java.util.concurrent

Class ThreadPoolExecutor

Implemented Interfaces:
Executor, ExecutorService
Known Direct 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 (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:

ThreadPoolExecutorgetPoolSize()getCorePoolSize()getMaximumPoolSize()execute(Runnable)Integer.MAX_VALUEsetCorePoolSize(int)setMaximumPoolSize(int)
prestartCoreThread()prestartAllCoreThreads()
ThreadFactoryExecutors.defaultThreadFactory()ThreadGroupNORM_PRIORITYThreadFactorynewThread
getKeepAliveTime(TimeUnit)setKeepAliveTime(long,TimeUnit)Long.MAX_VALUETimeUnitallowCoreThreadTimeOut
BlockingQueue
  • 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.
  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.
execute(Runnable)rejectedexecuteRejectedExecutionHandler.rejectedExecution(Runnable,ThreadPoolExecutor)RejectedExecutionHandler
  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.)
RejectedExecutionHandler
protectedbeforeExecute(Thread,Runnable)afterExecute(Runnable,Throwable)terminated()

getQueue()remove(Runnable)purge()
ANDshutdownshutdown()allowCoreThreadTimeOut

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 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 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 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 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.
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)
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
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
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

Future submit, Future submit, List> invokeAll, List> invokeAll, RunnableFuture newTaskFor, RunnableFuture newTaskFor, T invokeAny, T invokeAny, submit

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ThreadPoolExecutor

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue workQueue)
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 - 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

ThreadPoolExecutor

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

ThreadPoolExecutor

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue workQueue,
                          ThreadFactory threadFactory)
Creates a new ThreadPoolExecutor with the given initial parameters and default rejected execution handler.
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.

ThreadPoolExecutor

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue 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.

Method Details

afterExecute

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 RuntimeException or Error that caused execution to terminate abruptly.

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.

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.

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

allowCoreThreadTimeOut

public 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. 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 - true if should time out, else false
Throws:
IllegalArgumentException - if value is true and the current keep-alive time is not greater than zero.
Since:
1.6

allowsCoreThreadTimeOut

public 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. 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:
true if core threads are allowed to time out, else false
Since:
1.6

awaitTermination

public boolean awaitTermination(long timeout,
                                TimeUnit unit)
            throws InterruptedException
Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
Specified by:
awaitTermination in interface ExecutorService
Parameters:
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
true if this executor terminated and false if the timeout elapsed before termination
Throws:
InterruptedException - if interrupted while waiting

beforeExecute

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.

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 - the thread that will run task r.
r - the task that will be executed.

execute

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.
Specified by:
execute in interface Executor
Parameters:
command - the task to execute
Throws:
RejectedExecutionException - at discretion of RejectedExecutionHandler, if task cannot be accepted for execution
NullPointerException - if command is null

finalize

protected void finalize()
Invokes shutdown when this executor is no longer referenced.
Overrides:
finalize in interface Object

getActiveCount

public int getActiveCount()
Returns the approximate number of threads that are actively executing tasks.
Returns:
the number of threads

getCompletedTaskCount

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

getCorePoolSize

public int getCorePoolSize()
Returns the core number of threads.
Returns:
the core number of threads

getKeepAliveTime

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

getLargestPoolSize

public int getLargestPoolSize()
Returns the largest number of threads that have ever simultaneously been in the pool.
Returns:
the number of threads

getMaximumPoolSize

public int getMaximumPoolSize()
Returns the maximum allowed number of threads.
Returns:
the maximum allowed number of threads

getPoolSize

public int getPoolSize()
Returns the current number of threads in the pool.
Returns:
the number of threads

getQueue

public BlockingQueue 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

getRejectedExecutionHandler

public RejectedExecutionHandler getRejectedExecutionHandler()
Returns the current handler for unexecutable tasks.
Returns:
the current handler

getTaskCount

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

getThreadFactory

public ThreadFactory getThreadFactory()
Returns the thread factory used to create new threads.
Returns:
the current thread factory

isShutdown

public boolean isShutdown()
Returns true if this executor has been shut down.
Specified by:
isShutdown in interface ExecutorService
Returns:
true if this executor has been shut down

isTerminated

public boolean isTerminated()
Returns true if all tasks have completed following shut down. Note that isTerminated is never true unless either shutdown or shutdownNow was called first.
Specified by:
isTerminated in interface ExecutorService
Returns:
true if all tasks have completed following shut down

isTerminating

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.

prestartAllCoreThreads

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.

prestartCoreThread

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

purge

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.

remove

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

setCorePoolSize

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

setKeepAliveTime

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 or if time is zero and allowsCoreThreadTimeOut

setMaximumPoolSize

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 the new maximum is less than or equal to zero, or less than the core pool size

setRejectedExecutionHandler

public void setRejectedExecutionHandler(RejectedExecutionHandler handler)
Sets a new handler for unexecutable tasks.
Parameters:
handler - the new handler
Throws:
NullPointerException - if handler is null

setThreadFactory

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

shutdown

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.
Specified by:
shutdown in interface ExecutorService
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.

shutdownNow

public List 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.

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

Specified by:
shutdownNow in interface ExecutorService
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.

terminated

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.