JavaTM 2 Platform Std. Ed. v1.6.0
java.util.concurrent
Class ThreadPoolExecutor
java.lang.Object
java.util.concurrent.AbstractExecutorService
java.util.concurrent.ThreadPoolExecutor
- All Implemented Interfaces:
- Executor, ExecutorService
- Direct Known Subclasses:
- ScheduledThreadPoolExecutor
public class ThreadPoolExecutor - extends AbstractExecutorService
An ExecutorService that executes each submitted task using
one of possibly several pooled threads, normally configured
using Executors factory methods.
Thread pools address two different problems: they usually
provide improved performance when executing large numbers of
asynchronous tasks, due to reduced per-task invocation overhead,
and they provide a means of bounding and managing the resources,
including threads, consumed when executing a collection of tasks.
Each ThreadPoolExecutor also maintains some basic
statistics, such as the number of completed tasks.
To be useful across a wide range of contexts, this class
provides many adjustable parameters and extensibility
hooks. However, programmers are urged to use the more convenient
Executors factory methods Executors.newCachedThreadPool() (unbounded thread pool, with
automatic thread reclamation), Executors.newFixedThreadPool(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:
- Core and maximum pool sizes
- A 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(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) .
- On-demand construction
- By default, even core threads are initially created and
started only when new tasks arrive, but this can be overridden
dynamically using method
prestartCoreThread() or
prestartAllCoreThreads() .
You probably want to prestart threads if you construct the
pool with a non-empty queue.
- Creating new threads
- New threads are created using a
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.
- Keep-alive times
- If the pool currently has more than corePoolSize threads,
excess threads will be terminated if they have been idle for more
than the keepAliveTime (see
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.
- Queuing
- Any
BlockingQueue may be used to transfer and hold
submitted tasks. The use of this queue interacts with pool sizing:
- If fewer than corePoolSize threads are running, the Executor
always prefers adding a new thread
rather than queuing.
- If corePoolSize or more threads are running, the Executor
always prefers queuing a request rather than adding a new
thread.
- If a request cannot be queued, a new thread is created unless
this would exceed maximumPoolSize, in which case, the task will be
rejected.
There are three general strategies for queuing:
- Direct handoffs. A good default choice for a work
queue is a
SynchronousQueue that hands off tasks to threads
without otherwise holding them. Here, an attempt to queue a task
will fail if no threads are immediately available to run it, so a
new thread will be constructed. This policy avoids lockups when
handling sets of requests that might have internal dependencies.
Direct handoffs generally require unbounded maximumPoolSizes to
avoid rejection of new submitted tasks. This in turn admits the
possibility of unbounded thread growth when commands continue to
arrive on average faster than they can be processed.
- Unbounded queues. Using an unbounded queue (for
example a
LinkedBlockingQueue without a predefined
capacity) will cause new tasks to wait in the queue when all
corePoolSize threads are busy. Thus, no more than corePoolSize
threads will ever be created. (And the value of the maximumPoolSize
therefore doesn't have any effect.) This may be appropriate when
each task is completely independent of others, so tasks cannot
affect each others execution; for example, in a web page server.
While this style of queuing can be useful in smoothing out
transient bursts of requests, it admits the possibility of
unbounded work queue growth when commands continue to arrive on
average faster than they can be processed.
- Bounded queues. A bounded queue (for example, an
ArrayBlockingQueue ) helps prevent resource exhaustion when
used with finite maximumPoolSizes, but can be more difficult to
tune and control. Queue sizes and maximum pool sizes may be traded
off for each other: Using large queues and small pools minimizes
CPU usage, OS resources, and context-switching overhead, but can
lead to artificially low throughput. If tasks frequently block (for
example if they are I/O bound), a system may be able to schedule
time for more threads than you otherwise allow. Use of small queues
generally requires larger pool sizes, which keeps CPUs busier but
may encounter unacceptable scheduling overhead, which also
decreases throughput.
- Rejected tasks
- New tasks submitted in method
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:
- In the
default
ThreadPoolExecutor.AbortPolicy , the handler throws a
runtime RejectedExecutionException upon rejection.
- In
ThreadPoolExecutor.CallerRunsPolicy , the thread that invokes
execute itself runs the task. This provides a simple
feedback control mechanism that will slow down the rate that new
tasks are submitted.
- In
ThreadPoolExecutor.DiscardPolicy ,
a task that cannot be executed is simply dropped.
- In
ThreadPoolExecutor.DiscardOldestPolicy , if the executor is not
shut down, the task at the head of the work queue is dropped, and
then execution is retried (which can fail again, causing this to be
repeated.)
It is possible to define and use other kinds of RejectedExecutionHandler classes. Doing so requires some care
especially when policies are designed to work only under particular
capacity or queuing policies.
