2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain, as explained at
4 * http://creativecommons.org/licenses/publicdomain
7 package java.util.concurrent;
8 import java.util.concurrent.locks.*;
12 * An {@link ExecutorService} that executes each submitted task using
13 * one of possibly several pooled threads, normally configured
14 * using {@link Executors} factory methods.
16 * <p>Thread pools address two different problems: they usually
17 * provide improved performance when executing large numbers of
18 * asynchronous tasks, due to reduced per-task invocation overhead,
19 * and they provide a means of bounding and managing the resources,
20 * including threads, consumed when executing a collection of tasks.
21 * Each <tt>ThreadPoolExecutor</tt> also maintains some basic
22 * statistics, such as the number of completed tasks.
24 * <p>To be useful across a wide range of contexts, this class
25 * provides many adjustable parameters and extensibility
26 * hooks. However, programmers are urged to use the more convenient
27 * {@link Executors} factory methods {@link
28 * Executors#newCachedThreadPool} (unbounded thread pool, with
29 * automatic thread reclamation), {@link Executors#newFixedThreadPool}
30 * (fixed size thread pool) and {@link
31 * Executors#newSingleThreadExecutor} (single background thread), that
32 * preconfigure settings for the most common usage
33 * scenarios. Otherwise, use the following guide when manually
34 * configuring and tuning this class:
38 * <dt>Core and maximum pool sizes</dt>
40 * <dd>A <tt>ThreadPoolExecutor</tt> will automatically adjust the
42 * (see {@link ThreadPoolExecutor#getPoolSize})
43 * according to the bounds set by corePoolSize
44 * (see {@link ThreadPoolExecutor#getCorePoolSize})
47 * (see {@link ThreadPoolExecutor#getMaximumPoolSize}).
48 * When a new task is submitted in method {@link
49 * ThreadPoolExecutor#execute}, and fewer than corePoolSize threads
50 * are running, a new thread is created to handle the request, even if
51 * other worker threads are idle. If there are more than
52 * corePoolSize but less than maximumPoolSize threads running, a new
53 * thread will be created only if the queue is full. By setting
54 * corePoolSize and maximumPoolSize the same, you create a fixed-size
55 * thread pool. By setting maximumPoolSize to an essentially unbounded
56 * value such as <tt>Integer.MAX_VALUE</tt>, you allow the pool to
57 * accommodate an arbitrary number of concurrent tasks. Most typically,
58 * core and maximum pool sizes are set only upon construction, but they
59 * may also be changed dynamically using {@link
60 * ThreadPoolExecutor#setCorePoolSize} and {@link
61 * ThreadPoolExecutor#setMaximumPoolSize}. <dd>
63 * <dt> On-demand construction
65 * <dd> By default, even core threads are initially created and
66 * started only when new tasks arrive, but this can be overridden
67 * dynamically using method {@link
68 * ThreadPoolExecutor#prestartCoreThread} or
69 * {@link ThreadPoolExecutor#prestartAllCoreThreads}.
70 * You probably want to prestart threads if you construct the
71 * pool with a non-empty queue. </dd>
73 * <dt>Creating new threads</dt>
75 * <dd>New threads are created using a {@link
76 * java.util.concurrent.ThreadFactory}. If not otherwise specified, a
77 * {@link Executors#defaultThreadFactory} is used, that creates threads to all
78 * be in the same {@link ThreadGroup} and with the same
79 * <tt>NORM_PRIORITY</tt> priority and non-daemon status. By supplying
80 * a different ThreadFactory, you can alter the thread's name, thread
81 * group, priority, daemon status, etc. If a <tt>ThreadFactory</tt> fails to create
82 * a thread when asked by returning null from <tt>newThread</tt>,
83 * the executor will continue, but might
84 * not be able to execute any tasks. </dd>
86 * <dt>Keep-alive times</dt>
88 * <dd>If the pool currently has more than corePoolSize threads,
89 * excess threads will be terminated if they have been idle for more
90 * than the keepAliveTime (see {@link
91 * ThreadPoolExecutor#getKeepAliveTime}). This provides a means of
92 * reducing resource consumption when the pool is not being actively
93 * used. If the pool becomes more active later, new threads will be
94 * constructed. This parameter can also be changed dynamically using
95 * method {@link ThreadPoolExecutor#setKeepAliveTime}. Using a value
96 * of <tt>Long.MAX_VALUE</tt> {@link TimeUnit#NANOSECONDS} effectively
97 * disables idle threads from ever terminating prior to shut down. By
98 * default, the keep-alive policy applies only when there are more
99 * than corePoolSizeThreads. But method {@link
100 * ThreadPoolExecutor#allowCoreThreadTimeOut} can be used to apply
101 * this time-out policy to core threads as well, so long as
102 * the keepAliveTime value is non-zero. </dd>
106 * <dd>Any {@link BlockingQueue} may be used to transfer and hold
107 * submitted tasks. The use of this queue interacts with pool sizing:
111 * <li> If fewer than corePoolSize threads are running, the Executor
112 * always prefers adding a new thread
113 * rather than queuing.</li>
115 * <li> If corePoolSize or more threads are running, the Executor
116 * always prefers queuing a request rather than adding a new
119 * <li> If a request cannot be queued, a new thread is created unless
120 * this would exceed maximumPoolSize, in which case, the task will be
125 * There are three general strategies for queuing:
128 * <li> <em> Direct handoffs.</em> A good default choice for a work
129 * queue is a {@link SynchronousQueue} that hands off tasks to threads
130 * without otherwise holding them. Here, an attempt to queue a task
131 * will fail if no threads are immediately available to run it, so a
132 * new thread will be constructed. This policy avoids lockups when
133 * handling sets of requests that might have internal dependencies.
