java.util.concurrent
Class ThreadPoolExecutor

java.lang.Object   
  java.util.concurrent.AbstractExecutorService   
      java.util.concurrent.ThreadPoolExecutor

All Implemented Interfaces:

Executor , ExecutorService

Direct Known Subclasses:

ScheduledThreadPoolExecutor

public class ThreadPoolExecutor
extends AbstractExecutorService   

An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.

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 ThreadPoolExecutor also maintains some basic statistics, such as the number of completed tasks.

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 Executors factory methods Executors.newCachedThreadPool() (unbounded thread pool, with automatic thread reclamation), Executors.newFixedThreadPool(int) (fixed size thread pool) and 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:

Core and maximum pool sizes

A ThreadPoolExecutor will automatically adjust the pool size (see getPoolSize() ) according to the bounds set by corePoolSize (see getCorePoolSize() ) and maximumPoolSize (see getMaximumPoolSize() ). When a new task is submitted in method execute(java.lang.Runnable) , and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle. If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full. By setting corePoolSize and maximumPoolSize the same, you create a fixed-size thread pool. By setting maximumPoolSize to an essentially unbounded value such as Integer.MAX_VALUE, you allow the pool to accommodate an arbitrary number of concurrent tasks. Most typically, core and maximum pool sizes are set only upon construction, but they may also be changed dynamically using setCorePoolSize(int) and setMaximumPoolSize(int) .

On-demand construction

By default, even core threads are initially created and started only when needed by new tasks, but this can be overridden dynamically using method prestartCoreThread() or prestartAllCoreThreads() .

Creating new threads

New threads are created using a ThreadFactory . If not otherwise specified, a Executors.defaultThreadFactory() is used, that creates threads to all be in the same ThreadGroup and with the same NORM_PRIORITY priority and non-daemon status. By supplying a different ThreadFactory, you can alter the thread's name, thread group, priority, daemon status, etc. If a ThreadFactory fails to create a thread when asked by returning null from newThread, the executor will continue, but might not be able to execute any tasks.

Keep-alive times

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 getKeepAliveTime(java.util.concurrent.TimeUnit) ). This provides a means of reducing resource consumption when the pool is not being actively used. If the pool becomes more active later, new threads will be constructed. This parameter can also be changed dynamically using method setKeepAliveTime(long, java.util.concurrent.TimeUnit) . Using a value of Long.MAX_VALUE TimeUnit.NANOSECONDS effectively disables idle threads from ever terminating prior to shut down.

Queuing

Any BlockingQueue may be used to transfer and hold submitted tasks. The use of this queue interacts with pool sizing:

• If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.
• If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.
• If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.

There are three general strategies for queuing:

1. Direct handoffs. A good default choice for a work queue is a 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.
2. Unbounded queues. Using an unbounded queue (for example a 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.
3. Bounded queues. A bounded queue (for example, an 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.

Rejected tasks

New tasks submitted in method execute(java.lang.Runnable) will be rejected when the Executor has been shut down, and also when the Executor uses finite bounds for both maximum threads and work queue capacity, and is saturated. In either case, the execute method invokes the RejectedExecutionHandler.rejectedExecution(java.lang.Runnable, java.util.concurrent.ThreadPoolExecutor) method of its RejectedExecutionHandler . Four predefined handler policies are provided:

1. In the default ThreadPoolExecutor.AbortPolicy , the handler throws a runtime RejectedExecutionException upon rejection.
2. In ThreadPoolExecutor.CallerRunsPolicy , the thread that invokes execute itself runs the task. This provides a simple feedback control mechanism that will slow down the rate that new tasks are submitted.
3. In ThreadPoolExecutor.DiscardPolicy , a task that cannot be executed is simply dropped.
4. In 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.)

It is possible to define and use other kinds of RejectedExecutionHandler classes. Doing so requires some care especially when policies are designed to work only under particular capacity or queuing policies.

Hook methods

This class provides protected overridable beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.

