java.util.concurrent.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 newCachedThreadPool() (unbounded thread pool, with
automatic thread reclamation), newFixedThreadPool(int)
(fixed size thread pool) and newSingleThreadExecutor() (single background thread), that
preconfigure settings for the most common usage
scenarios. Otherwise, use the following guide when manually
configuring and tuning this class:
- 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), 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 needed by new tasks, but this can be overridden
dynamically using method prestartCoreThread() or
prestartAllCoreThreads().
- Creating new threads
- New threads are created using a ThreadFactory. If not otherwise specified, a
defaultThreadFactory() is used, that creates threads to all
be in the same ThreadGroup and with the same
NORM_PRIORITY priority and non-daemon status. By supplying
a different ThreadFactory, you can alter the thread's name, thread
group, priority, daemon status, etc.
- 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.
- 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 be queued in cases where 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
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.
- 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(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.
- 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.
Extension example. Most extensions of this class
override one or more of the protected hook methods. For example,
here is a subclass that adds a simple pause/resume feature:
class PausableThreadPoolExecutor extends ThreadPoolExecutor {
private boolean isPaused;
private ReentrantLock pauseLock = new ReentrantLock();
private Condition unpaused = pauseLock.newCondition();
public PausableThreadPoolExecutor(...) { super(...); }
protected void beforeExecute(Thread t, Runnable r) {
super.beforeExecute(t, r);
pauseLock.lock();
try {
while (isPaused) unpaused.await();
} catch(InterruptedException ie) {
t.interrupt();
} finally {
pauseLock.unlock();
}
}
public void pause() {
pauseLock.lock();
try {
isPaused = true;
} finally {
pauseLock.unlock();
}
}
public void resume() {
pauseLock.lock();
try {
isPaused = false;
unpaused.signalAll();
} finally {
pauseLock.unlock();
}
}
}
Nested Classes
Known Direct Subclasses
Summary
Public Constructors
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ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) |
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ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) |
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ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) |
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ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) |
Public Methods
Protected Methods
clone,
equals,
finalize,
getClass,
hashCode,
notify,
notifyAll,
toString,
wait,
wait,
wait
awaitTermination,
invokeAll,
invokeAll,
invokeAny,
invokeAny,
isShutdown,
isTerminated,
shutdown,
shutdownNow,
submit,
submit,
submit
Details
Public Constructors
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 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. |
public
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)
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. |
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.
|
public
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, 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. |
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.
|
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.
|
Public Methods
public
boolean
awaitTermination(long timeout, TimeUnit unit)
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 |
public
int
getActiveCount()
Returns the approximate number of threads that are actively
executing 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.
public
int
getCorePoolSize()
Returns the core number of threads.
Returns
- the core number of threads
public
long
getKeepAliveTime(TimeUnit unit)
Returns the thread keep-alive time, which is the amount of time
which threads in excess of the core pool size may remain
idle before being terminated.
Parameters
unit
| the desired time unit of the result |
public
int
getLargestPoolSize()
Returns the largest number of threads that have ever
simultaneously been in the pool.
public
int
getMaximumPoolSize()
Returns the maximum allowed number of threads.
Returns
- the maximum allowed number of threads
public
int
getPoolSize()
Returns the current number of threads in the pool.
Returns the task queue used by this executor. Access to the
task queue is intended primarily for debugging and monitoring.
This queue may be in active use. Retrieving the task queue
does not prevent queued tasks from executing.
Returns the current handler for unexecutable tasks.
public
long
getTaskCount()
Returns the approximate total number of tasks that have been
scheduled for 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 thread factory used to create new threads.
Returns
- the current thread factory
public
boolean
isShutdown()
public
boolean
isTerminated()
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
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
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
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.
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.
Returns
- true if the task was removed
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 |
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 |
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 |
public
void
setThreadFactory(ThreadFactory threadFactory)
Sets the thread factory used to create new threads.
Parameters
threadFactory
| the new thread factory |
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.
|
Attempts to stop all actively executing tasks, halts the
processing of waiting tasks, and returns a list of the tasks that were
awaiting execution.
This implementation cancels tasks via interrupt(), so if any tasks mask or fail to respond to
interrupts, they 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.
|
Protected Methods
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 exception
that caused execution to terminate abruptly. 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.
|
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. 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
finalize()
Invokes shutdown when this executor is no longer
referenced.
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.