134 * Direct handoffs generally require unbounded maximumPoolSizes to
135 * avoid rejection of new submitted tasks. This in turn admits the
136 * possibility of unbounded thread growth when commands continue to
137 * arrive on average faster than they can be processed. </li>
139 * <li><em> Unbounded queues.</em> Using an unbounded queue (for
140 * example a {@link LinkedBlockingQueue} without a predefined
141 * capacity) will cause new tasks to wait in the queue when all
142 * corePoolSize threads are busy. Thus, no more than corePoolSize
143 * threads will ever be created. (And the value of the maximumPoolSize
144 * therefore doesn't have any effect.) This may be appropriate when
145 * each task is completely independent of others, so tasks cannot
146 * affect each others execution; for example, in a web page server.
147 * While this style of queuing can be useful in smoothing out
148 * transient bursts of requests, it admits the possibility of
149 * unbounded work queue growth when commands continue to arrive on
150 * average faster than they can be processed. </li>
152 * <li><em>Bounded queues.</em> A bounded queue (for example, an
153 * {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
154 * used with finite maximumPoolSizes, but can be more difficult to
155 * tune and control. Queue sizes and maximum pool sizes may be traded
156 * off for each other: Using large queues and small pools minimizes
157 * CPU usage, OS resources, and context-switching overhead, but can
158 * lead to artificially low throughput. If tasks frequently block (for
159 * example if they are I/O bound), a system may be able to schedule
160 * time for more threads than you otherwise allow. Use of small queues
161 * generally requires larger pool sizes, which keeps CPUs busier but
162 * may encounter unacceptable scheduling overhead, which also
163 * decreases throughput. </li>
169 * <dt>Rejected tasks</dt>
171 * <dd> New tasks submitted in method {@link
172 * ThreadPoolExecutor#execute} will be <em>rejected</em> when the
173 * Executor has been shut down, and also when the Executor uses finite
174 * bounds for both maximum threads and work queue capacity, and is
175 * saturated. In either case, the <tt>execute</tt> method invokes the
176 * {@link RejectedExecutionHandler#rejectedExecution} method of its
177 * {@link RejectedExecutionHandler}. Four predefined handler policies
183 * default {@link ThreadPoolExecutor.AbortPolicy}, the handler throws a
184 * runtime {@link RejectedExecutionException} upon rejection. </li>
187 * ThreadPoolExecutor.CallerRunsPolicy}, the thread that invokes
188 * <tt>execute</tt> itself runs the task. This provides a simple
189 * feedback control mechanism that will slow down the rate that new
190 * tasks are submitted. </li>
192 * <li> In {@link ThreadPoolExecutor.DiscardPolicy},
193 * a task that cannot be executed is simply dropped. </li>
196 * ThreadPoolExecutor.DiscardOldestPolicy}, if the executor is not
197 * shut down, the task at the head of the work queue is dropped, and
198 * then execution is retried (which can fail again, causing this to be
203 * It is possible to define and use other kinds of {@link
204 * RejectedExecutionHandler} classes. Doing so requires some care
205 * especially when policies are designed to work only under particular
206 * capacity or queuing policies. </dd>
208 * <dt>Hook methods</dt>
210 * <dd>This class provides <tt>protected</tt> overridable {@link
211 * ThreadPoolExecutor#beforeExecute} and {@link
212 * ThreadPoolExecutor#afterExecute} methods that are called before and
213 * after execution of each task. These can be used to manipulate the
214 * execution environment; for example, reinitializing ThreadLocals,
215 * gathering statistics, or adding log entries. Additionally, method
216 * {@link ThreadPoolExecutor#terminated} can be overridden to perform
217 * any special processing that needs to be done once the Executor has
220 * <p>If hook or callback methods throw
221 * exceptions, internal worker threads may in turn fail and
222 * abruptly terminate.</dd>
224 * <dt>Queue maintenance</dt>
226 * <dd> Method {@link ThreadPoolExecutor#getQueue} allows access to
227 * the work queue for purposes of monitoring and debugging. Use of
228 * this method for any other purpose is strongly discouraged. Two
229 * supplied methods, {@link ThreadPoolExecutor#remove} and {@link
230 * ThreadPoolExecutor#purge} are available to assist in storage
231 * reclamation when large numbers of queued tasks become
234 * <dt>Finalization</dt>
236 * <dd> A pool that is no longer referenced in a program <em>AND</em>
237 * has no remaining threads will be <tt>shutdown</tt>
238 * automatically. If you would like to ensure that unreferenced pools
239 * are reclaimed even if users forget to call {@link
240 * ThreadPoolExecutor#shutdown}, then you must arrange that unused
241 * threads eventually die, by setting appropriate keep-alive times,
242 * using a lower bound of zero core threads and/or setting {@link
243 * ThreadPoolExecutor#allowCoreThreadTimeOut}. </dd> </dl>
245 * <p> <b>Extension example</b>. Most extensions of this class
246 * override one or more of the protected hook methods. For example,
247 * here is a subclass that adds a simple pause/resume feature:
250 * class PausableThreadPoolExecutor extends ThreadPoolExecutor {
251 * private boolean isPaused;
252 * private ReentrantLock pauseLock = new ReentrantLock();
253 * private Condition unpaused = pauseLock.newCondition();
255 * public PausableThreadPoolExecutor(...) { super(...); }
257 * protected void beforeExecute(Thread t, Runnable r) {
258 * super.beforeExecute(t, r);
261 * while (isPaused) unpaused.await();
262 * } catch (InterruptedException ie) {
265 * pauseLock.unlock();
269 * public void pause() {
274 * pauseLock.unlock();
278 * public void resume() {
282 * unpaused.signalAll();
284 * pauseLock.unlock();
292 public class ThreadPoolExecutor extends AbstractExecutorService {
294 * Only used to force toArray() to produce a Runnable[].
