/*
* @(#)ThreadPoolExecutor.java 1.19 06/08/07
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util.concurrent;
import java.util.concurrent.locks.*;
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 <tt>ThreadPoolExecutor</tt> 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</dt>
*
* <dd>A <tt>ThreadPoolExecutor</tt> will automatically adjust the
* pool size
* (see {@link ThreadPoolExecutor#getPoolSize})
* according to the bounds set by corePoolSize
* (see {@link ThreadPoolExecutor#getCorePoolSize})
* and
* maximumPoolSize
* (see {@link ThreadPoolExecutor#getMaximumPoolSize}).
* When a new task is submitted in method {@link
* ThreadPoolExecutor#execute}, 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 <tt>Integer.MAX_VALUE</tt>, 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
* ThreadPoolExecutor#setCorePoolSize} and {@link
* ThreadPoolExecutor#setMaximumPoolSize}. </dd>
*
* <dt>On-demand construction</dt>
*
* <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
* ThreadPoolExecutor#prestartCoreThread} or
* {@link ThreadPoolExecutor#prestartAllCoreThreads}.
* You probably want to prestart threads if you construct the
* pool with a non-empty queue. </dd>
*
* <dt>Creating new threads</dt>
*
* <dd>New threads are created using a {@link
* java.util.concurrent.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
* <tt>NORM_PRIORITY</tt> 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 <tt>ThreadFactory</tt> fails to create
* a thread when asked by returning null from <tt>newThread</tt>,
* the executor will continue, but might
* not be able to execute any tasks. </dd>
*
* <dt>Keep-alive times</dt>
*
* <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
* ThreadPoolExecutor#getKeepAliveTime}). 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 ThreadPoolExecutor#setKeepAliveTime}. Using a value
* of <tt>Long.MAX_VALUE</tt> {@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 corePoolSizeThreads. But method {@link
* ThreadPoolExecutor#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</dt>
*
* <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> <em> Direct handoffs.</em> 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><em> Unbounded queues.</em> 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><em>Bounded queues.</em> 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</dt>
*
* <dd> New tasks submitted in method {@link
* ThreadPoolExecutor#execute} will be <em>rejected</em> 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 <tt>execute</tt> method invokes the
* {@link RejectedExecutionHandler#rejectedExecution} 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
* <tt>execute</tt> 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</dt>
*
* <dd>This class provides <tt>protected</tt> overridable {@link
* ThreadPoolExecutor#beforeExecute} and {@link
* ThreadPoolExecutor#afterExecute} 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 ThreadPoolExecutor#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</dt>
*
* <dd> Method {@link ThreadPoolExecutor#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 ThreadPoolExecutor#remove} and {@link
* ThreadPoolExecutor#purge} are available to assist in storage
* reclamation when large numbers of queued tasks become
* cancelled.</dd>
*
* <dt>Finalization</dt>
*
* <dd> A pool that is no longer referenced in a program <em>AND</em>
* has no remaining threads will be <tt>shutdown</tt>
* automatically. If you would like to ensure that unreferenced pools
* are reclaimed even if users forget to call {@link
* ThreadPoolExecutor#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
* ThreadPoolExecutor#allowCoreThreadTimeOut(boolean)}. </dd> </dl>
*
* <p> <b>Extension example</b>. 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>
* 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 {
/**
* Permission for checking shutdown
*/
private static final RuntimePermission shutdownPerm =
new RuntimePermission("modifyThread");
/*
* A ThreadPoolExecutor manages a largish set of control fields.
* State changes in fields that affect execution control
* guarantees only occur within mainLock regions. These include
* fields runState, poolSize, corePoolSize, and maximumPoolSize
* However, these fields are also declared volatile, so can be
* read outside of locked regions. (Also, the workers Set is
* accessed only under lock).
*
* The other fields representing user control parameters do not
* affect execution invariants, so are declared volatile and
* allowed to change (via user methods) asynchronously with
* execution. These fields: allowCoreThreadTimeOut, keepAliveTime,
* the rejected execution handler, and threadFactory, are not
* updated within locks.
*
* The extensive use of volatiles here enables the most
* performance-critical actions, such as enqueuing and dequeuing
* tasks in the workQueue, to normally proceed without holding the
* mainLock when they see that the state allows actions, although,
* as described below, sometimes at the expense of re-checks
* following these actions.
