be submitted to an
 56  * {@link Executor} for execution.
 57  *
 58  * <p>In addition to serving as a standalone class, this class provides
 59  * {@code protected} functionality that may be useful when creating
 60  * customized task classes.
 61  *
 62  * @since 1.5
 63  * @author Doug Lea
 64  * @param <V> The result type returned by this FutureTask's {@code get} methods
 65  */
 66 public class FutureTask<V> implements RunnableFuture<V> {
 67     /*
 68      * Revision notes: This differs from previous versions of this
 69      * class that relied on AbstractQueuedSynchronizer, mainly to
 70      * avoid surprising users about retaining interrupt status during
 71      * cancellation races. Sync control in the current design relies
 72      * on a "state" field updated via CAS to track completion, along
 73      * with a simple Treiber stack to hold waiting threads.
 74      */
 75 
 76     /**
 77      * The run state of this task, initially NEW.  The run state
 78      * transitions to a terminal state only in methods set,
 79      * setException, and cancel.  During completion, state may take on
 80      * transient values of COMPLETING (while outcome is being set) or
 81      * INTERRUPTING (only while interrupting the runner to satisfy a
 82      * cancel(true)). Transitions from these intermediate to final
 83      * states use cheaper ordered/lazy writes because values are unique
 84      * and cannot be further modified.
 85      *
 86      * Possible state transitions:
 87      * NEW -> COMPLETING -> NORMAL
 88      * NEW -> COMPLETING -> EXCEPTIONAL
 89      * NEW -> CANCELLED
 90      * NEW -> INTERRUPTING -> INTERRUPTED
 91      */
 92     private volatile int state;
 93     private static final int NEW          = 0;
 94     private static final int COMPLETING   = 1;
 95     private static final int NORMAL       = 2;
 96     private static final int EXCEPTIONAL  = 3;
 97     private static final int CANCELLED    = 4;
 98     private static final int INTERRUPTING = 5;
 99     private static final int INTERRUPTED  = 6;
100 
101     /** The underlying callable; nulled out after running */
102     private Callable<V> callable;
103     /** The result to return or exception to throw from get() */
104     private Object outcome; // non-volatile, protected by state reads/writes
105     /** The thread running the callable; CASed during run() */
106     private volatile Thread runner;
107     /** Treiber stack of waiting threads */
108     private volatile WaitNode waiters;
109 
110     /**
111      * Returns result or throws exception for completed task.
112      *
113      * @param s completed state value
114      */
115     @SuppressWarnings("unchecked")
116     private V report(int s) throws ExecutionException {
117         Object x = outcome;
118         if (s == NORMAL)
119             return (V)x;
120         if (s >= CANCELLED)
121             throw new CancellationException();
122         throw new ExecutionException((Throwable)x);
123     }
124 
125     /**
126      * Creates a {@code FutureTask} that will, upon running, execute the
127      * given {@code Callable}.
128      *
129      * @param  callable the callable task
130      * @throws NullPointerException if the callable is null
131      */
132     public FutureTask(Callable<V> callable) {
133         if (callable == null)
134             throw new NullPointerException();
135         this.callable = callable;
136         this.state = NEW;       // ensure visibility of callable
137     }
138 
139     /**
140      * Creates a {@code FutureTask} that will, upon running, execute the
141      * given {@code Runnable}, and arrange that {@code get} will return the
142      * given result on successful completion.