296 private static final Runnable[] EMPTY_RUNNABLE_ARRAY = new Runnable[0];
299 * Permission for checking shutdown
301 private static final RuntimePermission shutdownPerm =
302 new RuntimePermission("modifyThread");
305 * Queue used for holding tasks and handing off to worker threads.
307 private final BlockingQueue<Runnable> workQueue;
310 * Lock held on updates to poolSize, corePoolSize, maximumPoolSize, and
313 private final ReentrantLock mainLock = new ReentrantLock();
316 * Wait condition to support awaitTermination
318 private final Condition termination = mainLock.newCondition();
321 * Set containing all worker threads in pool.
323 private final HashSet<Worker> workers = new HashSet<Worker>();
326 * Timeout in nanoseconds for idle threads waiting for work.
327 * Threads use this timeout only when there are more than
328 * corePoolSize present. Otherwise they wait forever for new work.
330 private volatile long keepAliveTime;
333 * If false (default) core threads stay alive even when idle.
334 * If true, core threads use keepAliveTime to time out waiting for work.
336 private volatile boolean allowCoreThreadTimeOut;
339 * Core pool size, updated only while holding mainLock,
340 * but volatile to allow concurrent readability even
343 private volatile int corePoolSize;
346 * Maximum pool size, updated only while holding mainLock
347 * but volatile to allow concurrent readability even
350 private volatile int maximumPoolSize;
353 * Current pool size, updated only while holding mainLock
354 * but volatile to allow concurrent readability even
357 private volatile int poolSize;
362 volatile int runState;
364 // Special values for runState
365 /** Normal, not-shutdown mode */
366 static final int RUNNING = 0;
367 /** Controlled shutdown mode */
368 static final int SHUTDOWN = 1;
369 /** Immediate shutdown mode */
370 static final int STOP = 2;
372 static final int TERMINATED = 3;
375 * Handler called when saturated or shutdown in execute.
377 private volatile RejectedExecutionHandler handler;
380 * Factory for new threads.
382 private volatile ThreadFactory threadFactory;
385 * Tracks largest attained pool size.
387 private int largestPoolSize;
390 * Counter for completed tasks. Updated only on termination of
393 private long completedTaskCount;
396 * The default rejected execution handler
398 private static final RejectedExecutionHandler defaultHandler =
402 * Invokes the rejected execution handler for the given command.
404 void reject(Runnable command) {
405 handler.rejectedExecution(command, this);
409 * Creates and returns a new thread running firstTask as its first
410 * task. Call only while holding mainLock.
411 * @param firstTask the task the new thread should run first (or
413 * @return the new thread, or null if threadFactory fails to create thread
415 private Thread addThread(Runnable firstTask) {
416 if (runState == TERMINATED) // Don't create thread if terminated
418 Worker w = new Worker(firstTask);
419 Thread t = threadFactory.newThread(w);
424 if (nt > largestPoolSize)
425 largestPoolSize = nt;
431 * Creates and starts a new thread running firstTask as its first
432 * task, only if fewer than corePoolSize threads are running.
433 * @param firstTask the task the new thread should run first (or
435 * @return true if successful.
437 private boolean addIfUnderCorePoolSize(Runnable firstTask) {
439 final ReentrantLock mainLock = this.mainLock;
442 if (poolSize < corePoolSize)
443 t = addThread(firstTask);
454 * Creates and starts a new thread only if fewer than maximumPoolSize
455 * threads are running. The new thread runs as its first task the
456 * next task in queue, or if there is none, the given task.
457 * @param firstTask the task the new thread should run first (or
459 * @return 0 if a new thread cannot be created, a positive number
460 * if firstTask will be run in a new thread, or a negative number
461 * if a new thread was created but is running some other task, in
462 * which case the caller must try some other way to run firstTask
463 * (perhaps by calling this method again).
465 private int addIfUnderMaximumPoolSize(Runnable firstTask) {
468 final ReentrantLock mainLock = this.mainLock;
471 if (poolSize < maximumPoolSize) {
472 Runnable next = workQueue.poll();
491 * Gets the next task for a worker thread to run.
499 // untimed wait if core and not allowing core timeout
500 if (poolSize <= corePoolSize && !allowCoreThreadTimeOut)
501 return workQueue.take();
503 long timeout = keepAliveTime;
504 if (timeout <= 0) // die immediately for 0 timeout
506 Runnable r = workQueue.poll(timeout, TimeUnit.NANOSECONDS);
509 if (poolSize > corePoolSize || allowCoreThreadTimeOut)
510 return null; // timed out
511 // Else, after timeout, the pool shrank. Retry
517 Runnable r = workQueue.poll();
521 // Check if can terminate
522 if (workQueue.isEmpty()) {
523 interruptIdleWorkers();
527 // Else there could still be delayed tasks in queue.
528 return workQueue.take();
536 } catch (InterruptedException ie) {
537 // On interruption, re-check runstate
543 * Wakes up all threads that might be waiting for tasks.
545 void interruptIdleWorkers() {
546 final ReentrantLock mainLock = this.mainLock;
549 for (Worker w : workers)
557 * Performs bookkeeping for a terminated worker thread.
