|
|
Java example source code file (ThreadPoolExecutor.java)
This example Java source code file (ThreadPoolExecutor.java) is included in the alvinalexander.com
"Java Source Code
Warehouse" project. The intent of this project is to help you "Learn
Java by Example" TM.
Learn more about this Java project at its project page.
The ThreadPoolExecutor.java Java example source code
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.*;
/**
* An {@link ExecutorService} that executes each submitted task using
* one of possibly several pooled threads, normally configured
* using {@link Executors} factory methods.
*
* <p>Thread pools address two different problems: they usually
* provide improved performance when executing large numbers of
* asynchronous tasks, due to reduced per-task invocation overhead,
* and they provide a means of bounding and managing the resources,
* including threads, consumed when executing a collection of tasks.
* Each {@code ThreadPoolExecutor} also maintains some basic
* statistics, such as the number of completed tasks.
*
* <p>To be useful across a wide range of contexts, this class
* provides many adjustable parameters and extensibility
* hooks. However, programmers are urged to use the more convenient
* {@link Executors} factory methods {@link
* Executors#newCachedThreadPool} (unbounded thread pool, with
* automatic thread reclamation), {@link Executors#newFixedThreadPool}
* (fixed size thread pool) and {@link
* Executors#newSingleThreadExecutor} (single background thread), that
* preconfigure settings for the most common usage
* scenarios. Otherwise, use the following guide when manually
* configuring and tuning this class:
*
* <dl>
*
* <dt>Core and maximum pool sizes
*
* <dd>A {@code ThreadPoolExecutor} will automatically adjust the
* pool size (see {@link #getPoolSize})
* according to the bounds set by
* corePoolSize (see {@link #getCorePoolSize}) and
* maximumPoolSize (see {@link #getMaximumPoolSize}).
*
* When a new task is submitted in method {@link #execute(Runnable)},
* and fewer than corePoolSize threads are running, a new thread is
* created to handle the request, even if other worker threads are
* idle. If there are more than corePoolSize but less than
* maximumPoolSize threads running, a new thread will be created only
* if the queue is full. By setting corePoolSize and maximumPoolSize
* the same, you create a fixed-size thread pool. By setting
* maximumPoolSize to an essentially unbounded value such as {@code
* Integer.MAX_VALUE}, you allow the pool to accommodate an arbitrary
* number of concurrent tasks. Most typically, core and maximum pool
* sizes are set only upon construction, but they may also be changed
* dynamically using {@link #setCorePoolSize} and {@link
* #setMaximumPoolSize}. </dd>
*
* <dt>On-demand construction
*
* <dd>By default, even core threads are initially created and
* started only when new tasks arrive, but this can be overridden
* dynamically using method {@link #prestartCoreThread} or {@link
* #prestartAllCoreThreads}. You probably want to prestart threads if
* you construct the pool with a non-empty queue. </dd>
*
* <dt>Creating new threads
*
* <dd>New threads are created using a {@link ThreadFactory}. If not
* otherwise specified, a {@link Executors#defaultThreadFactory} is
* used, that creates threads to all be in the same {@link
* ThreadGroup} and with the same {@code NORM_PRIORITY} priority and
* non-daemon status. By supplying a different ThreadFactory, you can
* alter the thread's name, thread group, priority, daemon status,
* etc. If a {@code ThreadFactory} fails to create a thread when asked
* by returning null from {@code newThread}, the executor will
* continue, but might not be able to execute any tasks. Threads
* should possess the "modifyThread" {@code RuntimePermission}. If
* worker threads or other threads using the pool do not possess this
* permission, service may be degraded: configuration changes may not
* take effect in a timely manner, and a shutdown pool may remain in a
* state in which termination is possible but not completed.</dd>
*
* <dt>Keep-alive times
*
* <dd>If the pool currently has more than corePoolSize threads,
* excess threads will be terminated if they have been idle for more
* than the keepAliveTime (see {@link #getKeepAliveTime(TimeUnit)}).