*/
/**
* runState provides the main lifecyle 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
* TERMINATED: Same as STOP, plus all threads have terminated
*
* 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 -> TERMINATED
* When both queue and pool are empty
* STOP -> TERMINATED
* When pool is empty
*/
volatile int runState;
static final int RUNNING = 0;
static final int SHUTDOWN = 1;
static final int STOP = 2;
static final int TERMINATED = 3;
/**
* The queue used for holding tasks and handing off to worker
* threads. Note that when using this queue, we do not require
* that workQueue.poll() returning null necessarily means that
* workQueue.isEmpty(), so must sometimes check both. 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 updates to poolSize, corePoolSize,
* maximumPoolSize, runState, and workers set.
*/
private final ReentrantLock mainLock = new ReentrantLock();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* Set containing all worker threads in pool. Accessed only when
* holding mainLock.
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
/**
* 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, updated only while holding mainLock, but
* volatile to allow concurrent readability even during updates.
*/
private volatile int corePoolSize;
/**
* Maximum pool size, updated only while holding mainLock but
* volatile to allow concurrent readability even during updates.
*/
private volatile int maximumPoolSize;
/**
* Current pool size, updated only while holding mainLock but
* volatile to allow concurrent readability even during updates.
*/
private volatile int poolSize;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* Factory for new threads. All threads are created using this
* factory (via method addThread). All callers must be prepared
* for addThread to fail by returning null, 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. On the other hand, no special
* precautions exist to handle OutOfMemoryErrors that might be
* thrown while trying to create threads, since there is generally
* no recourse from within this class.
*/
private volatile ThreadFactory threadFactory;
/**
* Tracks largest attained pool size.
*/
private int largestPoolSize;
/**
* Counter for completed tasks. Updated only on termination of
* worker threads.
*/
private long completedTaskCount;
/**
* The default rejected execution handler
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
// Constructors
/**
* Creates a new <tt>ThreadPoolExecutor</tt> 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.
* @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 keepAliveTime
* argument.
* @param workQueue the queue to use for holding tasks before they
* are executed. This queue will hold only the <tt>Runnable</tt>
* tasks submitted by the <tt>execute</tt> method.
* @throws IllegalArgumentException if corePoolSize or
* keepAliveTime less than zero, or if maximumPoolSize less than or
* equal to zero, or if corePoolSize greater than maximumPoolSize.
* @throws NullPointerException if <tt>workQueue</tt> is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
/**
* Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
* parameters and default rejected execution handler.
*
* @param corePoolSize the number of threads to keep in the
* pool, even if they are idle.
* @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 keepAliveTime
* argument.
* @param workQueue the queue to use for holding tasks before they
* are executed. This queue will hold only the <tt>Runnable</tt>
* tasks submitted by the <tt>execute</tt> method.
* @param threadFactory the factory to use when the executor
* creates a new thread.
* @throws IllegalArgumentException if corePoolSize or
* keepAliveTime less than zero, or if maximumPoolSize less than or
* equal to zero, or if corePoolSize greater than maximumPoolSize.
* @throws NullPointerException if <tt>workQueue</tt>
* or <tt>threadFactory</tt> are null.
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
/**
* Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
* parameters and default thread factory.
*
* @param corePoolSize the number of threads to keep in the
* pool, even if they are idle.
* @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 keepAliveTime
* argument.
* @param workQueue the queue to use for holding tasks before they
* are executed. This queue will hold only the <tt>Runnable</tt>
* tasks submitted by the <tt>execute</tt> method.
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached.
* @throws IllegalArgumentException if corePoolSize or
* keepAliveTime less than zero, or if maximumPoolSize less than or
* equal to zero, or if corePoolSize greater than maximumPoolSize.
* @throws NullPointerException if <tt>workQueue</tt>
* or <tt>handler</tt> are null.
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), handler);
}
/**
* Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the
* pool, even if they are idle.
* @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 keepAliveTime
* argument.
* @param workQueue the queue to use for holding tasks before they
* are executed. This queue will hold only the <tt>Runnable</tt>
* tasks submitted by the <tt>execute</tt> method.
* @param threadFactory the factory to use when the executor
* creates a new thread.
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached.
* @throws IllegalArgumentException if corePoolSize or
* keepAliveTime less than zero, or if maximumPoolSize less than or
* equal to zero, or if corePoolSize greater than maximumPoolSize.