558 * @param w the worker
560 void workerDone(Worker w) {
561 final ReentrantLock mainLock = this.mainLock;
564 completedTaskCount += w.completedTasks;
569 // Else, this is the last thread. Deal with potential shutdown.
571 int state = runState;
572 assert state != TERMINATED;
575 // If there are queued tasks but no threads, create
576 // replacement thread. We must create it initially
577 // idle to avoid orphaned tasks in case addThread
578 // fails. This also handles case of delayed tasks
579 // that will sometime later become runnable.
580 if (!workQueue.isEmpty()) {
581 Thread t = addThread(null);
587 // Otherwise, we can exit without replacement
588 if (state == RUNNING)
592 // Either state is STOP, or state is SHUTDOWN and there is
593 // no work to do. So we can terminate.
594 termination.signalAll();
595 runState = TERMINATED;
596 // fall through to call terminate() outside of lock.
601 assert runState == TERMINATED;
608 private class Worker implements Runnable {
611 * The runLock is acquired and released surrounding each task
612 * execution. It mainly protects against interrupts that are
613 * intended to cancel the worker thread from instead
614 * interrupting the task being run.
616 private final ReentrantLock runLock = new ReentrantLock();
619 * Initial task to run before entering run loop
621 private Runnable firstTask;
624 * Per thread completed task counter; accumulated
625 * into completedTaskCount upon termination.
627 volatile long completedTasks;
630 * Thread this worker is running in. Acts as a final field,
631 * but cannot be set until thread is created.
635 Worker(Runnable firstTask) {
636 this.firstTask = firstTask;
640 return runLock.isLocked();
644 * Interrupts thread if not running a task.
646 void interruptIfIdle() {
647 final ReentrantLock runLock = this.runLock;
648 if (runLock.tryLock()) {
658 * Interrupts thread even if running a task.
660 void interruptNow() {
665 * Runs a single task between before/after methods.
667 private void runTask(Runnable task) {
668 final ReentrantLock runLock = this.runLock;
671 // If not shutting down then clear an outstanding interrupt.
672 if (runState != STOP &&
673 Thread.interrupted() &&
674 runState == STOP) // Re-interrupt if stopped after clearing
677 beforeExecute(thread, task);
681 afterExecute(task, null);
683 } catch (RuntimeException ex) {
685 afterExecute(task, ex);
686 // Else the exception occurred within
687 // afterExecute itself in which case we don't
688 // want to call it again.
701 Runnable task = firstTask;
703 while (task != null || (task = getTask()) != null) {
705 task = null; // unnecessary but can help GC
716 * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
717 * parameters and default thread factory and rejected execution handler.
718 * It may be more convenient to use one of the {@link Executors} factory
719 * methods instead of this general purpose constructor.
721 * @param corePoolSize the number of threads to keep in the
722 * pool, even if they are idle.
723 * @param maximumPoolSize the maximum number of threads to allow in the
725 * @param keepAliveTime when the number of threads is greater than
726 * the core, this is the maximum time that excess idle threads
727 * will wait for new tasks before terminating.
728 * @param unit the time unit for the keepAliveTime
730 * @param workQueue the queue to use for holding tasks before they
731 * are executed. This queue will hold only the <tt>Runnable</tt>
732 * tasks submitted by the <tt>execute</tt> method.
733 * @throws IllegalArgumentException if corePoolSize, or
734 * keepAliveTime less than zero, or if maximumPoolSize less than or
735 * equal to zero, or if corePoolSize greater than maximumPoolSize.
736 * @throws NullPointerException if <tt>workQueue</tt> is null
738 public ThreadPoolExecutor(int corePoolSize,
742 BlockingQueue<Runnable> workQueue) {
743 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
744 Executors.defaultThreadFactory(), defaultHandler);
748 * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
749 * parameters and default rejected execution handler.
751 * @param corePoolSize the number of threads to keep in the
752 * pool, even if they are idle.
753 * @param maximumPoolSize the maximum number of threads to allow in the
755 * @param keepAliveTime when the number of threads is greater than
756 * the core, this is the maximum time that excess idle threads
757 * will wait for new tasks before terminating.
758 * @param unit the time unit for the keepAliveTime
760 * @param workQueue the queue to use for holding tasks before they
761 * are executed. This queue will hold only the <tt>Runnable</tt>
762 * tasks submitted by the <tt>execute</tt> method.
763 * @param threadFactory the factory to use when the executor
764 * creates a new thread.
765 * @throws IllegalArgumentException if corePoolSize, or
766 * keepAliveTime less than zero, or if maximumPoolSize less than or
767 * equal to zero, or if corePoolSize greater than maximumPoolSize.
768 * @throws NullPointerException if <tt>workQueue</tt>
769 * or <tt>threadFactory</tt> are null.
771 public ThreadPoolExecutor(int corePoolSize,
775 BlockingQueue<Runnable> workQueue,
776 ThreadFactory threadFactory) {
777 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
778 threadFactory, defaultHandler);
782 * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
783 * parameters and default thread factory.
785 * @param corePoolSize the number of threads to keep in the
786 * pool, even if they are idle.
787 * @param maximumPoolSize the maximum number of threads to allow in the
789 * @param keepAliveTime when the number of threads is greater than
790 * the core, this is the maximum time that excess idle threads
791 * will wait for new tasks before terminating.
792 * @param unit the time unit for the keepAliveTime
794 * @param workQueue the queue to use for holding tasks before they
795 * are executed. This queue will hold only the <tt>Runnable</tt>
796 * tasks submitted by the <tt>execute</tt> method.