* This provides a means of reducing resource consumption when the
* pool is not being actively used. If the pool becomes more active
* later, new threads will be constructed. This parameter can also be
* changed dynamically using method {@link #setKeepAliveTime(long,
* TimeUnit)}. Using a value of {@code Long.MAX_VALUE} {@link
* TimeUnit#NANOSECONDS} effectively disables idle threads from ever
* terminating prior to shut down. By default, the keep-alive policy
* applies only when there are more than corePoolSize threads. But
* method {@link #allowCoreThreadTimeOut(boolean)} can be used to
* apply this time-out policy to core threads as well, so long as the
* keepAliveTime value is non-zero. </dd>
*
* <dt>Queuing
*
* <dd>Any {@link BlockingQueue} may be used to transfer and hold
* submitted tasks. The use of this queue interacts with pool sizing:
*
* <ul>
*
* <li> If fewer than corePoolSize threads are running, the Executor
* always prefers adding a new thread
* rather than queuing.</li>
*
* <li> If corePoolSize or more threads are running, the Executor
* always prefers queuing a request rather than adding a new
* thread.</li>
*
* <li> If a request cannot be queued, a new thread is created unless
* this would exceed maximumPoolSize, in which case, the task will be
* rejected.</li>
*
* </ul>
*
* There are three general strategies for queuing:
* <ol>
*
* <li> Direct handoffs. A good default choice for a work
* queue is a {@link SynchronousQueue} that hands off tasks to threads
* without otherwise holding them. Here, an attempt to queue a task
* will fail if no threads are immediately available to run it, so a
* new thread will be constructed. This policy avoids lockups when
* handling sets of requests that might have internal dependencies.
* Direct handoffs generally require unbounded maximumPoolSizes to
* avoid rejection of new submitted tasks. This in turn admits the
* possibility of unbounded thread growth when commands continue to
* arrive on average faster than they can be processed. </li>
*
* <li> Unbounded queues. Using an unbounded queue (for
* example a {@link LinkedBlockingQueue} without a predefined
* capacity) will cause new tasks to wait in the queue when all
* corePoolSize threads are busy. Thus, no more than corePoolSize
* threads will ever be created. (And the value of the maximumPoolSize
* therefore doesn't have any effect.) This may be appropriate when
* each task is completely independent of others, so tasks cannot
* affect each others execution; for example, in a web page server.
* While this style of queuing can be useful in smoothing out
* transient bursts of requests, it admits the possibility of
* unbounded work queue growth when commands continue to arrive on
* average faster than they can be processed. </li>
*
* <li>Bounded queues. A bounded queue (for example, an
* {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
* used with finite maximumPoolSizes, but can be more difficult to
* tune and control. Queue sizes and maximum pool sizes may be traded
* off for each other: Using large queues and small pools minimizes
* CPU usage, OS resources, and context-switching overhead, but can
* lead to artificially low throughput. If tasks frequently block (for
* example if they are I/O bound), a system may be able to schedule
* time for more threads than you otherwise allow. Use of small queues
* generally requires larger pool sizes, which keeps CPUs busier but
* may encounter unacceptable scheduling overhead, which also
* decreases throughput. </li>
*
* </ol>
*
* </dd>
*
* <dt>Rejected tasks
*
* <dd>New tasks submitted in method {@link #execute(Runnable)} will be
* <em>rejected when the Executor has been shut down, and also when
* the Executor uses finite bounds for both maximum threads and work queue
* capacity, and is saturated. In either case, the {@code execute} method
* invokes the {@link
* RejectedExecutionHandler#rejectedExecution(Runnable, ThreadPoolExecutor)}
* method of its {@link RejectedExecutionHandler}. Four predefined handler
* policies are provided:
*
* <ol>
*
* <li> In the default {@link ThreadPoolExecutor.AbortPolicy}, the
* handler throws a runtime {@link RejectedExecutionException} upon
* rejection. </li>
*
* <li> In {@link ThreadPoolExecutor.CallerRunsPolicy}, the thread
* that invokes {@code execute} itself runs the task. This provides a
* simple feedback control mechanism that will slow down the rate that
* new tasks are submitted. </li>
*
* <li> In {@link ThreadPoolExecutor.DiscardPolicy}, a task that
* cannot be executed is simply dropped. </li>
*
* <li>In {@link ThreadPoolExecutor.DiscardOldestPolicy}, if the
* executor is not shut down, the task at the head of the work queue
* is dropped, and then execution is retried (which can fail again,
* causing this to be repeated.) </li>
*
* </ol>
*
* It is possible to define and use other kinds of {@link
* RejectedExecutionHandler} classes. Doing so requires some care
* especially when policies are designed to work only under particular
* capacity or queuing policies. </dd>
*
* <dt>Hook methods
*
* <dd>This class provides {@code protected} overridable
* {@link #beforeExecute(Thread, Runnable)} and
* {@link #afterExecute(Runnable, Throwable)} methods that are called
* before and after execution of each task. These can be used to
* manipulate the execution environment; for example, reinitializing
* ThreadLocals, gathering statistics, or adding log entries.