* @throws NullPointerException if <tt>workQueue</tt>
* or <tt>threadFactory</tt> or <tt>handler</tt> are null.
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
/*
* Support for execute().
*
* Method execute() and its helper methods handle the various
* cases encountered when new tasks are submitted. The main
* execute() method proceeds in 3 steps:
*
* 1. If it appears that fewer than corePoolSize threads are
* running, try to start a new thread with the given command as
* its first task. The check here errs on the side of caution.
* The call to addIfUnderCorePoolSize rechecks runState and pool
* size under lock (they change only under lock) so prevents false
* alarms that would add threads when it shouldn't, but may also
* fail to add them when they should. This is compensated within
* the following steps.
*
* 2. If a task can be successfully queued, then we are done, but
* still need to compensate for missing the fact that we should
* have added a thread (because existing ones died) or that
* shutdown occurred since entry into this method. So we recheck
* state and if necessary (in ensureQueuedTaskHandled) roll back
* the enqueuing if shut down, or start a new thread if there are
* none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. There's no guesswork here (addIfUnderMaximumPoolSize)
* since it is performed under lock. If it fails, we know we are
* shut down or saturated.
*
* The reason for taking this overall approach is to normally
* avoid holding mainLock during this method, which would be a
* serious scalability bottleneck. After warmup, almost all calls
* take step 2 in a way that entails no locking.
*/
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current <tt>RejectedExecutionHandler</tt>.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
* for execution
* @throws NullPointerException if command is null
*/
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
if (runState == RUNNING && workQueue.offer(command)) {
if (runState != RUNNING || poolSize == 0)
ensureQueuedTaskHandled(command);
}
else if (!addIfUnderMaximumPoolSize(command))
reject(command); // is shutdown or saturated
}
}
/**
* Creates and returns a new thread running firstTask as its first
* task. Call only while holding mainLock.
*
* @param firstTask the task the new thread should run first (or
* null if none)
* @return the new thread, or null if threadFactory fails to create thread
*/
private Thread addThread(Runnable firstTask) {
Worker w = new Worker(firstTask);
Thread t = threadFactory.newThread(w);
if (t != null) {
w.thread = t;
workers.add(w);
int nt = ++poolSize;
if (nt > largestPoolSize)
largestPoolSize = nt;
}
return t;
}
/**
* Creates and starts a new thread running firstTask as its first
* task, only if fewer than corePoolSize threads are running
* and the pool is not shut down.
* @param firstTask the task the new thread should run first (or
* null if none)
* @return true if successful
*/
private boolean addIfUnderCorePoolSize(Runnable firstTask) {
Thread t = null;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (poolSize < corePoolSize && runState == RUNNING)
t = addThread(firstTask);
} finally {
mainLock.unlock();
}
if (t == null)
return false;
t.start();
return true;
}
/**
* Creates and starts a new thread running firstTask as its first
* task, only if fewer than maximumPoolSize threads are running
* and pool is not shut down.
* @param firstTask the task the new thread should run first (or
* null if none)
* @return true if successful
*/
private boolean addIfUnderMaximumPoolSize(Runnable firstTask) {
Thread t = null;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (poolSize < maximumPoolSize && runState == RUNNING)
t = addThread(firstTask);
} finally {
mainLock.unlock();
}
if (t == null)
return false;
t.start();
return true;
}
/**
* Rechecks state after queuing a task. Called from execute when
* pool state has been observed to change after queuing a task. If
* the task was queued concurrently with a call to shutdownNow,
* and is still present in the queue, this task must be removed
* and rejected to preserve shutdownNow guarantees. Otherwise,
* this method ensures (unless addThread fails) that there is at
* least one live thread to handle this task
* @param command the task
*/
private void ensureQueuedTaskHandled(Runnable command) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
boolean reject = false;
Thread t = null;
try {
int state = runState;
if (state != RUNNING && workQueue.remove(command))
reject = true;
else if (state < STOP &&
poolSize < Math.max(corePoolSize, 1) &&
!workQueue.isEmpty())
t = addThread(null);
} finally {
mainLock.unlock();
}
if (reject)
reject(command);
else if (t != null)
t.start();
}
/**
* Invokes the rejected execution handler for the given command.
*/
void reject(Runnable command) {
handler.rejectedExecution(command, this);
}
/**
* Worker threads.