143      *
144      * @param runnable the runnable task
145      * @param result the result to return on successful completion. If
146      * you don't need a particular result, consider using
147      * constructions of the form:
148      * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
149      * @throws NullPointerException if the runnable is null
150      */
151     public FutureTask(Runnable runnable, V result) {
152         this.callable = Executors.callable(runnable, result);
153         this.state = NEW;       // ensure visibility of callable
154     }
155 
156     public boolean isCancelled() {
157         return state >= CANCELLED;
158     }
159 
160     public boolean isDone() {
161         return state != NEW;
162     }
163 
164     public boolean cancel(boolean mayInterruptIfRunning) {
165         if (!(state == NEW && STATE.compareAndSet
166               (this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
167             return false;
168         try {    // in case call to interrupt throws exception
169             if (mayInterruptIfRunning) {
170                 try {
171                     Thread t = runner;
172                     if (t != null)
173                         t.interrupt();
174                 } finally { // final state
175                     STATE.setRelease(this, INTERRUPTED);
176                 }
177             }
178         } finally {
179             finishCompletion();
180         }
181         return true;
182     }
183 
184     /**
185      * @throws CancellationException {@inheritDoc}
186      */
187     public V get() throws InterruptedException, ExecutionException {
188         int s = state;
189         if (s <= COMPLETING)
190             s = awaitDone(false, 0L);
191         return report(s);
192     }
193 
194     /**
195      * @throws CancellationException {@inheritDoc}
196      */
197     public V get(long timeout, TimeUnit unit)
198         throws InterruptedException, ExecutionException, TimeoutException {
199         if (unit == null)
200             throw new NullPointerException();
201         int s = state;
202         if (s <= COMPLETING &&
203             (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
204             throw new TimeoutException();
205         return report(s);
206     }
207 
208     /**
209      * Protected method invoked when this task transitions to state
210      * {@code isDone} (whether normally or via cancellation). The
211      * default implementation does nothing.  Subclasses may override
212      * this method to invoke completion callbacks or perform
213      * bookkeeping. Note that you can query status inside the
214      * implementation of this method to determine whether this task
215      * has been cancelled.
216      */
217     protected void done() { }
218 
219     /**
220      * Sets the result of this future to the given value unless
221      * this future has already been set or has been cancelled.
222      *
223      * <p>This method is invoked internally by the {@link #run} method
224      * upon successful completion of the computation.
225      *
226      * @param v the value
227      */
228     protected void set(V v) {
229         if (STATE.compareAndSet(this, NEW, COMPLETING)) {
230             outcome = v;
231             STATE.setRelease(this, NORMAL); // final state
232             finishCompletion();
233         }
234     }
235 
236     /**
237      * Causes this future to report an {@link ExecutionException}
238      * with the given throwable as its cause, unless this future has
239      * already been set or has been cancelled.
240      *
241      * <p>This method is invoked internally by the {@link #run} method
242      * upon failure of the computation.
243      *
244      * @param t the cause of failure
245      */
246     protected void setException(Throwable t) {
247         if (STATE.compareAndSet(this, NEW, COMPLETING)) {
248             outcome = t;
249             STATE.setRelease(this, EXCEPTIONAL); // final state
250             finishCompletion();
251         }
252     }
253 
254     public void run() {
255         if (state != NEW ||
256             !RUNNER.compareAndSet(this, null, Thread.currentThread()))
257             return;
258         try {
259             Callable<V> c = callable;
260             if (c != null && state == NEW) {
261                 V result;
262                 boolean ran;
263                 try {
264                     result = c.call();
265                     ran = true;
266                 } catch (Throwable ex) {
267                     result = null;
268                     ran = false;
269                     setException(ex);
270                 }
271                 if (ran)
272                     set(result);
273             }
274         } finally {
275             // runner must be non-null until state is settled to
276             // prevent concurrent calls to run()
277             runner = null;
278             // state must be re-read after nulling runner to prevent
279             // leaked interrupts
280             int s = state;
281             if (s >= INTERRUPTING)
282                 handlePossibleCancellationInterrupt(s);
283         }
284     }
285 
286     /**
287      * Executes the computation without setting its result, and then
288      * resets this future to initial state, failing to do so if the
289      * computation encounters an exception or is cancelled.  This is
290      * designed for use with tasks that intrinsically execute more
291      * than once.