* Additionally, method {@link #terminated} can be overridden to perform
* any special processing that needs to be done once the Executor has
* fully terminated.
*
* <p>If hook or callback methods throw exceptions, internal worker
* threads may in turn fail and abruptly terminate.</dd>
*
* <dt>Queue maintenance
*
* <dd>Method {@link #getQueue()} allows access to the work queue
* for purposes of monitoring and debugging. Use of this method for
* any other purpose is strongly discouraged. Two supplied methods,
* {@link #remove(Runnable)} and {@link #purge} are available to
* assist in storage reclamation when large numbers of queued tasks
* become cancelled.</dd>
*
* <dt>Finalization
*
* <dd>A pool that is no longer referenced in a program AND
* has no remaining threads will be {@code shutdown} automatically. If
* you would like to ensure that unreferenced pools are reclaimed even
* if users forget to call {@link #shutdown}, then you must arrange
* that unused threads eventually die, by setting appropriate
* keep-alive times, using a lower bound of zero core threads and/or
* setting {@link #allowCoreThreadTimeOut(boolean)}. </dd>
*
* </dl>
*
* <p>Extension example. Most extensions of this class
* override one or more of the protected hook methods. For example,
* here is a subclass that adds a simple pause/resume feature:
*
* <pre> {@code
* class PausableThreadPoolExecutor extends ThreadPoolExecutor {
* private boolean isPaused;
* private ReentrantLock pauseLock = new ReentrantLock();
* private Condition unpaused = pauseLock.newCondition();
*
* public PausableThreadPoolExecutor(...) { super(...); }
*
* protected void beforeExecute(Thread t, Runnable r) {
* super.beforeExecute(t, r);
* pauseLock.lock();
* try {
* while (isPaused) unpaused.await();
* } catch (InterruptedException ie) {
* t.interrupt();
* } finally {
* pauseLock.unlock();
* }
* }
*
* public void pause() {
* pauseLock.lock();
* try {
* isPaused = true;
* } finally {
* pauseLock.unlock();
* }
* }
*
* public void resume() {
* pauseLock.lock();
* try {
* isPaused = false;
* unpaused.signalAll();
* } finally {
* pauseLock.unlock();
* }
* }
* }}</pre>
*
* @since 1.5
* @author Doug Lea
*/
public class ThreadPoolExecutor extends AbstractExecutorService {
/**
* The main pool control state, ctl, is an atomic integer packing
* two conceptual fields
* workerCount, indicating the effective number of threads
* runState, indicating whether running, shutting down etc
*
* In order to pack them into one int, we limit workerCount to
* (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
* billion) otherwise representable. If this is ever an issue in
* the future, the variable can be changed to be an AtomicLong,
* and the shift/mask constants below adjusted. But until the need
* arises, this code is a bit faster and simpler using an int.
*
* The workerCount is the number of workers that have been
* permitted to start and not permitted to stop. The value may be
* transiently different from the actual number of live threads,
* for example when a ThreadFactory fails to create a thread when
* asked, and when exiting threads are still performing
* bookkeeping before terminating. The user-visible pool size is
* reported as the current size of the workers set.
*
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*
* Threads waiting in awaitTermination() will return when the
* state reaches TERMINATED.
*
* Detecting the transition from SHUTDOWN to TIDYING is less
* straightforward than you'd like because the queue may become
* empty after non-empty and vice versa during SHUTDOWN state, but
* we can only terminate if, after seeing that it is empty, we see
* that workerCount is 0 (which sometimes entails a recheck -- see
* below).