*
* Worker threads can start out life either with an initial first
* task, or without one. Normally, they are started with a first
* task. This enables execute(), etc 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 created either by
* users (prestartCoreThread and setCorePoolSize) or when methods
* ensureQueuedTaskHandled and tryTerminate notice that the queue
* is not empty but there are no active threads to handle them.
*
* After completing a task, workers try to get another one,
* via method getTask. If they cannot (i.e., getTask returns
* null), they exit, calling workerDone to update pool state.
*
* When starting to run a task, unless the pool is stopped, each
* worker thread ensures that it is not interrupted, and uses
* runLock to prevent the pool from interrupting it in the midst
* of execution. This shields user tasks from any interrupts that
* may otherwise be needed during shutdown (see method
* interruptIdleWorkers), unless the pool is stopping (via
* shutdownNow) in which case interrupts are let through to affect
* both tasks and workers. However, this shielding does not
* necessarily protect the workers from lagging interrupts from
* other user threads directed towards tasks that have already
* been completed. Thus, a worker thread may be interrupted
* needlessly (for example in getTask), in which case it rechecks
* pool state to see if it should exit.
*/
private final class Worker implements Runnable {
/**
* The runLock is acquired and released surrounding each task
* execution. It mainly protects against interrupts that are
* intended to cancel the worker thread from instead
* interrupting the task being run.
*/
private final ReentrantLock runLock = new ReentrantLock();
/**
* Initial task to run before entering run loop. Possibly null.
*/
private Runnable firstTask;
/**
* Per thread completed task counter; accumulated
* into completedTaskCount upon termination.
*/
volatile long completedTasks;
/**
* Thread this worker is running in. Acts as a final field,
* but cannot be set until thread is created.
*/
Thread thread;
Worker(Runnable firstTask) {
this.firstTask = firstTask;
}
boolean isActive() {
return runLock.isLocked();
}
/**
* Interrupts thread if not running a task.
*/
void interruptIfIdle() {
final ReentrantLock runLock = this.runLock;
if (runLock.tryLock()) {
try {
thread.interrupt();
} finally {
runLock.unlock();
}
}
}
/**
* Interrupts thread even if running a task.
*/
void interruptNow() {
thread.interrupt();
}
/**
* Runs a single task between before/after methods.
*/
private void runTask(Runnable task) {
final ReentrantLock runLock = this.runLock;
runLock.lock();
try {
/*
* Ensure that unless pool is stopping, this thread
* does not have its interrupt set. This requires a
* double-check of state in case the interrupt was
* cleared concurrently with a shutdownNow -- if so,
* the interrupt is re-enabled.
*/
if (runState < STOP &&
Thread.interrupted() &&
runState >= STOP)
thread.interrupt();
/*
* Track execution state to ensure that afterExecute
* is called only if task completed or threw
* exception. Otherwise, the caught runtime exception
* will have been thrown by afterExecute itself, in
* which case we don't want to call it again.
*/
boolean ran = false;
beforeExecute(thread, task);
try {
task.run();
ran = true;
afterExecute(task, null);
++completedTasks;
} catch (RuntimeException ex) {
if (!ran)
afterExecute(task, ex);
throw ex;
}
} finally {
runLock.unlock();
}
}
/**
* Main run loop
*/
public void run() {
try {
Runnable task = firstTask;
firstTask = null;
while (task != null || (task = getTask()) != null) {
runTask(task);
task = null;
}
} finally {
workerDone(this);
}
}
}
/* Utilities for worker thread control */
/**
* Gets the next task for a worker thread to run. The general
* approach is similar to execute() in that worker threads trying
* to get a task to run do so on the basis of prevailing state
* accessed outside of locks. This may cause them to choose the
* "wrong" action, such as trying to exit because no tasks
* appear to be available, or entering a take when the pool is in
* the process of being shut down. These potential problems are
* countered by (1) rechecking pool state (in workerCanExit)
* before giving up, and (2) interrupting other workers upon
* shutdown, so they can recheck state. All other user-based state
* changes (to allowCoreThreadTimeOut etc) are OK even when
* performed asynchronously wrt getTask.