292      *
293      * @return {@code true} if successfully run and reset
294      */
295     protected boolean runAndReset() {
296         if (state != NEW ||
297             !RUNNER.compareAndSet(this, null, Thread.currentThread()))
298             return false;
299         boolean ran = false;
300         int s = state;
301         try {
302             Callable<V> c = callable;
303             if (c != null && s == NEW) {
304                 try {
305                     c.call(); // don't set result
306                     ran = true;
307                 } catch (Throwable ex) {
308                     setException(ex);
309                 }
310             }
311         } finally {
312             // runner must be non-null until state is settled to
313             // prevent concurrent calls to run()
314             runner = null;
315             // state must be re-read after nulling runner to prevent
316             // leaked interrupts
317             s = state;
318             if (s >= INTERRUPTING)
319                 handlePossibleCancellationInterrupt(s);
320         }
321         return ran && s == NEW;
322     }
323 
324     /**
325      * Ensures that any interrupt from a possible cancel(true) is only
326      * delivered to a task while in run or runAndReset.
327      */
328     private void handlePossibleCancellationInterrupt(int s) {
329         // It is possible for our interrupter to stall before getting a
330         // chance to interrupt us.  Let's spin-wait patiently.
331         if (s == INTERRUPTING)
332             while (state == INTERRUPTING)
333                 Thread.yield(); // wait out pending interrupt
334 
335         // assert state == INTERRUPTED;
336 
337         // We want to clear any interrupt we may have received from
338         // cancel(true).  However, it is permissible to use interrupts
339         // as an independent mechanism for a task to communicate with
340         // its caller, and there is no way to clear only the
341         // cancellation interrupt.
342         //
343         // Thread.interrupted();
344     }
345 
346     /**
347      * Simple linked list nodes to record waiting threads in a Treiber
348      * stack.  See other classes such as Phaser and SynchronousQueue
349      * for more detailed explanation.
350      */
351     static final class WaitNode {
352         volatile Thread thread;
353         volatile WaitNode next;
354         WaitNode() { thread = Thread.currentThread(); }
355     }
356 
357     /**
358      * Removes and signals all waiting threads, invokes done(), and
359      * nulls out callable.
360      */
361     private void finishCompletion() {
362         // assert state > COMPLETING;
363         for (WaitNode q; (q = waiters) != null;) {
364             if (WAITERS.weakCompareAndSet(this, q, null)) {
365                 for (;;) {
366                     Thread t = q.thread;
367                     if (t != null) {
368                         q.thread = null;
369                         LockSupport.unpark(t);
370                     }
371                     WaitNode next = q.next;
372                     if (next == null)
373                         break;
374                     q.next = null; // unlink to help gc
375                     q = next;
376                 }
377                 break;
378             }
379         }
380 
381         done();
382 
383         callable = null;        // to reduce footprint
384     }
385 
386     /**
387      * Awaits completion or aborts on interrupt or timeout.