- Hook methods
- This class provides protected overridable
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.
- Queue maintenance
- Method
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.
- Finalization
- A pool that is no longer referenced in a program AND
has no remaining threads will be 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();
}
}
}
- Since:
- 1.5
Nested Class Summary |
static class |
ThreadPoolExecutor.AbortPolicy
A handler for rejected tasks that throws a
RejectedExecutionException. |
static class |
ThreadPoolExecutor.CallerRunsPolicy
A handler for rejected tasks that runs the rejected task
directly in the calling thread of the execute method,
unless the executor has been shut down, in which case the task
is discarded. |
static class |
ThreadPoolExecutor.DiscardOldestPolicy
A handler for rejected tasks that discards the oldest unhandled
request and then retries execute, unless the executor
is shut down, in which case the task is discarded. |
static class |
ThreadPoolExecutor.DiscardPolicy
A handler for rejected tasks that silently discards the
rejected task. |
Constructor Summary |
ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<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. |
ThreadPoolExecutor
public 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.
It may be more convenient to use one of the
Executors factory
methods instead of this general purpose constructor.
- Parameters:
corePoolSize - the number of threads to keep in the
pool, even if they are idle.maximumPoolSize - the maximum number of threads to allow in the
pool.keepAliveTime - when the number of threads is greater than
the core, this is the maximum time that excess idle threads
will wait for new tasks before terminating.unit - the time unit for the keepAliveTime
argument.workQueue - the queue to use for holding tasks before they
are executed. This queue will hold only the Runnable
tasks submitted by the execute method.
- Throws:
IllegalArgumentException - if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException - if workQueue is null
ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory)
- Creates a new ThreadPoolExecutor with the given initial
parameters and default rejected execution handler.
- Parameters:
corePoolSize - the number of threads to keep in the
pool, even if they are idle.maximumPoolSize - the maximum number of threads to allow in the
pool.keepAliveTime - when the number of threads is greater than
the core, this is the maximum time that excess idle threads
will wait for new tasks before terminating.unit - the time unit for the keepAliveTime
argument.workQueue - the queue to use for holding tasks before they
are executed. This queue will hold only the Runnable
tasks submitted by the execute method.threadFactory - the factory to use when the executor
creates a new thread.
- Throws:
IllegalArgumentException - if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException - if workQueue
or threadFactory are null.
ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler)
- Creates a new ThreadPoolExecutor with the given initial
parameters and default thread factory.
- Parameters:
corePoolSize - the number of threads to keep in the
pool, even if they are idle.maximumPoolSize - the maximum number of threads to allow in the
pool.keepAliveTime - when the number of threads is greater than
the core, this is the maximum time that excess idle threads
will wait for new tasks before terminating.unit - the time unit for the keepAliveTime
argument.workQueue - the queue to use for holding tasks before they
are executed. This queue will hold only the Runnable
tasks submitted by the execute method.handler - the handler to use when execution is blocked
because the thread bounds and queue capacities are reached.
- Throws:
IllegalArgumentException - if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException - if workQueue
or handler are null.
ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
- Creates a new ThreadPoolExecutor with the given initial
parameters.
- Parameters:
corePoolSize - the number of threads to keep in the
pool, even if they are idle.maximumPoolSize - the maximum number of threads to allow in the
pool.keepAliveTime - when the number of threads is greater than
the core, this is the maximum time that excess idle threads
will wait for new tasks before terminating.unit - the time unit for the keepAliveTime
argument.workQueue - the queue to use for holding tasks before they
are executed. This queue will hold only the Runnable
tasks submitted by the execute method.threadFactory - the factory to use when the executor
creates a new thread.handler - the handler to use when execution is blocked
because the thread bounds and queue capacities are reached.
- Throws:
IllegalArgumentException - if corePoolSize or
keepAliveTime less than zero, or if maximumPoolSize less than or
equal to zero, or if corePoolSize greater than maximumPoolSize.
NullPointerException - if workQueue
or threadFactory or handler are null.
execute
public void execute(Runnable command)
- Executes the given task sometime in the future. The task
may execute in a new thread or in an existing pooled thread.
If the task cannot be submitted for execution, either because this
executor has been shutdown or because its capacity has been reached,
the task is handled by the current RejectedExecutionHandler.
- Parameters:
command - the task to execute
- Throws:
RejectedExecutionException - at discretion of
RejectedExecutionHandler, if task cannot be accepted
for execution
NullPointerException - if command is null
shutdown
public void shutdown()
- Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be
accepted. Invocation has no additional effect if already shut
down.
- Throws:
SecurityException - if a security manager exists and
shutting down this ExecutorService may manipulate threads that
the caller is not permitted to modify because it does not hold
RuntimePermission ("modifyThread"),
or the security manager's checkAccess method denies access.
shutdownNow
public List<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. These tasks are drained (removed)
from the task queue upon return from this method.
