|
Java™ Platform Standard Ed. 6 |
|||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Object java.util.concurrent.AbstractExecutorService java.util.concurrent.ThreadPoolExecutor
public class ThreadPoolExecutor
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(int)
(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()
)
according to the bounds set by corePoolSize
(see getCorePoolSize()
)
and
maximumPoolSize
(see getMaximumPoolSize()
).
When a new task is submitted in method execute(java.lang.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
Executors.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(java.util.concurrent.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, java.util.concurrent.TimeUnit)
. Using a value
of Long.MAX_VALUE TimeUnit.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 corePoolSizeThreads. 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(java.lang.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
RejectedExecutionHandler.rejectedExecution(java.lang.Runnable, java.util.concurrent.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. beforeExecute(java.lang.Thread, java.lang.Runnable)
and afterExecute(java.lang.Runnable, java.lang.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(java.lang.Runnable)
and purge()
are available to assist in storage
reclamation when large numbers of queued tasks become
cancelled.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 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<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,
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)
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,
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 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()
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<Runnable> |
shutdownNow()
Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. |
protected void |
terminated()
Method invoked when the Executor has terminated. |
Methods inherited from class java.util.concurrent.AbstractExecutorService |
---|
invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskFor, submit, submit, submit |
Methods inherited from class java.lang.Object |
---|
clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
---|
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)
Executors
factory
methods instead of this general purpose constructor.
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.
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 nullpublic ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)
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.
IllegalArgumentException
- if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException
- if workQueue
or threadFactory are null.public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)
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.
IllegalArgumentException
- if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException
- if workQueue
or handler are null.public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)
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.
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 Detail |
---|
public void execute(Runnable command)
command
- the task to execute
RejectedExecutionException
- at discretion of
RejectedExecutionHandler, if task cannot be accepted
for execution
NullPointerException
- if command is nullpublic void shutdown()
SecurityException
- if a security manager exists and
shutting down this ExecutorService may manipulate threads that
the caller is not permitted to modify because it does not hold
RuntimePermission
("modifyThread"),
or the security manager's checkAccess method denies access.public List<Runnable> shutdownNow()
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.
SecurityException
- if a security manager exists and
shutting down this ExecutorService may manipulate threads that
the caller is not permitted to modify because it does not hold
RuntimePermission
("modifyThread"),
or the security manager's checkAccess method denies access.public boolean isShutdown()
ExecutorService
public boolean isTerminating()
public boolean isTerminated()
ExecutorService
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
ExecutorService
timeout
- the maximum time to waitunit
- the time unit of the timeout argument
InterruptedException
- if interrupted while waitingprotected void finalize()
finalize
in class Object
public void setThreadFactory(ThreadFactory threadFactory)
threadFactory
- the new thread factory
NullPointerException
- if threadFactory is nullgetThreadFactory()
public ThreadFactory getThreadFactory()
setThreadFactory(java.util.concurrent.ThreadFactory)
public void setRejectedExecutionHandler(RejectedExecutionHandler handler)
handler
- the new handler
NullPointerException
- if handler is nullgetRejectedExecutionHandler()
public RejectedExecutionHandler getRejectedExecutionHandler()
setRejectedExecutionHandler(java.util.concurrent.RejectedExecutionHandler)
public void setCorePoolSize(int corePoolSize)
corePoolSize
- the new core size
IllegalArgumentException
- if corePoolSize
less than zerogetCorePoolSize()
public int getCorePoolSize()
setCorePoolSize(int)
public boolean prestartCoreThread()
public int prestartAllCoreThreads()
public boolean allowsCoreThreadTimeOut()
public void allowCoreThreadTimeOut(boolean value)
value
- true if should time out, else false
IllegalArgumentException
- if value is true
and the current keep-alive time is not greater than zero.public void setMaximumPoolSize(int maximumPoolSize)
maximumPoolSize
- the new maximum
IllegalArgumentException
- if the new maximum is
less than or equal to zero, or
less than the core pool sizegetMaximumPoolSize()
public int getMaximumPoolSize()
setMaximumPoolSize(int)
public void setKeepAliveTime(long time, TimeUnit unit)
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 allowsCoreThreadTimeOutgetKeepAliveTime(java.util.concurrent.TimeUnit)
public long getKeepAliveTime(TimeUnit unit)
unit
- the desired time unit of the result
setKeepAliveTime(long, java.util.concurrent.TimeUnit)
public BlockingQueue<Runnable> getQueue()
public boolean remove(Runnable task)
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
public void purge()
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 int getPoolSize()
public int getActiveCount()
public int getLargestPoolSize()
public long getTaskCount()
public long getCompletedTaskCount()
protected void beforeExecute(Thread t, Runnable r)
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.protected void afterExecute(Runnable r, Throwable t)
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.
r
- the runnable that has completed.t
- the exception that caused termination, or null if
execution completed normally.protected void terminated()
|
Java™ Platform Standard Ed. 6 |
|||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
Copyright © 1993, 2010, Oracle and/or its affiliates. All rights reserved.