*
* @return the task
*/
Runnable getTask() {
for (;;) {
try {
int state = runState;
if (state > SHUTDOWN)
return null;
Runnable r;
if (state == SHUTDOWN) // Help drain queue
r = workQueue.poll();
else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
else
r = workQueue.take();
if (r != null)
return r;
if (workerCanExit()) {
if (runState >= SHUTDOWN) // Wake up others
interruptIdleWorkers();
return null;
}
// Else retry
} catch (InterruptedException ie) {
// On interruption, re-check runState
}
}
}
/**
* Check whether a worker thread that fails to get a task can
* exit. We allow a worker thread to die if the pool is stopping,
* or the queue is empty, or there is at least one thread to
* handle possibly non-empty queue, even if core timeouts are
* allowed.
*/
private boolean workerCanExit() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
boolean canExit;
try {
canExit = runState >= STOP ||
workQueue.isEmpty() ||
(allowCoreThreadTimeOut &&
poolSize > Math.max(1, corePoolSize));
} finally {
mainLock.unlock();
}
return canExit;
}
/**
* Wakes up all threads that might be waiting for tasks so they
* can check for termination. Note: this method is also called by
* ScheduledThreadPoolExecutor.
*/
void interruptIdleWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfIdle();
} finally {
mainLock.unlock();
}
}
/**
* Performs bookkeeping for an exiting worker thread.
* @param w the worker
*/
void workerDone(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
if (--poolSize == 0)
tryTerminate();
} finally {
mainLock.unlock();
}
}
/* Termination support. */
/**
* Transitions to TERMINATED state if either (SHUTDOWN and pool
* and queue empty) or (STOP and pool empty), otherwise unless
* stopped, ensuring that there is at least one live thread to
* handle queued tasks.
*
* This method is called from the three places in which
* termination can occur: in workerDone on exit of the last thread
* after pool has been shut down, or directly within calls to
* shutdown or shutdownNow, if there are no live threads.
*/
private void tryTerminate() {
if (poolSize == 0) {
int state = runState;
if (state < STOP && !workQueue.isEmpty()) {
state = RUNNING; // disable termination check below
Thread t = addThread(null);
if (t != null)
t.start();
}
if (state == STOP || state == SHUTDOWN) {
runState = TERMINATED;
termination.signalAll();
terminated();
}
}
}
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be
* accepted. Invocation has no additional effect if already shut
* down.
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate threads that
* the caller is not permitted to modify because it does not hold
* {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
* or the security manager's <tt>checkAccess</tt> method denies access.
*/
public void shutdown() {
/*
* Conceptually, shutdown is just a matter of changing the
* runState to SHUTDOWN, and then interrupting any worker
* threads that might be blocked in getTask() to wake them up
* so they can exit. Then, if there happen not to be any
* threads or tasks, we can directly terminate pool via
* tryTerminate. Else, the last worker to leave the building
* turns off the lights (in workerDone).
*
* But this is made more delicate because we must cooperate
* with the security manager (if present), which may implement
* policies that make more sense for operations on Threads
* than they do for ThreadPools. This requires 3 steps:
*
* 1. Making sure caller has permission to shut down threads
* in general (see shutdownPerm).
*
* 2. If (1) passes, making sure the caller is allowed to
* modify each of our threads. This might not be true even if
* first check passed, if the SecurityManager treats some
* threads specially. If this check passes, then we can try
* to set runState.
*
* 3. If both (1) and (2) pass, dealing with inconsistent
* security managers that allow checkAccess but then throw a
* SecurityException when interrupt() is invoked. In this
* third case, because we have already set runState, we can
* only try to back out from the shutdown as cleanly as
* possible. Some workers may have been killed but we remain
* in non-shutdown state (which may entail tryTerminate from
* workerDone starting a new worker to maintain liveness.)
*/
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(shutdownPerm);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (security != null) { // Check if caller can modify our threads
for (Worker w : workers)
security.checkAccess(w.thread);
}
int state = runState;
if (state < SHUTDOWN)
runState = SHUTDOWN;
try {
for (Worker w : workers) {
w.interruptIfIdle();
}
} catch (SecurityException se) { // Try to back out
runState = state;
// tryTerminate() here would be a no-op
throw se;
}
tryTerminate(); // Terminate now if pool and queue empty
} finally {
mainLock.unlock();
}
}
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks
* that were awaiting execution. These tasks are drained (removed)
* from the task queue upon return from this method.
*
* <p>There are no guarantees beyond best-effort attempts to stop
* processing actively executing tasks. This implementation
* cancels tasks via {@link Thread#interrupt}, so any task that
* fails to respond to interrupts may never terminate.