388      *
389      * @param timed true if use timed waits
390      * @param nanos time to wait, if timed
391      * @return state upon completion or at timeout
392      */
393     private int awaitDone(boolean timed, long nanos)
394         throws InterruptedException {
395         // The code below is very delicate, to achieve these goals:
396         // - call nanoTime exactly once for each call to park
397         // - if nanos <= 0L, return promptly without allocation or nanoTime
398         // - if nanos == Long.MIN_VALUE, don't underflow
399         // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
400         //   and we suffer a spurious wakeup, we will do no worse than
401         //   to park-spin for a while
402         long startTime = 0L;    // Special value 0L means not yet parked
403         WaitNode q = null;
404         boolean queued = false;
405         for (;;) {
406             int s = state;
407             if (s > COMPLETING) {
408                 if (q != null)
409                     q.thread = null;
410                 return s;
411             }
412             else if (s == COMPLETING)
413                 // We may have already promised (via isDone) that we are done
414                 // so never return empty-handed or throw InterruptedException
415                 Thread.yield();
416             else if (Thread.interrupted()) {
417                 removeWaiter(q);
418                 throw new InterruptedException();
419             }
420             else if (q == null) {
421                 if (timed && nanos <= 0L)
422                     return s;
423                 q = new WaitNode();
424             }
425             else if (!queued)
426                 queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
427             else if (timed) {
428                 final long parkNanos;
429                 if (startTime == 0L) { // first time
430                     startTime = System.nanoTime();
431                     if (startTime == 0L)
432                         startTime = 1L;
433                     parkNanos = nanos;
434                 } else {
435                     long elapsed = System.nanoTime() - startTime;
436                     if (elapsed >= nanos) {
437                         removeWaiter(q);
438                         return state;
439                     }
440                     parkNanos = nanos - elapsed;
441                 }
442                 // nanoTime may be slow; recheck before parking
443                 if (state < COMPLETING)
444                     LockSupport.parkNanos(this, parkNanos);
445             }
446             else
447                 LockSupport.park(this);
448         }
449     }
450 
451     /**
452      * Tries to unlink a timed-out or interrupted wait node to avoid
453      * accumulating garbage.  Internal nodes are simply unspliced
454      * without CAS since it is harmless if they are traversed anyway
455      * by releasers.  To avoid effects of unsplicing from already
456      * removed nodes, the list is retraversed in case of an apparent
457      * race.  This is slow when there are a lot of nodes, but we don't
458      * expect lists to be long enough to outweigh higher-overhead
459      * schemes.
460      */
461     private void removeWaiter(WaitNode node) {
462         if (node != null) {
463             node.thread = null;
464             retry:
465             for (;;) {          // restart on removeWaiter race
466                 for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
467                     s = q.next;
468                     if (q.thread != null)
469                         pred = q;
470                     else if (pred != null) {
471                         pred.next = s;
472                         if (pred.thread == null) // check for race
473                             continue retry;
474                     }
475                     else if (!WAITERS.compareAndSet(this, q, s))
476                         continue retry;
477                 }
478                 break;
479             }
480         }
481     }
482 
483     /**
484      * Returns a string representation of this FutureTask.
485      *
486      * @implSpec
487      * The default implementation returns a string identifying this
488      * FutureTask, as well as its completion state.  The state, in
489      * brackets, contains one of the strings {@code "Completed Normally"},
490      * {@code "Completed Exceptionally"}, {@code "Cancelled"}, or {@code
491      * "Not completed"}.
492      *
493      * @return a string representation of this FutureTask
494      */
495     public String toString() {
496         final String status;
497         switch (state) {
498         case NORMAL:
499             status = "[Completed normally]";
500             break;
501         case EXCEPTIONAL:
502             status = "[Completed exceptionally: " + outcome + "]";
503             break;
504         case CANCELLED:
505         case INTERRUPTING:
506         case INTERRUPTED:
507             status = "[Cancelled]";
508             break;
509         default:
510             final Callable<?> callable = this.callable;
511             status = (callable == null)
512                 ? "[Not completed]"
513                 : "[Not completed, task = " + callable + "]";
514         }
515         return super.toString() + status;
516     }
517 
518     // VarHandle mechanics
519     private static final VarHandle STATE;
520     private static final VarHandle RUNNER;
521     private static final VarHandle WAITERS;
522     static {
523         try {
524             MethodHandles.Lookup l = MethodHandles.lookup();
525             STATE = l.findVarHandle(FutureTask.class, "state", int.class);
526             RUNNER = l.findVarHandle(FutureTask.class, "runner", Thread.class);
527             WAITERS = l.findVarHandle(FutureTask.class, "waiters", WaitNode.class);
528         } catch (ReflectiveOperationException e) {
529             throw new ExceptionInInitializerError(e);
530         }
531 
532         // Reduce the risk of rare disastrous classloading in first call to
533         // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
534         Class<?> ensureLoaded = LockSupport.class;
535     }
536 
537 }