797 * @param handler the handler to use when execution is blocked
798 * because the thread bounds and queue capacities are reached.
799 * @throws IllegalArgumentException if corePoolSize, or
800 * keepAliveTime less than zero, or if maximumPoolSize less than or
801 * equal to zero, or if corePoolSize greater than maximumPoolSize.
802 * @throws NullPointerException if <tt>workQueue</tt>
803 * or <tt>handler</tt> are null.
805 public ThreadPoolExecutor(int corePoolSize,
809 BlockingQueue<Runnable> workQueue,
810 RejectedExecutionHandler handler) {
811 this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
812 Executors.defaultThreadFactory(), handler);
816 * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
819 * @param corePoolSize the number of threads to keep in the
820 * pool, even if they are idle.
821 * @param maximumPoolSize the maximum number of threads to allow in the
823 * @param keepAliveTime when the number of threads is greater than
824 * the core, this is the maximum time that excess idle threads
825 * will wait for new tasks before terminating.
826 * @param unit the time unit for the keepAliveTime
828 * @param workQueue the queue to use for holding tasks before they
829 * are executed. This queue will hold only the <tt>Runnable</tt>
830 * tasks submitted by the <tt>execute</tt> method.
831 * @param threadFactory the factory to use when the executor
832 * creates a new thread.
833 * @param handler the handler to use when execution is blocked
834 * because the thread bounds and queue capacities are reached.
835 * @throws IllegalArgumentException if corePoolSize, or
836 * keepAliveTime less than zero, or if maximumPoolSize less than or
837 * equal to zero, or if corePoolSize greater than maximumPoolSize.
838 * @throws NullPointerException if <tt>workQueue</tt>
839 * or <tt>threadFactory</tt> or <tt>handler</tt> are null.
841 public ThreadPoolExecutor(int corePoolSize,
845 BlockingQueue<Runnable> workQueue,
846 ThreadFactory threadFactory,
847 RejectedExecutionHandler handler) {
848 if (corePoolSize < 0 ||
849 maximumPoolSize <= 0 ||
850 maximumPoolSize < corePoolSize ||
852 throw new IllegalArgumentException();
853 if (workQueue == null || threadFactory == null || handler == null)
854 throw new NullPointerException();
855 this.corePoolSize = corePoolSize;
856 this.maximumPoolSize = maximumPoolSize;
857 this.workQueue = workQueue;
858 this.keepAliveTime = unit.toNanos(keepAliveTime);
859 this.threadFactory = threadFactory;
860 this.handler = handler;
865 * Executes the given task sometime in the future. The task
866 * may execute in a new thread or in an existing pooled thread.
868 * If the task cannot be submitted for execution, either because this
869 * executor has been shutdown or because its capacity has been reached,
870 * the task is handled by the current <tt>RejectedExecutionHandler</tt>.
872 * @param command the task to execute
873 * @throws RejectedExecutionException at discretion of
874 * <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
876 * @throws NullPointerException if command is null
878 public void execute(Runnable command) {
880 throw new NullPointerException();
882 if (runState != RUNNING) {
886 if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
888 if (workQueue.offer(command))
890 int status = addIfUnderMaximumPoolSize(command);
891 if (status > 0) // created new thread
893 if (status == 0) { // failed to create thread
897 // Retry if created a new thread but it is busy with another task
902 * Initiates an orderly shutdown in which previously submitted
903 * tasks are executed, but no new tasks will be
904 * accepted. Invocation has no additional effect if already shut
906 * @throws SecurityException if a security manager exists and
907 * shutting down this ExecutorService may manipulate threads that
908 * the caller is not permitted to modify because it does not hold
909 * {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
910 * or the security manager's <tt>checkAccess</tt> method denies access.
912 public void shutdown() {
913 // Fail if caller doesn't have modifyThread permission.
914 SecurityManager security = System.getSecurityManager();
915 if (security != null)
916 security.checkPermission(shutdownPerm);
918 boolean fullyTerminated = false;
919 final ReentrantLock mainLock = this.mainLock;
922 if (workers.size() > 0) {
923 // Check if caller can modify worker threads. This
924 // might not be true even if passed above check, if
925 // the SecurityManager treats some threads specially.
926 if (security != null) {
927 for (Worker w: workers)
928 security.checkAccess(w.thread);
931 int state = runState;
932 if (state == RUNNING) // don't override shutdownNow
936 for (Worker w: workers)
938 } catch (SecurityException se) {
939 // If SecurityManager allows above checks, but
940 // then unexpectedly throws exception when
941 // interrupting threads (which it ought not do),
942 // back out as cleanly as we can. Some threads may
943 // have been killed but we remain in non-shutdown
949 else { // If no workers, trigger full termination now
950 fullyTerminated = true;
951 runState = TERMINATED;
952 termination.signalAll();
963 * Attempts to stop all actively executing tasks, halts the
964 * processing of waiting tasks, and returns a list of the tasks
965 * that were awaiting execution.
967 * <p>There are no guarantees beyond best-effort attempts to stop
968 * processing actively executing tasks. This implementation
969 * cancels tasks via {@link Thread#interrupt}, so any task that
970 * fails to respond to interrupts may never terminate.
972 * @return list of tasks that never commenced execution
973 * @throws SecurityException if a security manager exists and
974 * shutting down this ExecutorService may manipulate threads that
975 * the caller is not permitted to modify because it does not hold
976 * {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
977 * or the security manager's <tt>checkAccess</tt> method denies access.
