AutoCloseable
, Executor
, ExecutorService
ScheduledThreadPoolExecutor
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(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:
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)
, if fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle. Else if fewer than maximumPoolSize threads are running, a new thread will be created to handle the request 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. Threads should possess the "modifyThread" RuntimePermission
. If worker threads or other threads using the pool do not possess this permission, service may be degraded: configuration changes may not take effect in a timely manner, and a shutdown pool may remain in a state in which termination is possible but not completed.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
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 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 RejectedExecutionHandler.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. This policy is designed only for those rare cases in which task completion is never relied upon. 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.) This policy is rarely acceptable. In nearly all cases, you should also cancel the task to cause an exception in any component waiting for its completion, and/or log the failure, as illustrated in ThreadPoolExecutor.DiscardOldestPolicy
documentation. 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, callback, or BlockingQueue methods throw exceptions, internal worker threads may in turn fail, abruptly terminate, and possibly be replaced.
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.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();
}
}
}
Modifier and Type | Class | Description |
---|---|---|
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 | Description |
---|---|
ThreadPoolExecutor |
Creates a new ThreadPoolExecutor with the given initial parameters, the default thread factory and the default rejected execution handler. |
ThreadPoolExecutor |
Creates a new ThreadPoolExecutor with the given initial parameters and the default thread factory. |
ThreadPoolExecutor |
Creates a new ThreadPoolExecutor with the given initial parameters and the default rejected execution handler. |
ThreadPoolExecutor |
Creates a new ThreadPoolExecutor with the given initial parameters. |
Modifier and Type | Method | Description |
---|---|---|
protected void |
afterExecute |
Method invoked upon completion of execution of the given Runnable. |
void |
allowCoreThreadTimeOut |
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 |
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 |
Method invoked prior to executing the given Runnable in the given thread. |
void |
execute |
Executes the given task sometime in the future. |
protected void |
finalize() |
Deprecated, for removal: This API element is subject to removal in a future version. Finalization has been deprecated for removal. |
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 |
Returns the thread keep-alive time, which is the amount of time that threads 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 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 |
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 |
Sets the core number of threads. |
void |
setKeepAliveTime |
Sets the thread keep-alive time, which is the amount of time that threads may remain idle before being terminated. |
void |
setMaximumPoolSize |
Sets the maximum allowed number of threads. |
void |
setRejectedExecutionHandler |
Sets a new handler for unexecutable tasks. |
void |
setThreadFactory |
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. |
String |
toString() |
Returns a string identifying this pool, as well as its state, including indications of run state and estimated worker and task counts. |
invokeAll, invokeAll, invokeAny, invokeAny, newTaskFor, newTaskFor, submit, submit, submit
clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait
close
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)
ThreadPoolExecutor
with the given initial parameters, the default thread factory and the default 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 setmaximumPoolSize
- the maximum number of threads to allow in the poolkeepAliveTime
- 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
argumentworkQueue
- 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 nullpublic ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)
ThreadPoolExecutor
with the given initial parameters and the default rejected execution handler.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is setmaximumPoolSize
- the maximum number of threads to allow in the poolkeepAliveTime
- 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
argumentworkQueue
- 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 threadIllegalArgumentException
- if one of the following holds:corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException
- if workQueue
or threadFactory
is nullpublic ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)
ThreadPoolExecutor
with the given initial parameters and the default thread factory.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is setmaximumPoolSize
- the maximum number of threads to allow in the poolkeepAliveTime
- 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
argumentworkQueue
- 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 reachedIllegalArgumentException
- if one of the following holds:corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException
- if workQueue
or handler
is nullpublic ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)
ThreadPoolExecutor
with the given initial parameters.corePoolSize
- the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut
is setmaximumPoolSize
- the maximum number of threads to allow in the poolkeepAliveTime
- 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
argumentworkQueue
- 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 threadhandler
- the handler to use when execution is blocked because the thread bounds and queue capacities are reachedIllegalArgumentException
- if one of the following holds:corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
NullPointerException
- if workQueue
or threadFactory
or handler
is nullpublic void execute(Runnable command)
RejectedExecutionHandler
.command
- the task to executeRejectedExecutionException
- at discretion of RejectedExecutionHandler
, if the task cannot be accepted for executionNullPointerException
- if command
is nullpublic void shutdown()
This method does not wait for previously submitted tasks to complete execution. Use awaitTermination
to do that.