If hook or callback methods throw exceptions, internal worker threads may in turn fail and abruptly terminate.

Queue maintenance

Method 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, remove(java.lang.Runnable) and purge() are available to assist in storage reclamation when large numbers of queued tasks become cancelled.

Extension example. Most extensions of this class override one or more of the protected hook methods. For example, here is a subclass that adds a simple pause/resume feature:

 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();
     }
   }
 }
 

Since:

1.5

Nested Class Summary

static class	ThreadPoolExecutor.AbortPolicy A handler for rejected tasks that throws a RejectedExecutionException.
static class	ThreadPoolExecutor.CallerRunsPolicy A handler for rejected tasks that runs the rejected task directly in the calling thread of the execute method, unless the executor has been shut down, in which case the task is discarded.
static class	ThreadPoolExecutor.DiscardOldestPolicy A handler for rejected tasks that discards the oldest unhandled request and then retries execute, unless the executor is shut down, in which case the task is discarded.
static class	ThreadPoolExecutor.DiscardPolicy A handler for rejected tasks that silently discards the rejected task.

Constructor Summary

ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue <Runnable > workQueue)
Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory and handler.

ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue <Runnable > workQueue, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters.

ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue <Runnable > workQueue, ThreadFactory threadFactory)
Creates a new ThreadPoolExecutor with the given initial parameters.

ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue <Runnable > workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters.

Method Summary

protected void	afterExecute (Runnable r, Throwable t) Method invoked upon completion of execution of the given Runnable.
boolean	awaitTermination (long timeout, TimeUnit unit) Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
protected void	beforeExecute (Thread t, Runnable r) Method invoked prior to executing the given Runnable in the given thread.
void	execute (Runnable command) Executes the given task sometime in the future.
protected void	finalize () Invokes shutdown when this executor is no longer referenced.
int	getActiveCount () Returns the approximate number of threads that are actively executing tasks.
long	getCompletedTaskCount () Returns the approximate total number of tasks that have completed execution.
int	getCorePoolSize () Returns the core number of threads.
long	getKeepAliveTime (TimeUnit unit) 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.
int	getLargestPoolSize () Returns the largest number of threads that have ever simultaneously been in the pool.
int	getMaximumPoolSize () Returns the maximum allowed number of threads.
int	getPoolSize () Returns the current number of threads in the pool.
BlockingQueue <Runnable >	getQueue () Returns the task queue used by this executor.
RejectedExecutionHandler	getRejectedExecutionHandler () Returns the current handler for unexecutable tasks.
long	getTaskCount () Returns the approximate total number of tasks that have been scheduled for execution.
ThreadFactory	getThreadFactory () Returns the thread factory used to create new threads.
boolean	isShutdown () Returns true if this executor has been shut down.
boolean	isTerminated () Returns true if all tasks have completed following shut down.
boolean	isTerminating () Returns true if this executor is in the process of terminating after shutdown or shutdownNow but has not completely terminated.
int	prestartAllCoreThreads () Starts all core threads, causing them to idly wait for work.
boolean	prestartCoreThread () Starts a core thread, causing it to idly wait for work.
void	purge () Tries to remove from the work queue all Future tasks that have been cancelled.
boolean	remove (Runnable task) 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.
void	setCorePoolSize (int corePoolSize) Sets the core number of threads.
void	setKeepAliveTime (long time, TimeUnit unit) Sets the time limit for which threads may remain idle before being terminated.
void	setMaximumPoolSize (int maximumPoolSize) Sets the maximum allowed number of threads.
void	setRejectedExecutionHandler (RejectedExecutionHandler handler) Sets a new handler for unexecutable tasks.
void	setThreadFactory (ThreadFactory threadFactory) Sets the thread factory used to create new threads.
void	shutdown () Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
List <Runnable >	shutdownNow () Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.
protected void	terminated () Method invoked when the Executor has terminated.

Methods inherited from class java.util.concurrent.AbstractExecutorService

invokeAll , invokeAll , invokeAny , invokeAny , submit