/*
* 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/licenses/publicdomain
*/
package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.*;
// BEGIN android-note
// Altered {@link TimeUnit#NANOSECONDS} because our javadoc tool doesn't
// like it.
// END android-note
/**
* 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
*
* <dd> By default, even core threads are initially created and
* started only when needed by new tasks, but this can be overridden
* dynamically using method {@link
* ThreadPoolExecutor#prestartCoreThread} or
* {@link ThreadPoolExecutor#prestartAllCoreThreads}. </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. </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> <code>TimeUnit.NANOSECONDS</code>
* effectively disables idle threads from ever terminating prior
* to shut down.
* </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 be queued in cases where 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.</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> </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 {
/**
* Only used to force toArray() to produce a Runnable[].
*/
private static final Runnable[] EMPTY_RUNNABLE_ARRAY = new Runnable[0];
/**
* Permission for checking shutdown
*/
private static final RuntimePermission shutdownPerm =
new RuntimePermission("modifyThread");
/**
* Queue used for holding tasks and handing off to worker threads.
*/
private final BlockingQueue<Runnable> workQueue;
/**
* Lock held on updates to poolSize, corePoolSize, maximumPoolSize, 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.
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
/**
* Timeout in nanoseconds for idle threads waiting for work.
* Threads use this timeout only when there are more than
* corePoolSize present. Otherwise they wait forever for new work.
*/
private volatile long keepAliveTime;
/**
* 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;
/**
* Lifecycle state
*/
volatile int runState;
// Special values for runState
/** Normal, not-shutdown mode */
static final int RUNNING = 0;
/** Controlled shutdown mode */
static final int SHUTDOWN = 1;
/** Immediate shutdown mode */
static final int STOP = 2;
/** Final state */
static final int TERMINATED = 3;
/**
* Handler called when saturated or shutdown in execute.
*/
private volatile RejectedExecutionHandler handler;
/**
* Factory for new threads.
*/
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();
/**
* Invoke the rejected execution handler for the given command.
*/
void reject(Runnable command) {
handler.rejectedExecution(command, this);
}
/**
* Create and return 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
*/
private Thread addThread(Runnable firstTask) {
Worker w = new Worker(firstTask);
Thread t = threadFactory.newThread(w);
w.thread = t;
workers.add(w);
int nt = ++poolSize;
if (nt > largestPoolSize)
largestPoolSize = nt;
return t;
}
/**
* Create and start a new thread running firstTask as its first
* task, only if fewer than corePoolSize threads are running.
* @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)
t = addThread(firstTask);
} finally {
mainLock.unlock();
}
if (t == null)
return false;
t.start();
return true;
}
/**
* Create and start a new thread only if fewer than maximumPoolSize
* threads are running. The new thread runs as its first task the
* next task in queue, or if there is none, the given task.
* @param firstTask the task the new thread should run first (or
* null if none)
* @return null on failure, else the first task to be run by new thread.
*/
private Runnable addIfUnderMaximumPoolSize(Runnable firstTask) {
Thread t = null;
Runnable next = null;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (poolSize < maximumPoolSize) {
next = workQueue.poll();
if (next == null)
next = firstTask;
t = addThread(next);
}
} finally {
mainLock.unlock();
}
if (t == null)
return null;
t.start();
return next;
}
/**
* Get the next task for a worker thread to run.
* @return the task
* @throws InterruptedException if interrupted while waiting for task
*/
Runnable getTask() throws InterruptedException {
for (;;) {
switch(runState) {
case RUNNING: {
if (poolSize <= corePoolSize) // untimed wait if core
return workQueue.take();
long timeout = keepAliveTime;
if (timeout <= 0) // die immediately for 0 timeout
return null;
Runnable r = workQueue.poll(timeout, TimeUnit.NANOSECONDS);
if (r != null)
return r;
if (poolSize > corePoolSize) // timed out
return null;
// else, after timeout, pool shrank so shouldn't die, so retry
break;
}
case SHUTDOWN: {
// Help drain queue
Runnable r = workQueue.poll();
if (r != null)
return r;
// Check if can terminate
if (workQueue.isEmpty()) {
interruptIdleWorkers();
return null;
}
// There could still be delayed tasks in queue.
// Wait for one, re-checking state upon interruption
try {
return workQueue.take();
} catch(InterruptedException ignore) {}
break;
}
case STOP:
return null;
default:
assert false;
}
}
}
/**
* Wake up all threads that might be waiting for tasks.
*/
void interruptIdleWorkers() {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
for (Worker w : workers)
w.interruptIfIdle();
} finally {
mainLock.unlock();
}
}
/**
* Perform bookkeeping for a terminated 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)
return;
// Else, this is the last thread. Deal with potential shutdown.
int state = runState;
assert state != TERMINATED;
if (state != STOP) {
// If there are queued tasks but no threads, create
// replacement.
Runnable r = workQueue.poll();
if (r != null) {
addThread(r).start();
return;
}
// If there are some (presumably delayed) tasks but
// none pollable, create an idle replacement to wait.
if (!workQueue.isEmpty()) {
addThread(null).start();
return;
}
// Otherwise, we can exit without replacement
if (state == RUNNING)
return;
}
// Either state is STOP, or state is SHUTDOWN and there is
// no work to do. So we can terminate.
termination.signalAll();
runState = TERMINATED;
// fall through to call terminate() outside of lock.