*
* @return list of tasks that never commenced execution
* @throws SecurityException if a security manager exists and
* shutting down this ExecutorService may manipulate threads that
* the caller is not permitted to modify because it does not hold
* {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
* or the security manager's <tt>checkAccess</tt> method denies access.
*/
public List<Runnable> shutdownNow() {
/*
* shutdownNow differs from shutdown only in that
* 1. runState is set to STOP,
* 2. all worker threads are interrupted, not just the idle ones, and
* 3. the queue is drained and returned.
*/
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(shutdownPerm);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (security != null) { // Check if caller can modify our threads
for (Worker w : workers)
security.checkAccess(w.thread);
}
int state = runState;
if (state < STOP)
runState = STOP;
try {
for (Worker w : workers) {
w.interruptNow();
}
} catch (SecurityException se) { // Try to back out
runState = state;
// tryTerminate() here would be a no-op
throw se;
}
List<Runnable> tasks = drainQueue();
tryTerminate(); // Terminate now if pool and queue empty
return tasks;
} finally {
mainLock.unlock();
}
}
/**
* Drains the task queue into a new list. Used by shutdownNow.
* Call only while holding main lock.
*/
private List<Runnable> drainQueue() {
List<Runnable> taskList = new ArrayList<Runnable>();
workQueue.drainTo(taskList);
/*
* If the queue is a DelayQueue or any other kind of queue
* for which poll or drainTo may fail to remove some elements,
* we need to manually traverse and remove remaining tasks.
* To guarantee atomicity wrt other threads using this queue,
* we need to create a new iterator for each element removed.
*/
while (!workQueue.isEmpty()) {
Iterator<Runnable> it = workQueue.iterator();
try {
if (it.hasNext()) {
Runnable r = it.next();
if (workQueue.remove(r))
taskList.add(r);
}
} catch (ConcurrentModificationException ignore) {
}
}
return taskList;
}
public boolean isShutdown() {
return runState != RUNNING;
}
/**
* Returns true if this executor is in the process of terminating
* after <tt>shutdown</tt> or <tt>shutdownNow</tt> but has not
* completely terminated. This method may be useful for
* debugging. A return of <tt>true</tt> reported a sufficient
* period after shutdown may indicate that submitted tasks have
* ignored or suppressed interruption, causing this executor not
* to properly terminate.
* @return true if terminating but not yet terminated
*/
public boolean isTerminating() {
int state = runState;
return state == SHUTDOWN || state == STOP;
}
public boolean isTerminated() {
return runState == TERMINATED;
}
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (;;) {
if (runState == TERMINATED)
return true;
if (nanos <= 0)
return false;
nanos = termination.awaitNanos(nanos);
}
} finally {
mainLock.unlock();
}
}
/**
* Invokes <tt>shutdown</tt> when this executor is no longer
* referenced.
*/
protected void finalize() {
shutdown();
}
/* Getting and setting tunable parameters */
/**
* Sets the thread factory used to create new threads.
*
* @param threadFactory the new thread factory
* @throws NullPointerException if threadFactory is null
* @see #getThreadFactory
*/
public void setThreadFactory(ThreadFactory threadFactory) {
if (threadFactory == null)
throw new NullPointerException();
this.threadFactory = threadFactory;
}
/**
* Returns the thread factory used to create new threads.
*
* @return the current thread factory
* @see #setThreadFactory
*/
public ThreadFactory getThreadFactory() {
return threadFactory;
}
/**
* Sets a new handler for unexecutable tasks.
*
* @param handler the new handler
* @throws NullPointerException if handler is null
* @see #getRejectedExecutionHandler
*/
public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
if (handler == null)
throw new NullPointerException();
this.handler = handler;
}
/**
* Returns the current handler for unexecutable tasks.
*
* @return the current handler
* @see #setRejectedExecutionHandler
*/
public RejectedExecutionHandler getRejectedExecutionHandler() {
return handler;
}
/**
* Sets the core number of threads. This overrides any value set
* in the constructor. If the new value is smaller than the
* current value, excess existing threads will be terminated when
* they next become idle. If larger, new threads will, if needed,
* be started to execute any queued tasks.