*/
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
/*
* Bit field accessors that don't require unpacking ctl.
* These depend on the bit layout and on workerCount being never negative.
*/
private static boolean runStateLessThan(int c, int s) {
return c < s;
}
private static boolean runStateAtLeast(int c, int s) {
return c >= s;
}
private static boolean isRunning(int c) {
return c < SHUTDOWN;
}
/**
* Attempts to CAS-increment the workerCount field of ctl.
*/
private boolean compareAndIncrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect + 1);
}
/**
* Attempts to CAS-decrement the workerCount field of ctl.
*/
private boolean compareAndDecrementWorkerCount(int expect) {
return ctl.compareAndSet(expect, expect - 1);
}
/**
* Decrements the workerCount field of ctl. This is called only on
* abrupt termination of a thread (see processWorkerExit). Other
* decrements are performed within getTask.
*/
private void decrementWorkerCount() {
do {} while (! compareAndDecrementWorkerCount(ctl.get()));
}
/**
* The queue used for holding tasks and handing off to worker
* threads. We do not require that workQueue.poll() returning
* null necessarily means that workQueue.isEmpty(), so rely
* solely on isEmpty to see if the queue is empty (which we must
* do for example when deciding whether to transition from
* SHUTDOWN to TIDYING). This accommodates special-purpose
* queues such as DelayQueues for which poll() is allowed to
* return null even if it may later return non-null when delays
* expire.
*/
private final BlockingQueue<Runnable> workQueue;
/**
* Lock held on access to workers set and related bookkeeping.
* While we could use a concurrent set of some sort, it turns out
* to be generally preferable to use a lock. Among the reasons is
* that this serializes interruptIdleWorkers, which avoids
* unnecessary interrupt storms, especially during shutdown.
* Otherwise exiting threads would concurrently interrupt those
* that have not yet interrupted. It also simplifies some of the
* associated statistics bookkeeping of largestPoolSize etc. We
* also hold mainLock on shutdown and shutdownNow, for the sake of
* ensuring workers set is stable while separately checking
* permission to interrupt and actually interrupting.
*/
private final ReentrantLock mainLock = new ReentrantLock();
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet<Worker> workers = new HashSet();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* Tracks largest attained pool size. Accessed only under
* mainLock.
*/
private int largestPoolSize;
/**
* Counter for completed tasks. Updated only on termination of
* worker threads. Accessed only under mainLock.
*/
private long completedTaskCount;
/*
* All user control parameters are declared as volatiles so that
* ongoing actions are based on freshest values, but without need
* for locking, since no internal invariants depend on them
* changing synchronously with respect to other actions.
*/
/**
* Factory for new threads. All threads are created using this
* factory (via method addWorker). All callers must be prepared
* for addWorker to fail, which may reflect a system or user's
* policy limiting the number of threads. Even though it is not
* treated as an error, failure to create threads may result in
* new tasks being rejected or existing ones remaining stuck in
* the queue.
*
* We go further and preserve pool invariants even in the face of
* errors such as OutOfMemoryError, that might be thrown while
* trying to create threads. Such errors are rather common due to
* the need to allocate a native stack in Thread.start, and users
* will want to perform clean pool shutdown to clean up. There
* will likely be enough memory available for the cleanup code to
* complete without encountering yet another OutOfMemoryError.
*/
private volatile ThreadFactory threadFactory;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* Timeout in nanoseconds for idle threads waiting for work.
* Threads use this timeout when there are more than corePoolSize
* present or if allowCoreThreadTimeOut. Otherwise they wait
* forever for new work.
*/
private volatile long keepAliveTime;
/**
* If false (default), core threads stay alive even when idle.
* If true, core threads use keepAliveTime to time out waiting
* for work.
*/
private volatile boolean allowCoreThreadTimeOut;
/**
* Core pool size is the minimum number of workers to keep alive
* (and not allow to time out etc) unless allowCoreThreadTimeOut
* is set, in which case the minimum is zero.
*/
private volatile int corePoolSize;
/**
* Maximum pool size. Note that the actual maximum is internally
* bounded by CAPACITY.
*/
private volatile int maximumPoolSize;
/**
* The default rejected execution handler
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
/**
* Permission required for callers of shutdown and shutdownNow.