} finally {
mainLock.unlock();
}
assert runState == TERMINATED;
terminated();
}
/**
* Worker threads
*/
private 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
*/
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();
}
/**
* Interrupt thread if not running a task
*/
void interruptIfIdle() {
final ReentrantLock runLock = this.runLock;
if (runLock.tryLock()) {
try {
thread.interrupt();
} finally {
runLock.unlock();
}
}
}
/**
* Cause thread to die even if running a task.
*/
void interruptNow() {
thread.interrupt();
}
/**
* Run a single task between before/after methods.
*/
private void runTask(Runnable task) {
final ReentrantLock runLock = this.runLock;
runLock.lock();
try {
// Abort now if immediate cancel. Otherwise, we have
// committed to run this task.
if (runState == STOP)
return;
Thread.interrupted(); // clear interrupt status on entry
boolean ran = false;
beforeExecute(thread, task);
try {
task.run();
ran = true;
afterExecute(task, null);
++completedTasks;
} catch(RuntimeException ex) {
if (!ran)
afterExecute(task, ex);
// Else the exception occurred within
// afterExecute itself in which case we don't
// want to call it again.
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; // unnecessary but can help GC
}
} catch(InterruptedException ie) {
// fall through
} finally {
workerDone(this);
}
}
}
// Public methods
/**
* Creates a new <tt>ThreadPoolExecutor</tt> with the given
* initial parameters and default thread factory and 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.
*
* @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.
*
* @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;
}
/**
* 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();
for (;;) {
if (runState != RUNNING) {
reject(command);
return;
}
if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
return;
if (workQueue.offer(command))
return;
Runnable r = addIfUnderMaximumPoolSize(command);
if (r == command)
return;
if (r == null) {
reject(command);
return;
}
// else retry
}
}
/**
* 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() {
// Fail if caller doesn't have modifyThread permission
SecurityManager security = System.getSecurityManager();
if (security != null)
java.security.AccessController.checkPermission(shutdownPerm);
boolean fullyTerminated = false;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (workers.size() > 0) {
// Check if caller can modify worker threads. This
// might not be true even if passed above check, if
// the SecurityManager treats some threads specially.
if (security != null) {
for (Worker w: workers)
security.checkAccess(w.thread);
}
int state = runState;
if (state == RUNNING) // don't override shutdownNow
runState = SHUTDOWN;
try {
for (Worker w: workers)
w.interruptIfIdle();
} catch(SecurityException se) {
// If SecurityManager allows above checks, but
// then unexpectedly throws exception when
// interrupting threads (which it ought not do),
// back out as cleanly as we can. Some threads may
// have been killed but we remain in non-shutdown
// state.
runState = state;
throw se;
}
}
else { // If no workers, trigger full termination now
fullyTerminated = true;
runState = TERMINATED;
termination.signalAll();
}
} finally {
mainLock.unlock();
}
if (fullyTerminated)
terminated();
}
/**
* Attempts to stop all actively executing tasks, halts the
* processing of waiting tasks, and returns a list of the tasks that were
* awaiting execution.
*
* <p>This implementation cancels tasks via {@link
* Thread#interrupt}, so if any tasks mask or fail to respond to
* interrupts, they 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() {
// Almost the same code as shutdown()
SecurityManager security = System.getSecurityManager();
if (security != null)
java.security.AccessController.checkPermission(shutdownPerm);
boolean fullyTerminated = false;
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (workers.size() > 0) {
if (security != null) {
for (Worker w: workers)
security.checkAccess(w.thread);
}
int state = runState;
if (state != TERMINATED)
runState = STOP;
try {
for (Worker w : workers)
w.interruptNow();
} catch(SecurityException se) {
runState = state; // back out;
throw se;
}
}
else { // If no workers, trigger full termination now
fullyTerminated = true;
runState = TERMINATED;
termination.signalAll();
}
} finally {
mainLock.unlock();
}
if (fullyTerminated)
terminated();
return Arrays.asList(workQueue.toArray(EMPTY_RUNNABLE_ARRAY));
}
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() {
return runState == 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();
}
/**
* 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;
}
/**
* 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;
}
}
/**
* 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) {
Runnable r;
while (extra++ < 0 && poolSize < corePoolSize &&
(r = workQueue.poll()) != null)
addThread(r).start();
}
else if (extra > 0 && poolSize > corePoolSize) {
Iterator<Worker> it = workers.iterator();
while (it.hasNext() &&
extra-- > 0 &&
poolSize > corePoolSize &&
workQueue.remainingCapacity() == 0)
it.next().interruptIfIdle();
}
} 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;
}
/**
* 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 maximumPoolSize less than zero or
* the {@link #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) {
Iterator<Worker> it = workers.iterator();
while (it.hasNext() &&
extra > 0 &&
poolSize > maximumPoolSize) {
it.next().interruptIfIdle();
--extra;
}
}
} 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
* @see #getKeepAliveTime
*/
public void setKeepAliveTime(long time, TimeUnit unit) {
if (time < 0)
throw new IllegalArgumentException();
this.keepAliveTime = unit.toNanos(time);
}
/**
* Returns the thread keep-alive time, which is the amount of time
* which 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);
}
/* 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 been
* scheduled for 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 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();
}
}
/**
* 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. 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 exception
* that caused execution to terminate abruptly. 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() { }
/**
* 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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