*
* @param corePoolSize the new core size
* @throws IllegalArgumentException if <tt>corePoolSize</tt>
* less than zero
* @see #getCorePoolSize
*/
public void setCorePoolSize(int corePoolSize) {
if (corePoolSize < 0)
throw new IllegalArgumentException();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int extra = this.corePoolSize - corePoolSize;
this.corePoolSize = corePoolSize;
if (extra < 0) {
int n = workQueue.size(); // don't add more threads than tasks
while (extra++ < 0 && n-- > 0 && poolSize < corePoolSize) {
Thread t = addThread(null);
if (t != null)
t.start();
else
break;
}
}
else if (extra > 0 && poolSize > corePoolSize) {
try {
Iterator<Worker> it = workers.iterator();
while (it.hasNext() &&
extra-- > 0 &&
poolSize > corePoolSize &&
workQueue.remainingCapacity() == 0)
it.next().interruptIfIdle();
} catch (SecurityException ignore) {
// Not an error; it is OK if the threads stay live
}
}
} finally {
mainLock.unlock();
}
}
/**
* Returns the core number of threads.
*
* @return the core number of threads
* @see #setCorePoolSize
*/
public int getCorePoolSize() {
return corePoolSize;
}
/**
* Starts a core thread, causing it to idly wait for work. This
* overrides the default policy of starting core threads only when
* new tasks are executed. This method will return <tt>false</tt>
* if all core threads have already been started.
* @return true if a thread was started
*/
public boolean prestartCoreThread() {
return addIfUnderCorePoolSize(null);
}
/**
* Starts all core threads, causing them to idly wait for work. This
* overrides the default policy of starting core threads only when
* new tasks are executed.
* @return the number of threads started
*/
public int prestartAllCoreThreads() {
int n = 0;
while (addIfUnderCorePoolSize(null))
++n;
return n;
}
/**
* Returns true if this pool allows core threads to time out and
* terminate if no tasks arrive within the keepAlive time, being
* replaced if needed when new tasks arrive. When true, the same
* keep-alive policy applying to non-core threads applies also to
* core threads. When false (the default), core threads are never
* terminated due to lack of incoming tasks.
* @return <tt>true</tt> if core threads are allowed to time out,
* else <tt>false</tt>
*
* @since 1.6
*/
public boolean allowsCoreThreadTimeOut() {
return allowCoreThreadTimeOut;
}
/**
* Sets the policy governing whether core threads may time out and
* terminate if no tasks arrive within the keep-alive time, being
* replaced if needed when new tasks arrive. When false, core
* threads are never terminated due to lack of incoming
* tasks. When true, the same keep-alive policy applying to
* non-core threads applies also to core threads. To avoid
* continual thread replacement, the keep-alive time must be
* greater than zero when setting <tt>true</tt>. This method
* should in general be called before the pool is actively used.
* @param value <tt>true</tt> if should time out, else <tt>false</tt>
* @throws IllegalArgumentException if value is <tt>true</tt>
* and the current keep-alive time is not greater than zero.
*
* @since 1.6
*/
public void allowCoreThreadTimeOut(boolean value) {
if (value && keepAliveTime <= 0)
throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
allowCoreThreadTimeOut = value;
}
/**
* Sets the maximum allowed number of threads. This overrides any
* value set in the constructor. If the new value is smaller than
* the current value, excess existing threads will be
* terminated when they next become idle.
*
* @param maximumPoolSize the new maximum
* @throws IllegalArgumentException if the new maximum is
* less than or equal to zero, or
* less than the {@linkplain #getCorePoolSize core pool size}
* @see #getMaximumPoolSize
*/
public void setMaximumPoolSize(int maximumPoolSize) {
if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
throw new IllegalArgumentException();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
int extra = this.maximumPoolSize - maximumPoolSize;
this.maximumPoolSize = maximumPoolSize;
if (extra > 0 && poolSize > maximumPoolSize) {
try {
Iterator<Worker> it = workers.iterator();
while (it.hasNext() &&
extra > 0 &&
poolSize > maximumPoolSize) {
it.next().interruptIfIdle();
--extra;
}
} catch (SecurityException ignore) {
// Not an error; it is OK if the threads stay live
}
}
} finally {
mainLock.unlock();
}
}
/**
* Returns the maximum allowed number of threads.
*
* @return the maximum allowed number of threads
* @see #setMaximumPoolSize
*/
public int getMaximumPoolSize() {
return maximumPoolSize;
}
/**
* Sets the time limit for which threads may remain idle before
* being terminated. If there are more than the core number of
* threads currently in the pool, after waiting this amount of
* time without processing a task, excess threads will be
* terminated. This overrides any value set in the constructor.
