Android APIs
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 new tasks arrive, but this can be overridden dynamically using method prestartCoreThread() or prestartAllCoreThreads(). You probably want to prestart threads if you construct the pool with a non-empty queue.
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. 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(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 NANOSECONDS effectively disables idle threads from ever terminating prior to shut down. By default, the keep-alive policy applies only when there are more than corePoolSize threads. But method 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.
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 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.
  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.

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(Runnable) and purge() are available to assist in storage reclamation when large numbers of queued tasks become cancelled.
Finalization
A pool that is no longer referenced in a program AND has no remaining threads will be shutdown automatically. If you would like to ensure that unreferenced pools are reclaimed even if users forget to call 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 allowCoreThreadTimeOut(boolean).

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
class ThreadPoolExecutor.AbortPolicy A handler for rejected tasks that throws a RejectedExecutionException
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. 
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. 
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 rejected execution handler.
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)
Creates a new ThreadPoolExecutor with the given initial parameters and default rejected execution handler.
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory.
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
void allowCoreThreadTimeOut(boolean value)
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.
boolean allowsCoreThreadTimeOut()
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.
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 that 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 ever 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.
String toString()
Returns a string identifying this pool, as well as its state, including indications of run state and estimated worker and task counts.
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 and it has no threads.
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.ExecutorService
From interface java.util.concurrent.Executor

Public Constructors

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

Added in API level 1

Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory and rejected execution handler. It may be more convenient to use one of the Executors factory methods instead of this general purpose constructor.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize int: the maximum number of threads to allow in the pool
keepAliveTime long: 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 TimeUnit: the time unit for the keepAliveTime argument
workQueue BlockingQueue: 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 one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException if workQueue is null

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

Added in API level 1

Creates a new ThreadPoolExecutor with the given initial parameters and default rejected execution handler.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize int: the maximum number of threads to allow in the pool
keepAliveTime long: 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 TimeUnit: the time unit for the keepAliveTime argument
workQueue BlockingQueue: 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 ThreadFactory: the factory to use when the executor creates a new thread
Throws
IllegalArgumentException if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException if workQueue or threadFactory is null

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

Added in API level 1

Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize int: the maximum number of threads to allow in the pool
keepAliveTime long: 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 TimeUnit: the time unit for the keepAliveTime argument
workQueue BlockingQueue: 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 RejectedExecutionHandler: the handler to use when execution is blocked because the thread bounds and queue capacities are reached
Throws
IllegalArgumentException if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException if workQueue or handler is null

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

Added in API level 1

Creates a new ThreadPoolExecutor with the given initial parameters.

Parameters
corePoolSize int: the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize int: the maximum number of threads to allow in the pool
keepAliveTime long: 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 TimeUnit: the time unit for the keepAliveTime argument
workQueue BlockingQueue: 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 ThreadFactory: the factory to use when the executor creates a new thread
handler RejectedExecutionHandler: the handler to use when execution is blocked because the thread bounds and queue capacities are reached
Throws
IllegalArgumentException if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException if workQueue or threadFactory or handler is null

Public Methods

public void allowCoreThreadTimeOut (boolean value)

Added in API level 9

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 true. This method should in general be called before the pool is actively used.

Parameters
value boolean: true if should time out, else false
Throws
IllegalArgumentException if value is true and the current keep-alive time is not greater than zero

public boolean allowsCoreThreadTimeOut ()

Added in API level 9

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.

Returns
boolean true if core threads are allowed to time out, else false

public boolean awaitTermination (long timeout, TimeUnit unit)

Added in API level 1

Parameters
timeout long
unit TimeUnit
Returns
boolean
Throws
InterruptedException

public void execute (Runnable command)

Added in API level 1

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

public int getActiveCount ()

Added in API level 1

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

Returns
int the number of threads

public long getCompletedTaskCount ()

Added in API level 1

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
long the number of tasks

public int getCorePoolSize ()

Added in API level 1

Returns the core number of threads.

Returns
int the core number of threads

public long getKeepAliveTime (TimeUnit unit)

Added in API level 1

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.

Parameters
unit TimeUnit: the desired time unit of the result
Returns
long the time limit

public int getLargestPoolSize ()

Added in API level 1

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

Returns
int the number of threads

public int getMaximumPoolSize ()

Added in API level 1

Returns the maximum allowed number of threads.

Returns
int the maximum allowed number of threads

public int getPoolSize ()

Added in API level 1

Returns the current number of threads in the pool.