979 public List<Runnable> shutdownNow() {
980 // Almost the same code as shutdown()
981 SecurityManager security = System.getSecurityManager();
982 if (security != null)
983 security.checkPermission(shutdownPerm);
985 boolean fullyTerminated = false;
986 final ReentrantLock mainLock = this.mainLock;
989 if (workers.size() > 0) {
990 if (security != null) {
991 for (Worker w: workers)
992 security.checkAccess(w.thread);
995 int state = runState;
996 if (state != TERMINATED)
999 for (Worker w : workers)
1001 } catch (SecurityException se) {
1002 runState = state; // back out;
1006 else { // If no workers, trigger full termination now
1007 fullyTerminated = true;
1008 runState = TERMINATED;
1009 termination.signalAll();
1014 if (fullyTerminated)
1016 return Arrays.asList(workQueue.toArray(EMPTY_RUNNABLE_ARRAY));
1019 public boolean isShutdown() {
1020 return runState != RUNNING;
1024 * Returns true if this executor is in the process of terminating
1025 * after <tt>shutdown</tt> or <tt>shutdownNow</tt> but has not
1026 * completely terminated. This method may be useful for
1027 * debugging. A return of <tt>true</tt> reported a sufficient
1028 * period after shutdown may indicate that submitted tasks have
1029 * ignored or suppressed interruption, causing this executor not
1030 * to properly terminate.
1031 * @return true if terminating but not yet terminated.
1033 public boolean isTerminating() {
1034 return runState == STOP;
1037 public boolean isTerminated() {
1038 return runState == TERMINATED;
1041 public boolean awaitTermination(long timeout, TimeUnit unit)
1042 throws InterruptedException {
1043 long nanos = unit.toNanos(timeout);
1044 final ReentrantLock mainLock = this.mainLock;
1048 if (runState == TERMINATED)
1052 nanos = termination.awaitNanos(nanos);
1060 * Invokes <tt>shutdown</tt> when this executor is no longer
1063 protected void finalize() {
1068 * Sets the thread factory used to create new threads.
1070 * @param threadFactory the new thread factory
1071 * @throws NullPointerException if threadFactory is null
1072 * @see #getThreadFactory
1074 public void setThreadFactory(ThreadFactory threadFactory) {
1075 if (threadFactory == null)
1076 throw new NullPointerException();
1077 this.threadFactory = threadFactory;
1081 * Returns the thread factory used to create new threads.
1083 * @return the current thread factory
1084 * @see #setThreadFactory
1086 public ThreadFactory getThreadFactory() {
1087 return threadFactory;
1091 * Sets a new handler for unexecutable tasks.
1093 * @param handler the new handler
1094 * @throws NullPointerException if handler is null
1095 * @see #getRejectedExecutionHandler
1097 public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
1098 if (handler == null)
1099 throw new NullPointerException();
1100 this.handler = handler;
1104 * Returns the current handler for unexecutable tasks.
1106 * @return the current handler
1107 * @see #setRejectedExecutionHandler
1109 public RejectedExecutionHandler getRejectedExecutionHandler() {
1114 * Returns the task queue used by this executor. Access to the
1115 * task queue is intended primarily for debugging and monitoring.
1116 * This queue may be in active use. Retrieving the task queue
1117 * does not prevent queued tasks from executing.
1119 * @return the task queue
1121 public BlockingQueue<Runnable> getQueue() {
1126 * Removes this task from the executor's internal queue if it is
1127 * present, thus causing it not to be run if it has not already
1130 * <p> This method may be useful as one part of a cancellation
1131 * scheme. It may fail to remove tasks that have been converted
1132 * into other forms before being placed on the internal queue. For
1133 * example, a task entered using <tt>submit</tt> might be
1134 * converted into a form that maintains <tt>Future</tt> status.
1135 * However, in such cases, method {@link ThreadPoolExecutor#purge}
1136 * may be used to remove those Futures that have been cancelled.
1138 * @param task the task to remove
1139 * @return true if the task was removed
1141 public boolean remove(Runnable task) {
1142 return getQueue().remove(task);
1147 * Tries to remove from the work queue all {@link Future}
1148 * tasks that have been cancelled. This method can be useful as a
1149 * storage reclamation operation, that has no other impact on
1150 * functionality. Cancelled tasks are never executed, but may
1151 * accumulate in work queues until worker threads can actively
1152 * remove them. Invoking this method instead tries to remove them now.
1153 * However, this method may fail to remove tasks in
1154 * the presence of interference by other threads.
1156 public void purge() {
1157 // Fail if we encounter interference during traversal
1159 Iterator<Runnable> it = getQueue().iterator();
1160 while (it.hasNext()) {
1161 Runnable r = it.next();
1162 if (r instanceof Future<?>) {
1163 Future<?> c = (Future<?>)r;
1164 if (c.isCancelled())
1169 catch (ConcurrentModificationException ex) {
1175 * Sets the core number of threads. This overrides any value set
1176 * in the constructor. If the new value is smaller than the
1177 * current value, excess existing threads will be terminated when
1178 * they next become idle. If larger, new threads will, if needed,
1179 * be started to execute any queued tasks.
1181 * @param corePoolSize the new core size
1182 * @throws IllegalArgumentException if <tt>corePoolSize</tt>
1184 * @see #getCorePoolSize
1186 public void setCorePoolSize(int corePoolSize) {
1187 if (corePoolSize < 0)
1188 throw new IllegalArgumentException();
1189 final ReentrantLock mainLock = this.mainLock;
1192 int extra = this.corePoolSize - corePoolSize;
1193 this.corePoolSize = corePoolSize;
1195 int n = workQueue.size();
1196 // We have to create initially-idle threads here
1197 // because we otherwise have no recourse about
1198 // what to do with a dequeued task if addThread fails.
