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FlexDoc/Javadoc 2.0 Demo Java Doc |
Class ThreadPoolExecutor
java.util.concurrent.ThreadPoolExecutor
All Implemented Interfaces:
Direct Known Subclasses:
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:
- 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(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).
- 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. 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.
- 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(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.
- 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(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:
- 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. This policy is designed only for
those rare cases in which task completion is never relied upon.
- 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.) 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.
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(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.
- 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(Runnable) and purge() are available to
assist in storage reclamation when large numbers of queued tasks
become cancelled.
- Reclamation
- A pool that is no longer referenced in a program AND
has no remaining threads may be reclaimed (garbage collected)
without being explicitly shutdown. You can configure a pool to
allow all unused threads to 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
Author:
Doug Lea
Nested Class Summary |
static class |
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static class |
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.
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static class |
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.
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static class |
A handler for rejected tasks that silently discards the
rejected task.
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Constructor Summary |
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Creates a new ThreadPoolExecutor with the given initial
parameters.
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Method Summary |
protected void |
Method invoked upon completion of execution of the given Runnable.
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void |
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.
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boolean |
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.
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boolean |
Blocks until all tasks have completed execution after a shutdown
request, or the timeout occurs, or the current thread is
interrupted, whichever happens first.
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protected void |
Method invoked prior to executing the given Runnable in the
given thread.
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void |
Executes the given task sometime in the future.
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protected void |
Deprecated.
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int |
Returns the approximate number of threads that are actively
executing tasks.
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long |
Returns the approximate total number of tasks that have
completed execution.
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int |
Returns the core number of threads.
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long |
Returns the thread keep-alive time, which is the amount of time
that threads may remain idle before being terminated.
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int |
Returns the largest number of threads that have ever
simultaneously been in the pool.
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int |
Returns the maximum allowed number of threads.
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int |
Returns the current number of threads in the pool.
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Returns the task queue used by this executor.
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Returns the current handler for unexecutable tasks.
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long |
Returns the approximate total number of tasks that have ever been
scheduled for execution.
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Returns the thread factory used to create new threads.
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boolean |
Returns true if this executor has been shut down.
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boolean |
Returns true if all tasks have completed following shut down.
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boolean |
Returns true if this executor is in the process of terminating
after shutdown() or shutdownNow() but has not
completely terminated.
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int |
Starts all core threads, causing them to idly wait for work.
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boolean |
Starts a core thread, causing it to idly wait for work.
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void |
Tries to remove from the work queue all Future
tasks that have been cancelled.
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boolean |
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.
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void |
Sets the core number of threads.
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void |
Sets the thread keep-alive time, which is the amount of time
that threads may remain idle before being terminated.
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void |
Sets the maximum allowed number of threads.
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void |
Sets a new handler for unexecutable tasks.
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void |
Sets the thread factory used to create new threads.
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void |
Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted.
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Attempts to stop all actively executing tasks, halts the
processing of waiting tasks, and returns a list of the tasks
that were awaiting execution.
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protected void |
Method invoked when the Executor has terminated.
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Returns a string identifying this pool, as well as its state,
including indications of run state and estimated worker and
task counts.
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Methods inherited from class java.lang. Object |
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public ThreadPoolExecutor |
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Creates a new
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.
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.
Throws:
IllegalArgumentException - if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
public ThreadPoolExecutor |
|
Creates a new
ThreadPoolExecutor with the given initial
parameters and the
default rejected execution handler.
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
Throws:
IllegalArgumentException - if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
public ThreadPoolExecutor |
|
Creates a new
ThreadPoolExecutor with the given initial
parameters and the
default thread factory.
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.
handler - the handler to use when execution is blocked
because the thread bounds and queue capacities are reached
Throws:
IllegalArgumentException - if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
public ThreadPoolExecutor |
|
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, 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
Throws:
IllegalArgumentException - if one of the following holds:
corePoolSize < 0
keepAliveTime < 0
maximumPoolSize <= 0
maximumPoolSize < corePoolSize
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.
Specified by:
Parameters:
command - the task to execute
Throws:
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.
Specified by:
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.
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
interrupts tasks via Thread.interrupt(); any task that
fails to respond to interrupts may never terminate.
Specified by:
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.
public boolean isShutdown |
() |
Returns true if this executor has been shut down.