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()
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 interrupts tasks via Thread.interrupt()
; 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
true
if this executor has been shut down.true
if this executor has been shut downpublic boolean isTerminating()
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 terminatedpublic boolean isTerminated()
ExecutorService
true
if all tasks have completed following shut down. Note that isTerminated
is never true
unless either shutdown
or shutdownNow
was called first.true
if all tasks have completed following shut downpublic boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
ExecutorService
timeout
- the maximum time to waitunit
- the time unit of the timeout argumenttrue
if this executor terminated and false
if the timeout elapsed before terminationInterruptedException
- if interrupted while waiting@Deprecated(since="9", forRemoval=true) protected void finalize()
Object.finalize()
for background information and details about migration options.Object
finalize
method to dispose of system resources or to perform other cleanup. When running in a Java virtual machine in which finalization has been disabled or removed, the garbage collector will never call finalize()
. In a Java virtual machine in which finalization is enabled, the garbage collector might call finalize
only after an indefinite delay.
The general contract of finalize
is that it is invoked if and when the Java virtual machine has determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, except as a result of an action taken by the finalization of some other object or class which is ready to be finalized. The finalize
method may take any action, including making this object available again to other threads; the usual purpose of finalize
, however, is to perform cleanup actions before the object is irrevocably discarded. For example, the finalize method for an object that represents an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.
The finalize
method of class Object
performs no special action; it simply returns normally. Subclasses of Object
may override this definition.
The Java programming language does not guarantee which thread will invoke the finalize
method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the finalize
method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The finalize
method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize
method causes the finalization of this object to be halted, but is otherwise ignored.
public void setThreadFactory(ThreadFactory threadFactory)
threadFactory
- the new thread factoryNullPointerException
- if threadFactory is nullpublic ThreadFactory getThreadFactory()
public void setRejectedExecutionHandler(RejectedExecutionHandler handler)
handler
- the new handlerNullPointerException
- if handler is nullpublic RejectedExecutionHandler getRejectedExecutionHandler()
public void setCorePoolSize(int corePoolSize)
corePoolSize
- the new core sizeIllegalArgumentException
- if corePoolSize < 0
or corePoolSize
is greater than the maximum pool size
public int getCorePoolSize()
public boolean prestartCoreThread()
false
if all core threads have already been started.true
if a thread was startedpublic int prestartAllCoreThreads()
public boolean allowsCoreThreadTimeOut()
true
if core threads are allowed to time out, else false
public void allowCoreThreadTimeOut(boolean value)
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 zeropublic void setMaximumPoolSize(int maximumPoolSize)
maximumPoolSize
- the new maximumIllegalArgumentException
- if the new maximum is less than or equal to zero, or less than the core pool size
public int getMaximumPoolSize()
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
argumentIllegalArgumentException
- if time
less than zero or if time
is zero and allowsCoreThreadTimeOut
public long getKeepAliveTime(TimeUnit unit)
unit
- the desired time unit of the resultpublic 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 removetrue
if the task was removedpublic 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()
public String toString()
protected void beforeExecute(Thread t, Runnable r)
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 executedprotected void afterExecute(Runnable r, Throwable t)
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<?>
&& ((Future<?>)r).isDone()) {
try {
Object result = ((Future<?>) r).get();
} catch (CancellationException ce) {
t = ce;
} catch (ExecutionException ee) {
t = ee.getCause();
} catch (InterruptedException ie) {
// ignore/reset
Thread.currentThread().interrupt();
}
}
if (t != null)
System.out.println(t);
}
}
r
- the runnable that has completedt
- the exception that caused termination, or null if execution completed normallyprotected void terminated()
super.terminated
within this method.
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Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/ThreadPoolExecutor.html