Returns
int the number of threads

public BlockingQueue<Runnable> getQueue ()

Added in API level 1

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
BlockingQueue<Runnable> the task queue

public RejectedExecutionHandler getRejectedExecutionHandler ()

Added in API level 1

Returns the current handler for unexecutable tasks.

Returns
RejectedExecutionHandler the current handler

public long getTaskCount ()

Added in API level 1

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.

Returns
long the number of tasks

public ThreadFactory getThreadFactory ()

Added in API level 1

Returns the thread factory used to create new threads.

Returns
ThreadFactory the current thread factory

public boolean isShutdown ()

Added in API level 1

Returns
boolean

public boolean isTerminated ()

Added in API level 1

Returns
boolean

public boolean isTerminating ()

Added in API level 1

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
boolean true if terminating but not yet terminated

public int prestartAllCoreThreads ()

Added in API level 1

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
int the number of threads started

public boolean prestartCoreThread ()

Added in API level 1

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
boolean true if a thread was started

public void purge ()

Added in API level 1

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)

Added in API level 1

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 Runnable: the task to remove
Returns
boolean true if the task was removed

public void setCorePoolSize (int corePoolSize)

Added in API level 1

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 int: the new core size
Throws
IllegalArgumentException if corePoolSize < 0

See also:

public void setKeepAliveTime (long time, TimeUnit unit)

Added in API level 1

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 long: the time to wait. A time value of zero will cause excess threads to terminate immediately after executing tasks.
unit TimeUnit: the time unit of the time argument
Throws
IllegalArgumentException if time less than zero or if time is zero and allowsCoreThreadTimeOut

public void setMaximumPoolSize (int maximumPoolSize)

Added in API level 1

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 int: the new maximum
Throws
IllegalArgumentException if the new maximum is less than or equal to zero, or less than the core pool size

public void setRejectedExecutionHandler (RejectedExecutionHandler handler)

Added in API level 1

Sets a new handler for unexecutable tasks.

Parameters
handler RejectedExecutionHandler: the new handler
Throws
NullPointerException if handler is null

public void setThreadFactory (ThreadFactory threadFactory)

Added in API level 1

Sets the thread factory used to create new threads.

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

See also:

public void shutdown ()

Added in API level 1

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.

This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

public List<Runnable> shutdownNow ()

Added in API level 1

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.

This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. This implementation cancels tasks via interrupt(), so any task that fails to respond to interrupts may never terminate.

Returns
List<Runnable>

public String toString ()

Added in API level 1

Returns a string identifying this pool, as well as its state, including indications of run state and estimated worker and task counts.

Returns
String a string identifying this pool, as well as its state

Protected Methods

protected void afterExecute (Runnable r, Throwable t)

Added in API level 1

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 RuntimeException or Error that caused execution to terminate abruptly.

This implementation does nothing, but may be customized in subclasses. Note: To properly nest multiple overridings, subclasses should generally invoke super.afterExecute at the beginning of this method.

Note: When actions are enclosed in tasks (such as FutureTask) either explicitly or via methods such as submit, these task objects catch and maintain computational exceptions, and so they do not cause abrupt termination, and the internal exceptions are not passed to this method. If you would like to trap both kinds of failures in this method, you can further probe for such cases, as in this sample subclass that prints either the direct cause or the underlying exception if a task has been aborted:

 class ExtendedExecutor extends ThreadPoolExecutor {
   // ...
   protected void afterExecute(Runnable r, Throwable t) {
     super.afterExecute(r, t);
     if (t == null && r instanceof Future) {
       try {
         Object result = ((Future) r).get();
        catch (CancellationException ce) {
           t = ce;
       } catch (ExecutionException ee) {
           t = ee.getCause();
       } catch (InterruptedException ie) {
           Thread.currentThread().interrupt(); // ignore/reset
       }
     }
     if (t != null)
       System.out.println(t);
   }
 }}

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

protected void beforeExecute (Thread t, Runnable r)

Added in API level 1

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.

This implementation does nothing, but may be customized in subclasses. Note: To properly nest multiple overridings, subclasses should generally invoke super.beforeExecute at the end of this method.

Parameters
t Thread: the thread that will run task r
r Runnable: the task that will be executed

protected void finalize ()

Added in API level 1

Invokes shutdown when this executor is no longer referenced and it has no threads.

protected void terminated ()

Added in API level 1

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.