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.
- Returns:
- list of tasks that never commenced execution
- Throws:
SecurityException - if a security manager exists and
shutting down this ExecutorService may manipulate threads that
the caller is not permitted to modify because it does not hold
RuntimePermission ("modifyThread"),
or the security manager's checkAccess method denies access.
isShutdown
public boolean isShutdown()
- Description copied from interface:
ExecutorService
- Returns true if this executor has been shut down.
- Returns:
- true if this executor has been shut down
isTerminating
public boolean isTerminating()
- Returns true if this executor is in the process of terminating
after shutdown or shutdownNow but has not
completely terminated. This method may be useful for
debugging. A return of true reported a sufficient
period after shutdown may indicate that submitted tasks have
ignored or suppressed interruption, causing this executor not
to properly terminate.
- Returns:
- true if terminating but not yet terminated
isTerminated
public boolean isTerminated()
- Description copied from interface:
ExecutorService
- Returns true if all tasks have completed following shut down.
Note that isTerminated is never true unless
either shutdown or shutdownNow was called first.
- Returns:
- true if all tasks have completed following shut down
awaitTermination
public boolean awaitTermination(long timeout,
TimeUnit unit)
throws InterruptedException
- Description copied from interface:
ExecutorService
- Blocks until all tasks have completed execution after a shutdown
request, or the timeout occurs, or the current thread is
interrupted, whichever happens first.
- 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
finalize
protected void finalize()
- Invokes shutdown when this executor is no longer
referenced.
- Overrides:
finalize in class Object
setThreadFactory
public void setThreadFactory(ThreadFactory threadFactory)
- Sets the thread factory used to create new threads.
- Parameters:
threadFactory - the new thread factory
- Throws:
NullPointerException - if threadFactory is null- See Also:
getThreadFactory()
getThreadFactory
public ThreadFactory getThreadFactory()
- Returns the thread factory used to create new threads.
- Returns:
- the current thread factory
- See Also:
setThreadFactory(java.util.concurrent.ThreadFactory)
setRejectedExecutionHandler
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()
getRejectedExecutionHandler
public RejectedExecutionHandler getRejectedExecutionHandler()
- Returns the current handler for unexecutable tasks.
- Returns:
- the current handler
- See Also:
setRejectedExecutionHandler(java.util.concurrent.RejectedExecutionHandler)
setCorePoolSize
public void setCorePoolSize(int corePoolSize)
- Sets the core number of threads. This overrides any value set
in the constructor. If the new value is smaller than the
current value, excess existing threads will be terminated when
they next become idle. If larger, new threads will, if needed,
be started to execute any queued tasks.
- Parameters:
corePoolSize - the new core size
- Throws:
IllegalArgumentException - if corePoolSize
less than zero- See Also:
getCorePoolSize()
getCorePoolSize
public int getCorePoolSize()
- Returns the core number of threads.
- Returns:
- the core number of threads
- See Also:
setCorePoolSize(int)
prestartCoreThread
public boolean prestartCoreThread()
- Starts a core thread, causing it to idly wait for work. This
overrides the default policy of starting core threads only when
new tasks are executed. This method will return false
if all core threads have already been started.
- Returns:
- true if a thread was started
prestartAllCoreThreads
public int prestartAllCoreThreads()
- Starts all core threads, causing them to idly wait for work. This
overrides the default policy of starting core threads only when
new tasks are executed.
- Returns:
- the number of threads started
allowsCoreThreadTimeOut
public boolean allowsCoreThreadTimeOut()
- Returns true if this pool allows core threads to time out and
terminate if no tasks arrive within the keepAlive time, being
replaced if needed when new tasks arrive. When true, the same
keep-alive policy applying to non-core threads applies also to
core threads. When false (the default), core threads are never
terminated due to lack of incoming tasks.
- Returns:
- true if core threads are allowed to time out,
else false
- Since:
- 1.6
allowCoreThreadTimeOut
public void allowCoreThreadTimeOut(boolean value)
- Sets the policy governing whether core threads may time out and
terminate if no tasks arrive within the keep-alive time, being
replaced if needed when new tasks arrive. When false, core
threads are never terminated due to lack of incoming
tasks. When true, the same keep-alive policy applying to
non-core threads applies also to core threads. To avoid
continual thread replacement, the keep-alive time must be
greater than zero when setting true. This method
should in general be called before the pool is actively used.
- Parameters:
value - true if should time out, else false
- Throws:
IllegalArgumentException - if value is true
and the current keep-alive time is not greater than zero.- Since:
- 1.6
setMaximumPoolSize
public void setMaximumPoolSize(int maximumPoolSize)
- Sets the maximum allowed number of threads. This overrides any
value set in the constructor. If the new value is smaller than
the current value, excess existing threads will be
terminated when they next become idle.