* We additionally require (see checkShutdownAccess) that callers
* have permission to actually interrupt threads in the worker set
* (as governed by Thread.interrupt, which relies on
* ThreadGroup.checkAccess, which in turn relies on
* SecurityManager.checkAccess). Shutdowns are attempted only if
* these checks pass.
*
* All actual invocations of Thread.interrupt (see
* interruptIdleWorkers and interruptWorkers) ignore
* SecurityExceptions, meaning that the attempted interrupts
* silently fail. In the case of shutdown, they should not fail
* unless the SecurityManager has inconsistent policies, sometimes
* allowing access to a thread and sometimes not. In such cases,
* failure to actually interrupt threads may disable or delay full
* termination. Other uses of interruptIdleWorkers are advisory,
* and failure to actually interrupt will merely delay response to
* configuration changes so is not handled exceptionally.
*/
private static final RuntimePermission shutdownPerm =
new RuntimePermission("modifyThread");
/**
* Class Worker mainly maintains interrupt control state for
* threads running tasks, along with other minor bookkeeping.
* This class opportunistically extends AbstractQueuedSynchronizer
* to simplify acquiring and releasing a lock surrounding each
* task execution. This protects against interrupts that are
* intended to wake up a worker thread waiting for a task from
* instead interrupting a task being run. We implement a simple
* non-reentrant mutual exclusion lock rather than use
* ReentrantLock because we do not want worker tasks to be able to
* reacquire the lock when they invoke pool control methods like
* setCorePoolSize. Additionally, to suppress interrupts until
* the thread actually starts running tasks, we initialize lock
* state to a negative value, and clear it upon start (in
* runWorker).
*/
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
// Lock methods
//
// The value 0 represents the unlocked state.
// The value 1 represents the locked state.
protected boolean isHeldExclusively() {
return getState() != 0;
}
protected boolean tryAcquire(int unused) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
protected boolean tryRelease(int unused) {
setExclusiveOwnerThread(null);
setState(0);
return true;
}
public void lock() { acquire(1); }
public boolean tryLock() { return tryAcquire(1); }
public void unlock() { release(1); }
public boolean isLocked() { return isHeldExclusively(); }
void interruptIfStarted() {
Thread t;
if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
/*
* Methods for setting control state
*/
/**
* Transitions runState to given target, or leaves it alone if
* already at least the given target.
*
* @param targetState the desired state, either SHUTDOWN or STOP
* (but not TIDYING or TERMINATED -- use tryTerminate for that)
*/
private void advanceRunState(int targetState) {
for (;;) {
int c = ctl.get();
if (runStateAtLeast(c, targetState) ||
ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
break;
}
}
/**
* Transitions to TERMINATED state if either (SHUTDOWN and pool
* and queue empty) or (STOP and pool empty). If otherwise
* eligible to terminate but workerCount is nonzero, interrupts an
* idle worker to ensure that shutdown signals propagate. This
* method must be called following any action that might make
* termination possible -- reducing worker count or removing tasks
* from the queue during shutdown. The method is non-private to
* allow access from ScheduledThreadPoolExecutor.
*/
final void tryTerminate() {
for (;;) {
int c = ctl.get();
if (isRunning(c) ||
runStateAtLeast(c, TIDYING) ||
(runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
return;
if (workerCountOf(c) != 0) { // Eligible to terminate
interruptIdleWorkers(ONLY_ONE);
return;
}
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
try {
terminated();
} finally {
ctl.set(ctlOf(TERMINATED, 0));
termination.signalAll();
}
return;
}
} finally {
mainLock.unlock();
}
// else retry on failed CAS
}
}
/*
* Methods for controlling interrupts to worker threads.
*/
/**
* If there is a security manager, makes sure caller has
* permission to shut down threads in general (see shutdownPerm).
* If this passes, additionally makes sure the caller is allowed
* to interrupt each worker thread. This might not be true even if
* first check passed, if the SecurityManager treats some threads
* specially.
*/
private void checkShutdownAccess() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkPermission(shutdownPerm);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
security.checkAccess(w.thread);
} finally {
mainLock.unlock();
}
}
}
/**
* Interrupts all threads, even if active. Ignores SecurityExceptions
* (in which case some threads may remain uninterrupted).