1199 while (extra++ < 0 && n-- > 0 && poolSize < corePoolSize ) {
1200 Thread t = addThread(null);
1207 else if (extra > 0 && poolSize > corePoolSize) {
1208 Iterator<Worker> it = workers.iterator();
1209 while (it.hasNext() &&
1211 poolSize > corePoolSize &&
1212 workQueue.remainingCapacity() == 0)
1213 it.next().interruptIfIdle();
1221 * Returns the core number of threads.
1223 * @return the core number of threads
1224 * @see #setCorePoolSize
1226 public int getCorePoolSize() {
1227 return corePoolSize;
1231 * Starts a core thread, causing it to idly wait for work. This
1232 * overrides the default policy of starting core threads only when
1233 * new tasks are executed. This method will return <tt>false</tt>
1234 * if all core threads have already been started.
1235 * @return true if a thread was started
1237 public boolean prestartCoreThread() {
1238 return addIfUnderCorePoolSize(null);
1242 * Starts all core threads, causing them to idly wait for work. This
1243 * overrides the default policy of starting core threads only when
1244 * new tasks are executed.
1245 * @return the number of threads started.
1247 public int prestartAllCoreThreads() {
1249 while (addIfUnderCorePoolSize(null))
1255 * Returns true if this pool allows core threads to time out and
1256 * terminate if no tasks arrive within the keepAlive time, being
1257 * replaced if needed when new tasks arrive. When true, the same
1258 * keep-alive policy applying to non-core threads applies also to
1259 * core threads. When false (the default), core threads are never
1260 * terminated due to lack of incoming tasks.
1261 * @return <tt>true</tt> if core threads are allowed to time out,
1262 * else <tt>false</tt>
1266 public boolean allowsCoreThreadTimeOut() {
1267 return allowCoreThreadTimeOut;
1271 * Sets the policy governing whether core threads may time out and
1272 * terminate if no tasks arrive within the keep-alive time, being
1273 * replaced if needed when new tasks arrive. When false, core
1274 * threads are never terminated due to lack of incoming
1275 * tasks. When true, the same keep-alive policy applying to
1276 * non-core threads applies also to core threads. To avoid
1277 * continual thread replacement, the keep-alive time must be
1278 * greater than zero when setting <tt>true</tt>. This method
1279 * should in general be called before the pool is actively used.
1280 * @param value <tt>true</tt> if should time out, else <tt>false</tt>
1281 * @throws IllegalArgumentException if value is <tt>true</tt>
1282 * and the current keep-alive time is not greater than zero.
1286 public void allowCoreThreadTimeOut(boolean value) {
1287 if (value && keepAliveTime <= 0)
1288 throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
1290 allowCoreThreadTimeOut = value;
1294 * Sets the maximum allowed number of threads. This overrides any
1295 * value set in the constructor. If the new value is smaller than
1296 * the current value, excess existing threads will be
1297 * terminated when they next become idle.
1299 * @param maximumPoolSize the new maximum
1300 * @throws IllegalArgumentException if the new maximum is
1301 * less than or equal to zero, or
1302 * less than the {@linkplain #getCorePoolSize core pool size}
1303 * @see #getMaximumPoolSize
1305 public void setMaximumPoolSize(int maximumPoolSize) {
1306 if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
1307 throw new IllegalArgumentException();
1308 final ReentrantLock mainLock = this.mainLock;
1311 int extra = this.maximumPoolSize - maximumPoolSize;
1312 this.maximumPoolSize = maximumPoolSize;
1313 if (extra > 0 && poolSize > maximumPoolSize) {
1314 Iterator<Worker> it = workers.iterator();
1315 while (it.hasNext() &&
1317 poolSize > maximumPoolSize) {
1318 it.next().interruptIfIdle();
1328 * Returns the maximum allowed number of threads.
1330 * @return the maximum allowed number of threads
1331 * @see #setMaximumPoolSize
1333 public int getMaximumPoolSize() {
1334 return maximumPoolSize;
1338 * Sets the time limit for which threads may remain idle before
1339 * being terminated. If there are more than the core number of
1340 * threads currently in the pool, after waiting this amount of
1341 * time without processing a task, excess threads will be
1342 * terminated. This overrides any value set in the constructor.
1343 * @param time the time to wait. A time value of zero will cause
1344 * excess threads to terminate immediately after executing tasks.
1345 * @param unit the time unit of the time argument
1346 * @throws IllegalArgumentException if time less than zero or
1347 * if time is zero and allowsCoreThreadTimeOut
1348 * @see #getKeepAliveTime
1350 public void setKeepAliveTime(long time, TimeUnit unit) {
1352 throw new IllegalArgumentException();
1353 if (time == 0 && allowsCoreThreadTimeOut())
1354 throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
1355 this.keepAliveTime = unit.toNanos(time);
1359 * Returns the thread keep-alive time, which is the amount of time
1360 * which threads in excess of the core pool size may remain
1361 * idle before being terminated.
1363 * @param unit the desired time unit of the result
1364 * @return the time limit
1365 * @see #setKeepAliveTime
1367 public long getKeepAliveTime(TimeUnit unit) {
1368 return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
1374 * Returns the current number of threads in the pool.
1376 * @return the number of threads
1378 public int getPoolSize() {
1383 * Returns the approximate number of threads that are actively
1386 * @return the number of threads
1388 public int getActiveCount() {
1389 final ReentrantLock mainLock = this.mainLock;
1393 for (Worker w : workers) {
1404 * Returns the largest number of threads that have ever
1405 * simultaneously been in the pool.
