public class

ThreadPoolExecutor

extends AbstractExecutorService
java.lang.Object
   ↳ java.util.concurrent.AbstractExecutorService
     ↳ java.util.concurrent.ThreadPoolExecutor
Known Direct Subclasses

Class Overview

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 newCachedThreadPool() (unbounded thread pool, with automatic thread reclamation), newFixedThreadPool(int) (fixed size thread pool) and 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(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 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.
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(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, 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(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 rejectedExecution(Runnable, 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(Thread, Runnable) and afterExecute(Runnable, Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment, for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.
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(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();
     }
   }
 }
 

Summary

Nested Classes
public class ThreadPoolExecutor.AbortPolicy A handler for rejected tasks that throws a RejectedExecutionException
public 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. 
public 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. 
public class ThreadPoolExecutor.DiscardPolicy A handler for rejected tasks that silently discards the rejected task. 
Public Constructors
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, ThreadFactory threadFactory)
Creates a new ThreadPoolExecutor with the given initial parameters.
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, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters.
Public Methods
boolean awaitTermination(long timeout, TimeUnit unit)
void execute(Runnable command)
Executes the given task sometime in the future.
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()
boolean isTerminated()
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 Methods
void afterExecute(Runnable r, Throwable t)
Method invoked upon completion of execution of the given Runnable.
void beforeExecute(Thread t, Runnable r)
Method invoked prior to executing the given Runnable in the given thread.
void finalize()
Invokes shutdown when this executor is no longer referenced.
void terminated()
Method invoked when the Executor has terminated.
[Expand]
Inherited Methods
From class java.util.concurrent.AbstractExecutorService
From class java.lang.Object
From interface java.util.concurrent.Executor
From interface java.util.concurrent.ExecutorService

Public Constructors

public 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. It may be more convenient to use one of the Executors factory methods instead of this general purpose constructor.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle.
maximumPoolSize the maximum number of threads to allow in the pool.
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.
unit the time unit for the keepAliveTime argument.
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute 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.
NullPointerException if workQueue is null

public ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)

Creates a new ThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle.
maximumPoolSize the maximum number of threads to allow in the pool.
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.
unit the time unit for the keepAliveTime argument.
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
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.
NullPointerException if workQueue or threadFactory are null.

public ThreadPoolExecutor (int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)

Creates a new ThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle.
maximumPoolSize the maximum number of threads to allow in the pool.
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.
unit the time unit for the keepAliveTime argument.
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
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.
NullPointerException if workQueue or handler are null.

public 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.

Parameters
corePoolSize the number of threads to keep in the pool, even if they are idle.
maximumPoolSize the maximum number of threads to allow in the pool.
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.
unit the time unit for the keepAliveTime argument.
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
threadFactory the factory to use when the executor creates a new thread.
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.
NullPointerException if workQueue or threadFactory or handler are null.

Public Methods

public boolean awaitTermination (long timeout, TimeUnit unit)

public void execute (Runnable command)

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 RejectedExecutionHandler.

Parameters
command the task to execute
Throws
RejectedExecutionException at discretion of RejectedExecutionHandler, if task cannot be accepted for execution
NullPointerException if command is null

public int getActiveCount ()

Returns the approximate number of threads that are actively executing tasks.

Returns
  • the number of threads

public long getCompletedTaskCount ()

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.

Returns
  • the number of tasks

public int getCorePoolSize ()

Returns the core number of threads.

Returns
  • the core number of threads

public 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.

Parameters
unit the desired time unit of the result
Returns
  • the time limit

public int getLargestPoolSize ()

Returns the largest number of threads that have ever simultaneously been in the pool.

Returns
  • the number of threads

public int getMaximumPoolSize ()

Returns the maximum allowed number of threads.

Returns
  • the maximum allowed number of threads

public int getPoolSize ()

Returns the current number of threads in the pool.

Returns
  • the number of threads

public BlockingQueue<Runnable> getQueue ()

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.

Returns
  • the task queue

public RejectedExecutionHandler getRejectedExecutionHandler ()

Returns the current handler for unexecutable tasks.

Returns
  • the current handler

public long getTaskCount ()

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.

Returns
  • the number of tasks

public ThreadFactory getThreadFactory ()

Returns the thread factory used to create new threads.

Returns
  • the current thread factory

public boolean isShutdown ()

public boolean isTerminated ()

public boolean isTerminating ()

Returns true if this executor is in the process of terminating after shutdown or shutdownNow but has not completely terminated. This method may be useful for debugging. A return of true reported a sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption, causing this executor not to properly terminate.

Returns
  • true if terminating but not yet terminated.

public int prestartAllCoreThreads ()

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.

Returns
  • the number of threads started.

public boolean prestartCoreThread ()

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 false if all core threads have already been started.

Returns
  • true if a thread was started

public void purge ()

Tries to remove from the work queue all 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 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.

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 submit might be converted into a form that maintains Future status. However, in such cases, method purge() may be used to remove those Futures that have been cancelled.

Parameters
task the task to remove
Returns
  • true if the task was removed

public void setCorePoolSize (int corePoolSize)

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.

Parameters
corePoolSize the new core size
Throws
IllegalArgumentException if corePoolSize less than zero

public void setKeepAliveTime (long time, TimeUnit unit)

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.

Parameters
time the time to wait. A time value of zero will cause excess threads to terminate immediately after executing tasks.
unit the time unit of the time argument
Throws
IllegalArgumentException if time less than zero

public void setMaximumPoolSize (int maximumPoolSize)

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.

Parameters
maximumPoolSize the new maximum
Throws
IllegalArgumentException if maximumPoolSize less than zero or the core pool size

public void setRejectedExecutionHandler (RejectedExecutionHandler handler)

Sets a new handler for unexecutable tasks.

Parameters
handler the new handler
Throws
NullPointerException if handler is null

public void setThreadFactory (ThreadFactory threadFactory)

Sets the thread factory used to create new threads.

Parameters
threadFactory the new thread factory
Throws
NullPointerException if threadFactory is null

public void shutdown ()

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 RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

public 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.

This implementation cancels tasks via interrupt(), so if any tasks mask or fail to respond to interrupts, they may never terminate.

Returns
  • 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 RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

Protected Methods

protected void afterExecute (Runnable r, Throwable t)

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 super.afterExecute at the beginning of this method.

Parameters
r the runnable that has completed.
t the exception that caused termination, or null if execution completed normally.

protected void beforeExecute (Thread t, Runnable r)

Method invoked prior to executing the given Runnable in the given thread. This method is invoked by thread t that will execute task r, and may be used to re-initialize ThreadLocals, or to perform logging. Note: To properly nest multiple overridings, subclasses should generally invoke super.beforeExecute at the end of this method.

Parameters
t the thread that will run task r.
r the task that will be executed.

protected void finalize ()

Invokes shutdown when this executor is no longer referenced.

protected void terminated ()

Method invoked when the Executor has terminated. Default implementation does nothing. Note: To properly nest multiple overridings, subclasses should generally invoke super.terminated within this method.