*/
private void interruptWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfStarted();
} finally {
mainLock.unlock();
}
}
/**
* Interrupts threads that might be waiting for tasks (as
* indicated by not being locked) so they can check for
* termination or configuration changes. Ignores
* SecurityExceptions (in which case some threads may remain
* uninterrupted).
*
* @param onlyOne If true, interrupt at most one worker. This is
* called only from tryTerminate when termination is otherwise
* enabled but there are still other workers. In this case, at
* most one waiting worker is interrupted to propagate shutdown
* signals in case all threads are currently waiting.
* Interrupting any arbitrary thread ensures that newly arriving
* workers since shutdown began will also eventually exit.
* To guarantee eventual termination, it suffices to always
* interrupt only one idle worker, but shutdown() interrupts all
* idle workers so that redundant workers exit promptly, not
* waiting for a straggler task to finish.
*/
private void interruptIdleWorkers(boolean onlyOne) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers) {
Thread t = w.thread;
if (!t.isInterrupted() && w.tryLock()) {
try {
t.interrupt();
} catch (SecurityException ignore) {
} finally {
w.unlock();
}
}
if (onlyOne)
break;
}
} finally {
mainLock.unlock();
}
}
/**
* Common form of interruptIdleWorkers, to avoid having to
* remember what the boolean argument means.
*/
private void interruptIdleWorkers() {
interruptIdleWorkers(false);
}
private static final boolean ONLY_ONE = true;
/*
* Misc utilities, most of which are also exported to
* ScheduledThreadPoolExecutor
*/
/**
* Invokes the rejected execution handler for the given command.
* Package-protected for use by ScheduledThreadPoolExecutor.
*/
final void reject(Runnable command) {
handler.rejectedExecution(command, this);
}
/**
* Performs any further cleanup following run state transition on
* invocation of shutdown. A no-op here, but used by
* ScheduledThreadPoolExecutor to cancel delayed tasks.
*/
void onShutdown() {
}
/**
* State check needed by ScheduledThreadPoolExecutor to
* enable running tasks during shutdown.
*
* @param shutdownOK true if should return true if SHUTDOWN
*/
final boolean isRunningOrShutdown(boolean shutdownOK) {
int rs = runStateOf(ctl.get());
return rs == RUNNING || (rs == SHUTDOWN && shutdownOK);
}
/**
* Drains the task queue into a new list, normally using
* drainTo. But if the queue is a DelayQueue or any other kind of
* queue for which poll or drainTo may fail to remove some
* elements, it deletes them one by one.
*/
private List<Runnable> drainQueue() {
BlockingQueue<Runnable> q = workQueue;
ArrayList<Runnable> taskList = new ArrayList();
q.drainTo(taskList);
if (!q.isEmpty()) {
for (Runnable r : q.toArray(new Runnable[0])) {
if (q.remove(r))
taskList.add(r);
}
}
return taskList;
}
/*
* Methods for creating, running and cleaning up after workers
*/
/**
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
/**
* Rolls back the worker thread creation.
* - removes worker from workers, if present
* - decrements worker count
* - rechecks for termination, in case the existence of this
* worker was holding up termination
*/
private void addWorkerFailed(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (w != null)
workers.remove(w);
decrementWorkerCount();
tryTerminate();
} finally {
mainLock.unlock();
}
}
/**
* Performs cleanup and bookkeeping for a dying worker. Called
* only from worker threads. Unless completedAbruptly is set,
* assumes that workerCount has already been adjusted to account
* for exit. This method removes thread from worker set, and
* possibly terminates the pool or replaces the worker if either
* it exited due to user task exception or if fewer than
* corePoolSize workers are running or queue is non-empty but
* there are no workers.
*
* @param w the worker
* @param completedAbruptly if the worker died due to user exception
*/
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) {
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
/**
* Performs blocking or timed wait for a task, depending on
* current configuration settings, or returns null if this worker
* must exit because of any of:
* 1. There are more than maximumPoolSize workers (due to
* a call to setMaximumPoolSize).
* 2. The pool is stopped.
* 3. The pool is shutdown and the queue is empty.
