java.lang.Object | ||
↳ | java.util.concurrent.AbstractExecutorService | |
↳ | java.util.concurrent.ThreadPoolExecutor |
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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:
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)
. prestartCoreThread()
or prestartAllCoreThreads()
. You probably want to prestart threads if
you construct the pool with a non-empty queue. 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.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. BlockingQueue
may be used to transfer and hold
submitted tasks. The use of this queue interacts with pool sizing:
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)
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:
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
classes. Doing so requires some care
especially when policies are designed to work only under particular
capacity or queuing policies. 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.
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.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();
}
}
}}
Nested Classes | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
ThreadPoolExecutor.AbortPolicy |
A handler for rejected tasks that throws a
RejectedExecutionException .
|
||||||||||
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.
|
||||||||||
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.
|
||||||||||
ThreadPoolExecutor.DiscardPolicy | A handler for rejected tasks that silently discards the rejected task. |
Public Constructors | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Creates a new
ThreadPoolExecutor with the given initial
parameters and default thread factory and rejected execution handler.
| |||||||||||
Creates a new
ThreadPoolExecutor with the given initial
parameters and default rejected execution handler.
| |||||||||||
Creates a new
ThreadPoolExecutor with the given initial
parameters and default thread factory.
| |||||||||||
Creates a new
ThreadPoolExecutor with the given initial
parameters.
|
Public Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
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.
| |||||||||||
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.
| |||||||||||
Executes the given task sometime in the future.
| |||||||||||
Returns the approximate number of threads that are actively
executing tasks.
| |||||||||||
Returns the approximate total number of tasks that have
completed execution.
| |||||||||||
Returns the core number of threads.
| |||||||||||
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.
| |||||||||||
Returns the largest number of threads that have ever
simultaneously been in the pool.
| |||||||||||
Returns the maximum allowed number of threads.
| |||||||||||
Returns the current number of threads in the pool.
| |||||||||||
Returns the task queue used by this executor.
| |||||||||||
Returns the current handler for unexecutable tasks.
| |||||||||||
Returns the approximate total number of tasks that have ever been
scheduled for execution.
| |||||||||||
Returns the thread factory used to create new threads.
| |||||||||||
Returns true if this executor is in the process of terminating
after
shutdown() or shutdownNow() but has not
completely terminated.
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Starts all core threads, causing them to idly wait for work.
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Starts a core thread, causing it to idly wait for work.
| |||||||||||
Tries to remove from the work queue all
Future
tasks that have been cancelled.
| |||||||||||
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.
| |||||||||||
Sets the core number of threads.
| |||||||||||
Sets the time limit for which threads may remain idle before
being terminated.
| |||||||||||
Sets the maximum allowed number of threads.
| |||||||||||
Sets a new handler for unexecutable tasks.
| |||||||||||
Sets the thread factory used to create new threads.
| |||||||||||
Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted.
| |||||||||||
Attempts to stop all actively executing tasks, halts the
processing of waiting tasks, and returns a list of the tasks
that were awaiting execution.
| |||||||||||
Returns a string identifying this pool, as well as its state,
including indications of run state and estimated worker and
task counts.
|
Protected Methods | |||||||||||
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Method invoked upon completion of execution of the given Runnable.
| |||||||||||
Method invoked prior to executing the given Runnable in the
given thread.
| |||||||||||
Invokes
shutdown when this executor is no longer
referenced and it has no threads.
| |||||||||||
Method invoked when the Executor has terminated.
|
[Expand]
Inherited Methods | |||||||||||
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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.
corePoolSize | the number of threads to keep in the pool, even
if they are idle, unless allowCoreThreadTimeOut is set |
---|---|
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. |
IllegalArgumentException | if one of the following holds:corePoolSize < 0 keepAliveTime < 0 maximumPoolSize <= 0 maximumPoolSize < corePoolSize |
---|---|
NullPointerException | if workQueue is null
|
Creates a new ThreadPoolExecutor
with the given initial
parameters and default rejected execution handler.
corePoolSize | the number of threads to keep in the pool, even
if they are idle, unless allowCoreThreadTimeOut is set |
---|---|
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 |
IllegalArgumentException | if one of the following holds:corePoolSize < 0 keepAliveTime < 0 maximumPoolSize <= 0 maximumPoolSize < corePoolSize |
---|---|
NullPointerException | if workQueue
or threadFactory is null
|
Creates a new ThreadPoolExecutor
with the given initial
parameters and default thread factory.
corePoolSize | the number of threads to keep in the pool, even
if they are idle, unless allowCoreThreadTimeOut is set |
---|---|
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 |
IllegalArgumentException | if one of the following holds:corePoolSize < 0 keepAliveTime < 0 maximumPoolSize <= 0 maximumPoolSize < corePoolSize |
---|---|
NullPointerException | if workQueue
or handler is null
|
Creates a new ThreadPoolExecutor
with the given initial
parameters.
corePoolSize | the number of threads to keep in the pool, even
if they are idle, unless allowCoreThreadTimeOut is set |
---|---|
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 |
IllegalArgumentException | if one of the following holds:corePoolSize < 0 keepAliveTime < 0 maximumPoolSize <= 0 maximumPoolSize < corePoolSize |
---|---|
NullPointerException | if workQueue
or threadFactory or handler is null
|
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.
value | true if should time out, else false |
---|
IllegalArgumentException | if value is true
and the current keep-alive time is not greater than zero |
---|
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.
true
if core threads are allowed to time out,
else false
InterruptedException |
---|
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
.
command | the task to execute |
---|
RejectedExecutionException | at discretion of
RejectedExecutionHandler , if the task
cannot be accepted for execution |
---|---|
NullPointerException | if command is null
|
Returns the approximate number of threads that are actively executing tasks.
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 core number of threads.
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.
unit | the desired time unit of the result |
---|
Returns the largest number of threads that have ever simultaneously been in the pool.
Returns the maximum allowed number of threads.
Returns the current number of threads in the pool.
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 current handler for unexecutable tasks.
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 the thread factory used to create new threads.
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.
true
if terminating but not yet terminated
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.
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.
true
if a thread was started
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.
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.
task | the task to remove |
---|
true
if the task was removed
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.
corePoolSize | the new core size |
---|
IllegalArgumentException | if corePoolSize < 0 |
---|
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.
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 |
IllegalArgumentException | if time less than zero or
if time is zero and allowsCoreThreadTimeOut |
---|
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.
maximumPoolSize | the new maximum |
---|
IllegalArgumentException | if the new maximum is
less than or equal to zero, or
less than the core pool size |
---|
Sets a new handler for unexecutable tasks.
handler | the new handler |
---|
NullPointerException | if handler is null |
---|
Sets the thread factory used to create new threads.
threadFactory | the new thread factory |
---|
NullPointerException | if threadFactory is null |
---|
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.
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 a string identifying this pool, as well as its state, including indications of run state and estimated worker and task counts.
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);
}
}}
r | the runnable that has completed |
---|---|
t | the exception that caused termination, or null if execution completed normally |
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.
t | the thread that will run task r |
---|---|
r | the task that will be executed |
Invokes shutdown
when this executor is no longer
referenced and it has no threads.
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.