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java.lang.Object
java.util.concurrent.AbstractExecutorService
java.util.concurrent.ThreadPoolExecutor
public class ThreadPoolExecutor
extends AbstractExecutorService
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:
getPoolSize()
getCorePoolSize()
getMaximumPoolSize()
execute(Runnable)
Integer.MAX_VALUEsetCorePoolSize(int)
setMaximumPoolSize(int)
prestartCoreThread()
prestartAllCoreThreads()
ThreadFactory
Executors.defaultThreadFactory()
ThreadGroup
NORM_PRIORITYThreadFactorynewThreadgetKeepAliveTime(TimeUnit)
setKeepAliveTime(long,TimeUnit)
Long.MAX_VALUETimeUnit
allowCoreThreadTimeOut
BlockingQueue
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. 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. 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
ThreadPoolExecutor.AbortPolicy
, the handler throws a
runtime RejectedExecutionException
upon rejection. 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. ThreadPoolExecutor.DiscardPolicy
,
a task that cannot be executed is simply dropped. 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
beforeExecute(Thread,Runnable)
afterExecute(Runnable,Throwable)
terminated()
getQueue()
remove(Runnable)
purge()
shutdown()
allowCoreThreadTimeOut
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(); } } }
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Methods inherited from class java.util.concurrent.AbstractExecutorService | |
Future , Future , List , List , RunnableFuture , RunnableFuture , 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 |
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueworkQueue)
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 theExecutors
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, BlockingQueueworkQueue, 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.
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueworkQueue, 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.
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueworkQueue, 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.
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 asFutureTask
) 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.
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
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
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 waitunit
- 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
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.
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 executionNullPointerException
- 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
- See Also:
setCorePoolSize(int)
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
- See Also:
setKeepAliveTime(long,TimeUnit)
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
- See Also:
setMaximumPoolSize(int)
public int getPoolSize()
Returns the current number of threads in the pool.
- Returns:
- the number of threads
public BlockingQueuegetQueue()
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
- See Also:
setThreadFactory(ThreadFactory)
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
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
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 allFuture
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, methodpurge()
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
- See Also:
getCorePoolSize()
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
- See Also:
getKeepAliveTime(TimeUnit)
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
- See Also:
getMaximumPoolSize()
public void setRejectedExecutionHandler(RejectedExecutionHandler handler)
Sets a new handler for unexecutable tasks.
- Parameters:
handler
- the new handler
- Throws:
NullPointerException
- if handler is null
- See Also:
getRejectedExecutionHandler()
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
- See Also:
getThreadFactory()
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 holdRuntimePermission
("modifyThread"), or the security manager's checkAccess method denies access.
public ListshutdownNow()
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 viaThread.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 holdRuntimePermission
("modifyThread"), or the security manager's checkAccess method denies access.
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.
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