* 4. This worker timed out waiting for a task, and timed-out
* workers are subject to termination (that is,
* {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
* both before and after the timed wait, and if the queue is
* non-empty, this worker is not the last thread in the pool.
*
* @return task, or null if the worker must exit, in which case
* workerCount is decremented
*/
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
/**
* Main worker run loop. Repeatedly gets tasks from queue and
* executes them, while coping with a number of issues:
*
* 1. We may start out with an initial task, in which case we
* don't need to get the first one. Otherwise, as long as pool is
* running, we get tasks from getTask. If it returns null then the
* worker exits due to changed pool state or configuration
* parameters. Other exits result from exception throws in
* external code, in which case completedAbruptly holds, which
* usually leads processWorkerExit to replace this thread.
*
* 2. Before running any task, the lock is acquired to prevent
* other pool interrupts while the task is executing, and then we
* ensure that unless pool is stopping, this thread does not have
* its interrupt set.
*
* 3. Each task run is preceded by a call to beforeExecute, which
* might throw an exception, in which case we cause thread to die
* (breaking loop with completedAbruptly true) without processing
* the task.
*
* 4. Assuming beforeExecute completes normally, we run the task,
* gathering any of its thrown exceptions to send to afterExecute.
* We separately handle RuntimeException, Error (both of which the
* specs guarantee that we trap) and arbitrary Throwables.
* Because we cannot rethrow Throwables within Runnable.run, we
* wrap them within Errors on the way out (to the thread's
* UncaughtExceptionHandler). Any thrown exception also
* conservatively causes thread to die.
*
* 5. After task.run completes, we call afterExecute, which may
* also throw an exception, which will also cause thread to
* die. According to JLS Sec 14.20, this exception is the one that
* will be in effect even if task.run throws.
*
* The net effect of the exception mechanics is that afterExecute
* and the thread's UncaughtExceptionHandler have as accurate
* information as we can provide about any problems encountered by
* user code.
*
* @param w the worker
*/
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
// Public constructors and methods
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory and rejected execution handler.
* It may be more convenient to use one of the {@link Executors} factory
* methods instead of this general purpose constructor.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0})r).isCancelled())
it.remove();
}
} catch (ConcurrentModificationException fallThrough) {
// Take slow path if we encounter interference during traversal.
// Make copy for traversal and call remove for cancelled entries.
// The slow path is more likely to be O(N*N).
for (Object r : q.toArray())
if (r instanceof Future<?> && ((Future)r).isCancelled())
q.remove(r);
}
tryTerminate(); // In case SHUTDOWN and now empty
}
/* Statistics */
/**
* Returns the current number of threads in the pool.
*
* @return the number of threads
*/
public int getPoolSize() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Remove rare and surprising possibility of
// isTerminated() && getPoolSize() > 0
return runStateAtLeast(ctl.get(), TIDYING) ? 0
: workers.size();
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate number of threads that are actively
* executing tasks.
*
* @return the number of threads
*/
public int getActiveCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int n = 0;
for (Worker w : workers)
if (w.isLocked())
++n;
return n;
} finally {
mainLock.unlock();
}
}
/**
* Returns the largest number of threads that have ever
* simultaneously been in the pool.
*
* @return the number of threads
*/
public int getLargestPoolSize() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
return largestPoolSize;
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate total number of tasks that have ever been
* scheduled for execution. Because the states of tasks and
* threads may change dynamically during computation, the returned
* value is only an approximation.
*
* @return the number of tasks
*/
public long getTaskCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
long n = completedTaskCount;
for (Worker w : workers) {
n += w.completedTasks;
if (w.isLocked())
++n;
}
return n + workQueue.size();
} finally {
mainLock.unlock();
}
}
/**
* Returns the approximate total number of tasks that have
* completed execution. Because the states of tasks and threads
* may change dynamically during computation, the returned value
* is only an approximation, but one that does not ever decrease
* across successive calls.
*
* @return the number of tasks
*/
public long getCompletedTaskCount() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
long n = completedTaskCount;
for (Worker w : workers)
n += w.completedTasks;
return n;
} finally {
mainLock.unlock();
}
}
/**
* Returns a string identifying this pool, as well as its state,
* including indications of run state and estimated worker and
* task counts.