- Parameters:
maximumPoolSize - the new maximum
- Throws:
IllegalArgumentException - if the new maximum is
less than or equal to zero, or
less than the core pool size- See Also:
getMaximumPoolSize()
getMaximumPoolSize
public int getMaximumPoolSize()
- Returns the maximum allowed number of threads.
- Returns:
- the maximum allowed number of threads
- See Also:
setMaximumPoolSize(int)
setKeepAliveTime
public void setKeepAliveTime(long time,
TimeUnit unit)
- Sets the time limit for which threads may remain idle before
being terminated. If there are more than the core number of
threads currently in the pool, after waiting this amount of
time without processing a task, excess threads will be
terminated. This overrides any value set in the constructor.
- Parameters:
time - the time to wait. A time value of zero will cause
excess threads to terminate immediately after executing tasks.unit - the time unit of the time argument
- Throws:
IllegalArgumentException - if time less than zero or
if time is zero and allowsCoreThreadTimeOut- See Also:
getKeepAliveTime(java.util.concurrent.TimeUnit)
getKeepAliveTime
public 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.
- Parameters:
unit - the desired time unit of the result
- Returns:
- the time limit
- See Also:
setKeepAliveTime(long, java.util.concurrent.TimeUnit)
getQueue
public BlockingQueue<Runnable> getQueue()
- Returns the task queue used by this executor. Access to the
task queue is intended primarily for debugging and monitoring.
This queue may be in active use. Retrieving the task queue
does not prevent queued tasks from executing.
- Returns:
- the task queue
remove
public boolean remove(Runnable task)
- Removes this task from the executor's internal queue if it is
present, thus causing it not to be run if it has not already
started.
This method may be useful as one part of a cancellation
scheme. It may fail to remove tasks that have been converted
into other forms before being placed on the internal queue. For
example, a task entered using submit might be
converted into a form that maintains Future status.
However, in such cases, method purge()
may be used to remove those Futures that have been cancelled.
- Parameters:
task - the task to remove
- Returns:
- true if the task was removed
purge
public void purge()
- Tries to remove from the work queue all
Future
tasks that have been cancelled. This method can be useful as a
storage reclamation operation, that has no other impact on
functionality. Cancelled tasks are never executed, but may
accumulate in work queues until worker threads can actively
remove them. Invoking this method instead tries to remove them now.
However, this method may fail to remove tasks in
the presence of interference by other threads.
getPoolSize
public int getPoolSize()
- Returns the current number of threads in the pool.
- Returns:
- the number of threads
getActiveCount
public int getActiveCount()
- Returns the approximate number of threads that are actively
executing tasks.
- Returns:
- the number of threads
getLargestPoolSize
public int getLargestPoolSize()
- Returns the largest number of threads that have ever
simultaneously been in the pool.
- Returns:
- the number of threads
getTaskCount
public long getTaskCount()
- 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 number of tasks
getCompletedTaskCount
public long getCompletedTaskCount()
- Returns the approximate total number of tasks that have
completed execution. Because the states of tasks and threads
may change dynamically during computation, the returned value
is only an approximation, but one that does not ever decrease
across successive calls.
- Returns:
- the number of tasks
beforeExecute
protected void beforeExecute(Thread t,
Runnable r)
- Method invoked prior to executing the given Runnable in the
given thread. This method is invoked by thread t that
will execute task r, and may be used to re-initialize
ThreadLocals, or to perform logging.
This implementation does nothing, but may be customized in
subclasses. Note: To properly nest multiple overridings, subclasses
should generally invoke super.beforeExecute at the end of
this method.
- Parameters:
t - the thread that will run task r.r - the task that will be executed.
afterExecute
protected void afterExecute(Runnable r,
Throwable t)
- Method invoked upon completion of execution of the given Runnable.
This method is invoked by the thread that executed the task. If
non-null, the Throwable is the uncaught RuntimeException
or Error that caused execution to terminate abruptly.
Note: When actions are enclosed in tasks (such as
FutureTask ) either explicitly or via methods such as
submit, these task objects catch and maintain
computational exceptions, and so they do not cause abrupt
termination, and the internal exceptions are not
passed to this method.
This implementation does nothing, but may be customized in
subclasses. Note: To properly nest multiple overridings, subclasses
should generally invoke super.afterExecute at the
beginning of this method.
- Parameters:
r - the runnable that has completed.t - the exception that caused termination, or null if
execution completed normally.
terminated
protected void terminated()
- Method invoked when the Executor has terminated. Default
implementation does nothing. Note: To properly nest multiple
overridings, subclasses should generally invoke
super.terminated within this method.
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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