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
(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 ThreadPoolExecutor#getPoolSize
)
according to the bounds set by corePoolSize
(see ThreadPoolExecutor#getCorePoolSize
)
and
maximumPoolSize
(see ThreadPoolExecutor#getMaximumPoolSize
).
When a new task is submitted in method ThreadPoolExecutor#execute
, 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 ThreadPoolExecutor#setCorePoolSize
and ThreadPoolExecutor#setMaximumPoolSize
.
-
- 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 ThreadPoolExecutor#prestartCoreThread
or
ThreadPoolExecutor#prestartAllCoreThreads
.
- Creating new threads
- New threads are created using a java.util.concurrent.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 ThreadPoolExecutor#getKeepAliveTime
). 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 ThreadPoolExecutor#setKeepAliveTime
. 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 ThreadPoolExecutor#execute
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
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 ThreadPoolExecutor#beforeExecute
and ThreadPoolExecutor#afterExecute
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
ThreadPoolExecutor#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 ThreadPoolExecutor#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, ThreadPoolExecutor#remove
and ThreadPoolExecutor#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();
}
}
}
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.
Creates a new ThreadPoolExecutor with the given initial
parameters.
Creates a new ThreadPoolExecutor with the given initial
parameters.
Creates a new ThreadPoolExecutor with the given initial
parameters.
Indicates whether some other object is "equal to" this one.
The equals method implements an equivalence relation
on non-null object references:
- It is reflexive: for any non-null reference value
x, x.equals(x) should return
true.
- It is symmetric: for any non-null reference values
x and y, x.equals(y)
should return true if and only if
y.equals(x) returns true.
- It is transitive: for any non-null reference values
x, y, and z, if
x.equals(y) returns true and
y.equals(z) returns true, then
x.equals(z) should return true.
- It is consistent: for any non-null reference values
x and y, multiple invocations of
x.equals(y) consistently return true
or consistently return false, provided no
information used in equals comparisons on the
objects is modified.
- For any non-null reference value
x,
x.equals(null) should return false.
The equals method for class Object implements
the most discriminating possible equivalence relation on objects;
that is, for any non-null reference values x and
y, this method returns true if and only
if x and y refer to the same object
(x == y has the value true).
Note that it is generally necessary to override the hashCode
method whenever this method is overridden, so as to maintain the
general contract for the hashCode method, which states
that equal objects must have equal hash codes.
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.
Returns the approximate number of threads that are actively
executing tasks.
Returns the runtime class of an object. That Class
object is the object that is locked by static synchronized
methods of the represented class.
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 core number of threads.
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.
Returns the largest number of threads that have ever
simultaneously been in the pool.
Returns the maximum allowed number of threads.
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.
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 a hash code value for the object. This method is
supported for the benefit of hashtables such as those provided by
java.util.Hashtable.
The general contract of hashCode is:
- Whenever it is invoked on the same object more than once during
an execution of a Java application, the hashCode method
must consistently return the same integer, provided no information
used in equals comparisons on the object is modified.
This integer need not remain consistent from one execution of an
application to another execution of the same application.
- If two objects are equal according to the equals(Object)
method, then calling the
hashCode method on each of
the two objects must produce the same integer result.
- It is not required that if two objects are unequal
according to the
method, then calling the hashCode method on each of the
two objects must produce distinct integer results. However, the
programmer should be aware that producing distinct integer results
for unequal objects may improve the performance of hashtables.
As much as is reasonably practical, the hashCode method defined by
class Object does return distinct integers for distinct
objects. (This is typically implemented by converting the internal
address of the object into an integer, but this implementation
technique is not required by the
JavaTM programming language.)
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.
Wakes up a single thread that is waiting on this object's
monitor. If any threads are waiting on this object, one of them
is chosen to be awakened. The choice is arbitrary and occurs at
the discretion of the implementation. A thread waits on an object's
monitor by calling one of the
wait methods.
The awakened thread will not be able to proceed until the current
thread relinquishes the lock on this object. The awakened thread will
compete in the usual manner with any other threads that might be
actively competing to synchronize on this object; for example, the
awakened thread enjoys no reliable privilege or disadvantage in being
the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. A thread becomes the owner of the
object's monitor in one of three ways:
- By executing a synchronized instance method of that object.