* @param time the time to wait. A time value of zero will cause
* excess threads to terminate immediately after executing tasks.
* @param unit the time unit of the time argument
* @throws IllegalArgumentException if time less than zero or
* if time is zero and allowsCoreThreadTimeOut
* @see #getKeepAliveTime
*/
public void setKeepAliveTime(long time, TimeUnit unit) {
if (time < 0)
throw new IllegalArgumentException();
if (time == 0 && allowsCoreThreadTimeOut())
throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
this.keepAliveTime = unit.toNanos(time);
}
/**
* Returns the thread keep-alive time, which is the amount of time
* that threads in excess of the core pool size may remain
* idle before being terminated.
*
* @param unit the desired time unit of the result
* @return the time limit
* @see #setKeepAliveTime
*/
public long getKeepAliveTime(TimeUnit unit) {
return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
}
/* User-level queue utilities */
/**
* Returns the task queue used by this executor. Access to the
* task queue is intended primarily for debugging and monitoring.
* This queue may be in active use. Retrieving the task queue
* does not prevent queued tasks from executing.
*
* @return the task queue
*/
public BlockingQueue<Runnable> getQueue() {
return workQueue;
}
/**
* Removes this task from the executor's internal queue if it is
* present, thus causing it not to be run if it has not already
* started.
*
* <p> This method may be useful as one part of a cancellation
* scheme. It may fail to remove tasks that have been converted
* into other forms before being placed on the internal queue. For
* example, a task entered using <tt>submit</tt> might be
* converted into a form that maintains <tt>Future</tt> status.
* However, in such cases, method {@link ThreadPoolExecutor#purge}
* may be used to remove those Futures that have been cancelled.
*
* @param task the task to remove
* @return true if the task was removed
*/
public boolean remove(Runnable task) {
return getQueue().remove(task);
}
/**
* Tries to remove from the work queue all {@link Future}
* tasks that have been cancelled. This method can be useful as a
* storage reclamation operation, that has no other impact on
* functionality. Cancelled tasks are never executed, but may
* accumulate in work queues until worker threads can actively
* remove them. Invoking this method instead tries to remove them now.
* However, this method may fail to remove tasks in
* the presence of interference by other threads.
*/
public void purge() {
// Fail if we encounter interference during traversal
try {
Iterator<Runnable> it = getQueue().iterator();
while (it.hasNext()) {
Runnable r = it.next();
if (r instanceof Future<?>) {
Future<?> c = (Future<?>)r;
if (c.isCancelled())
it.remove();
}
}
}
catch (ConcurrentModificationException ex) {
return;
}
}
/* Statistics */
/**
* Returns the current number of threads in the pool.
*
* @return the number of threads
*/
public int getPoolSize() {
return poolSize;
}
/**
* 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.isActive())
++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.isActive())
++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();
}
}
/* Extension hooks */
/**
* Method invoked prior to executing the given Runnable in the
* given thread. This method is invoked by thread <tt>t</tt> that
* will execute task <tt>r</tt>, 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 <tt>super.beforeExecute</tt> at the end of
* this method.
*
* @param t the thread that will run task 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 <tt>RuntimeException</tt>
* or <tt>Error</tt> that caused execution to terminate abruptly.
*
* <p><b>Note:</b> When actions are enclosed in tasks (such as
* {@link FutureTask}) either explicitly or via methods such as
* <tt>submit</tt>, these task objects catch and maintain
* computational exceptions, and so they do not cause abrupt
* termination, and the internal exceptions are <em>not</em>
* passed to this method.
*
* <p>This implementation does nothing, but may be customized in
* subclasses. Note: To properly nest multiple overridings, subclasses
* should generally invoke <tt>super.afterExecute</tt> at the
* beginning of this method.
*
* @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
* <tt>super.terminated</tt> 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 <tt>execute</tt> method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a <tt>CallerRunsPolicy</tt>.
*/
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
* <tt>RejectedExecutionException</tt>.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an <tt>AbortPolicy</tt>.
*/
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();
}
}
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a <tt>DiscardPolicy</tt>.
*/
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 <tt>execute</tt>, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a <tt>DiscardOldestPolicy</tt> 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);
}
}
}
}
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