Specified by:
Returns:
true if this executor has been shut down
public boolean isTerminating |
() |
Returns true if this executor is in the process of terminating
after
shutdown() or
shutdownNow() but has not
completely terminated. This method may be useful for
debugging. A return of
true reported a sufficient
period after shutdown may indicate that submitted tasks have
ignored or suppressed interruption, causing this executor not
to properly terminate.
Returns:
true if terminating but not yet terminated
public boolean isTerminated |
() |
Returns true if all tasks have completed following shut down.
Note that isTerminated is never true unless
either shutdown or shutdownNow was called first.
Specified by:
Returns:
true if all tasks have completed following shut down
public boolean awaitTermination |
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throws |
|
Blocks until all tasks have completed execution after a shutdown
request, or the timeout occurs, or the current thread is
interrupted, whichever happens first.
Specified by:
Parameters:
timeout - the maximum time to wait
unit - the time unit of the timeout argument
Returns:
true if this executor terminated and
false if the timeout elapsed before termination
Throws:
protected void finalize |
() |
Deprecated.
Description copied from class:
Object
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object.
A subclass overrides the
finalize method to dispose of
system resources or to perform other cleanup.
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.
Implementation Note:
Previous versions of this class had a finalize method
that shut down this executor, but in this version, finalize
does nothing.
Overrides:
See Also:
public void setThreadFactory |
|
Sets the thread factory used to create new threads.
Parameters:
threadFactory - the new thread factory
Throws:
See Also:
Returns the thread factory used to create new threads.
Returns:
the current thread factory
See Also:
setRejectedExecutionHandler
public void setRejectedExecutionHandler |
|
Sets a new handler for unexecutable tasks.
Parameters:
handler - the new handler
Throws:
See Also:
getRejectedExecutionHandler
Returns the current handler for unexecutable tasks.
Returns:
the current handler
See Also:
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:
See Also:
public int getCorePoolSize |
() |
Returns the core number of threads.
Returns:
the core number of threads
See Also:
public boolean prestartCoreThread |
() |
Starts a core thread, causing it to idly wait for work. This
overrides the default policy of starting core threads only when
new tasks are executed. This method will return
false
if all core threads have already been started.
Returns:
true if a thread was started
public int prestartAllCoreThreads |
() |
Starts all core threads, causing them to idly wait for work. This
overrides the default policy of starting core threads only when
new tasks are executed.
Returns:
the number of threads started
public boolean allowsCoreThreadTimeOut |
() |
Returns true if this pool allows core threads to time out and
terminate if no tasks arrive within the keepAlive time, being
replaced if needed when new tasks arrive. When true, the same
keep-alive policy applying to non-core threads applies also to
core threads. When false (the default), core threads are never
terminated due to lack of incoming tasks.
Returns:
true if core threads are allowed to time out,
else false
Since:
1.6
public 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:
Since:
1.6
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:
See Also:
public int getMaximumPoolSize |
() |
Returns the maximum allowed number of threads.
Returns:
the maximum allowed number of threads
See Also:
public void setKeepAliveTime |
|
Sets the thread keep-alive time, which is the amount of time
that threads may remain idle before being terminated.
Threads that wait this amount of time without processing a
task will be terminated if there are more than the core
number of threads currently in the pool, or if this pool
allows core thread timeout.
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:
See Also:
public long getKeepAliveTime |
|
Returns the thread keep-alive time, which is the amount of time
that threads may remain idle before being terminated.
Threads that wait this amount of time without processing a
task will be terminated if there are more than the core
number of threads currently in the pool, or if this pool
allows core thread timeout.
Parameters:
unit - the desired time unit of the result
Returns:
the time limit
See Also:
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
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
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.
public int getPoolSize |
() |
Returns the current number of threads in the pool.
Returns:
the number of threads
public int getActiveCount |
() |
Returns the approximate number of threads that are actively
executing tasks.
Returns:
the number of threads
public int getLargestPoolSize |
() |
Returns the largest number of threads that have ever
simultaneously been in the pool.
Returns:
the number of threads
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
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
Returns a string identifying this pool, as well as its state,
including indications of run state and estimated worker and
task counts.
Overrides:
Returns:
a string identifying this pool, as well as its state
protected void beforeExecute |
|
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
protected void afterExecute |
|
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<?>
&& ((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);
}
}
Parameters:
r - the runnable that has completed
t - the exception that caused termination, or null if
execution completed normally
protected void terminated |
() |
Method invoked when the Executor has terminated. Default
implementation does nothing. Note: To properly nest multiple
overridings, subclasses should generally invoke
super.terminated within this method.
|
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