1407 * @return the number of threads
1409 public int getLargestPoolSize() {
1410 final ReentrantLock mainLock = this.mainLock;
1413 return largestPoolSize;
1420 * Returns the approximate total number of tasks that have been
1421 * scheduled for execution. Because the states of tasks and
1422 * threads may change dynamically during computation, the returned
1423 * value is only an approximation, but one that does not ever
1424 * decrease across successive calls.
1426 * @return the number of tasks
1428 public long getTaskCount() {
1429 final ReentrantLock mainLock = this.mainLock;
1432 long n = completedTaskCount;
1433 for (Worker w : workers) {
1434 n += w.completedTasks;
1438 return n + workQueue.size();
1445 * Returns the approximate total number of tasks that have
1446 * completed execution. Because the states of tasks and threads
1447 * may change dynamically during computation, the returned value
1448 * is only an approximation, but one that does not ever decrease
1449 * across successive calls.
1451 * @return the number of tasks
1453 public long getCompletedTaskCount() {
1454 final ReentrantLock mainLock = this.mainLock;
1457 long n = completedTaskCount;
1458 for (Worker w : workers)
1459 n += w.completedTasks;
1467 * Method invoked prior to executing the given Runnable in the
1468 * given thread. This method is invoked by thread <tt>t</tt> that
1469 * will execute task <tt>r</tt>, and may be used to re-initialize
1470 * ThreadLocals, or to perform logging.
1472 * <p>This implementation does nothing, but may be customized in
1473 * subclasses. Note: To properly nest multiple overridings, subclasses
1474 * should generally invoke <tt>super.beforeExecute</tt> at the end of
1477 * @param t the thread that will run task r.
1478 * @param r the task that will be executed.
1480 protected void beforeExecute(Thread t, Runnable r) { }
1483 * Method invoked upon completion of execution of the given Runnable.
1484 * This method is invoked by the thread that executed the task. If
1485 * non-null, the Throwable is the uncaught <tt>RuntimeException</tt>
1486 * or <tt>Error</tt> that caused execution to terminate abruptly.
1488 * <p><b>Note:</b> When actions are enclosed in tasks (such as
1489 * {@link FutureTask}) either explicitly or via methods such as
1490 * <tt>submit</tt>, these task objects catch and maintain
1491 * computational exceptions, and so they do not cause abrupt
1492 * termination, and the internal exceptions are <em>not</em>
1493 * passed to this method.
1495 * <p>This implementation does nothing, but may be customized in
1496 * subclasses. Note: To properly nest multiple overridings, subclasses
1497 * should generally invoke <tt>super.afterExecute</tt> at the
1498 * beginning of this method.
1500 * @param r the runnable that has completed.
1501 * @param t the exception that caused termination, or null if
1502 * execution completed normally.
1504 protected void afterExecute(Runnable r, Throwable t) { }
1507 * Method invoked when the Executor has terminated. Default
1508 * implementation does nothing. Note: To properly nest multiple
1509 * overridings, subclasses should generally invoke
1510 * <tt>super.terminated</tt> within this method.
1512 protected void terminated() { }
1515 * A handler for rejected tasks that runs the rejected task
1516 * directly in the calling thread of the <tt>execute</tt> method,
1517 * unless the executor has been shut down, in which case the task
1520 public static class CallerRunsPolicy implements RejectedExecutionHandler {
1522 * Creates a <tt>CallerRunsPolicy</tt>.
1524 public CallerRunsPolicy() { }
1527 * Executes task r in the caller's thread, unless the executor
1528 * has been shut down, in which case the task is discarded.
1529 * @param r the runnable task requested to be executed
1530 * @param e the executor attempting to execute this task
1532 public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
1533 if (!e.isShutdown()) {
1540 * A handler for rejected tasks that throws a
1541 * <tt>RejectedExecutionException</tt>.
1543 public static class AbortPolicy implements RejectedExecutionHandler {
1545 * Creates an <tt>AbortPolicy</tt>.
1547 public AbortPolicy() { }
1550 * Always throws RejectedExecutionException.
1551 * @param r the runnable task requested to be executed
1552 * @param e the executor attempting to execute this task
1553 * @throws RejectedExecutionException always.
1555 public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
1556 throw new RejectedExecutionException();
1561 * A handler for rejected tasks that silently discards the
1564 public static class DiscardPolicy implements RejectedExecutionHandler {
1566 * Creates a <tt>DiscardPolicy</tt>.
1568 public DiscardPolicy() { }
1571 * Does nothing, which has the effect of discarding task r.
1572 * @param r the runnable task requested to be executed
1573 * @param e the executor attempting to execute this task
1575 public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
1580 * A handler for rejected tasks that discards the oldest unhandled
1581 * request and then retries <tt>execute</tt>, unless the executor
1582 * is shut down, in which case the task is discarded.
1584 public static class DiscardOldestPolicy implements RejectedExecutionHandler {
1586 * Creates a <tt>DiscardOldestPolicy</tt> for the given executor.
1588 public DiscardOldestPolicy() { }
1591 * Obtains and ignores the next task that the executor
1592 * would otherwise execute, if one is immediately available,
1593 * and then retries execution of task r, unless the executor
1594 * is shut down, in which case task r is instead discarded.
1595 * @param r the runnable task requested to be executed
1596 * @param e the executor attempting to execute this task
1598 public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
1599 if (!e.isShutdown()) {
1600 e.getQueue().poll();