- By executing the body of a
synchronized statement
that synchronizes on the object.
- For objects of type
Class, by executing a
synchronized static method of that class.
Only one thread at a time can own an object's monitor.
Wakes up all threads that are waiting on this object's monitor. A
thread waits on an object's monitor by calling one of the
wait methods.
The awakened threads will not be able to proceed until the current
thread relinquishes the lock on this object. The awakened threads
will compete in the usual manner with any other threads that might
be actively competing to synchronize on this object; for example,
the awakened threads enjoy no reliable privilege or disadvantage in
being the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
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.
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.
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.
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 ThreadPoolExecutor#purge
may be used to remove those Futures that have been cancelled.
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.
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.
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.
Sets a new handler for unexecutable tasks.
Sets the thread factory used to create new threads.
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.
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 Thread#interrupt
, so if any tasks mask or fail to respond to
interrupts, they may never terminate.
Returns a string representation of the object. In general, the
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
The toString method for class Object
returns a string consisting of the name of the class of which the
object is an instance, the at-sign character `@', and
the unsigned hexadecimal representation of the hash code of the
object. In other words, this method returns a string equal to the
value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
Causes current thread to wait until another thread invokes the
method or the
method for this object.
In other words, this method behaves exactly as if it simply
performs the call
wait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up
either through a call to the notify method or the
notifyAll method. The thread then waits until it can
re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are
possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the
notify method for a
description of the ways in which a thread can become the owner of
a monitor.
Causes current thread to wait until either another thread invokes the
method or the
method for this object, or a
specified amount of time has elapsed.
The current thread must own this object's monitor.
This method causes the current thread (call it T) to
place itself in the wait set for this object and then to relinquish
any and all synchronization claims on this object. Thread T
becomes disabled for thread scheduling purposes and lies dormant
until one of four things happens:
- Some other thread invokes the notify method for this
object and thread T happens to be arbitrarily chosen as
the thread to be awakened.
- Some other thread invokes the notifyAll method for this
object.
- Some other thread interrupts
thread T.
- The specified amount of real time has elapsed, more or less. If
timeout is zero, however, then real time is not taken into
consideration and the thread simply waits until notified.
The thread
T is then removed from the wait set for this
object and re-enabled for thread scheduling. It then competes in the
usual manner with other threads for the right to synchronize on the
object; once it has gained control of the object, all its
synchronization claims on the object are restored to the status quo
ante - that is, to the situation as of the time that the
wait
method was invoked. Thread
T then returns from the
invocation of the
wait method. Thus, on return from the
wait method, the synchronization state of the object and of
thread
T is exactly as it was when the
wait method
was invoked.
A thread can also wake up without being notified, interrupted, or
timing out, a so-called spurious wakeup. While this will rarely
occur in practice, applications must guard against it by testing for
the condition that should have caused the thread to be awakened, and
continuing to wait if the condition is not satisfied. In other words,
waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic, see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's "Effective Java Programming
Language Guide" (Addison-Wesley, 2001).
If the current thread is
interrupted
by another thread
while it is waiting, then an InterruptedException is thrown.
This exception is not thrown until the lock status of this object has
been restored as described above.
Note that the wait method, as it places the current thread
into the wait set for this object, unlocks only this object; any
other objects on which the current thread may be synchronized remain
locked while the thread waits.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
Causes current thread to wait until another thread invokes the
method or the
method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.
This method is similar to the wait method of one
argument, but it allows finer control over the amount of time to
wait for a notification before giving up. The amount of real time,
measured in nanoseconds, is given by:
1000000*timeout+nanos
In all other respects, this method does the same thing as the
method
of one argument. In particular,
wait(0, 0) means the same thing as wait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until either of the
following two conditions has occurred:
- Another thread notifies threads waiting on this object's monitor
to wake up either through a call to the
notify method
or the notifyAll method.
- The timeout period, specified by
timeout
milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the
monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are
possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout, nanos);
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the
notify method for a
description of the ways in which a thread can become the owner of
a monitor.