*
* @return a string identifying this pool, as well as its state
*/
public String toString() {
long ncompleted;
int nworkers, nactive;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
ncompleted = completedTaskCount;
nactive = 0;
nworkers = workers.size();
for (Worker w : workers) {
ncompleted += w.completedTasks;
if (w.isLocked())
++nactive;
}
} finally {
mainLock.unlock();
}
int c = ctl.get();
String rs = (runStateLessThan(c, SHUTDOWN) ? "Running" :
(runStateAtLeast(c, TERMINATED) ? "Terminated" :
"Shutting down"));
return super.toString() +
"[" + rs +
", pool size = " + nworkers +
", active threads = " + nactive +
", queued tasks = " + workQueue.size() +
", completed tasks = " + ncompleted +
"]";
}
/* Extension hooks */
/**
* Method invoked prior to executing the given Runnable in the
* given thread. This method is invoked by thread {@code t} that
* will execute task {@code r}, and may be used to re-initialize
* ThreadLocals, or to perform logging.
*
* <p>This implementation does nothing, but may be customized in
* subclasses. Note: To properly nest multiple overridings, subclasses
* should generally invoke {@code super.beforeExecute} at the end of
* this method.
*
* @param t the thread that will run task {@code r}
* @param r the task that will be executed
*/
protected void beforeExecute(Thread t, Runnable r) { }
/**
* Method invoked upon completion of execution of the given Runnable.
* This method is invoked by the thread that executed the task. If
* non-null, the Throwable is the uncaught {@code RuntimeException}
* or {@code Error} that caused execution to terminate abruptly.
*
* <p>This implementation does nothing, but may be customized in
* subclasses. Note: To properly nest multiple overridings, subclasses
* should generally invoke {@code super.afterExecute} at the
* beginning of this method.
*
* <p>Note: When actions are enclosed in tasks (such as
* {@link FutureTask}) either explicitly or via methods such as
* {@code submit}, these task objects catch and maintain
* computational exceptions, and so they do not cause abrupt
* termination, and the internal exceptions are <em>not
* passed to this method. If you would like to trap both kinds of
* failures in this method, you can further probe for such cases,
* as in this sample subclass that prints either the direct cause
* or the underlying exception if a task has been aborted:
*
* <pre> {@code
* class ExtendedExecutor extends ThreadPoolExecutor {
* // ...
* protected void afterExecute(Runnable r, Throwable t) {
* super.afterExecute(r, t);
* if (t == null && r instanceof Future<?>) {
* try {
* Object result = ((Future<?>) r).get();
* } catch (CancellationException ce) {
* t = ce;
* } catch (ExecutionException ee) {
* t = ee.getCause();
* } catch (InterruptedException ie) {
* Thread.currentThread().interrupt(); // ignore/reset
* }
* }
* if (t != null)
* System.out.println(t);
* }
* }}</pre>
*
* @param r the runnable that has completed
* @param t the exception that caused termination, or null if
* execution completed normally
*/
protected void afterExecute(Runnable r, Throwable t) { }
/**
* Method invoked when the Executor has terminated. Default
* implementation does nothing. Note: To properly nest multiple
* overridings, subclasses should generally invoke
* {@code super.terminated} within this method.
*/
protected void terminated() { }
/* Predefined RejectedExecutionHandlers */
/**
* A handler for rejected tasks that runs the rejected task
* directly in the calling thread of the {@code execute} method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code CallerRunsPolicy}.
*/
public CallerRunsPolicy() { }
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
}
/**
* A handler for rejected tasks that throws a
* {@code RejectedExecutionException}.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an {@code AbortPolicy}.
*/
public AbortPolicy() { }
/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardPolicy}.
*/
public DiscardPolicy() { }
/**
* Does nothing, which has the effect of discarding task r.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}
/**
* A handler for rejected tasks that discards the oldest unhandled
* request and then retries {@code execute}, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/
public DiscardOldestPolicy() { }
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}
}
Other Java examples (source code examples)
Here is a short list of links related to this Java ThreadPoolExecutor.java source code file:
|
The search page
Other Java source code examples at this package level
Click here to learn more about this project
