ExecutorCompletionService
public
class
ExecutorCompletionService
extends Object
implements
CompletionService<V>
A CompletionService
that uses a supplied Executor
to execute tasks. This class arranges that submitted tasks are,
upon completion, placed on a queue accessible using take
.
The class is lightweight enough to be suitable for transient use
when processing groups of tasks.
Usage Examples.
Suppose you have a set of solvers for a certain problem, each
returning a value of some type Result
, and would like to
run them concurrently, processing the results of each of them that
return a non-null value, in some method use(Result r)
. You
could write this as:
void solve(Executor e,
Collection<Callable<Result>> solvers)
throws InterruptedException, ExecutionException {
CompletionService<Result> cs
= new ExecutorCompletionService<>(e);
solvers.forEach(cs::submit);
for (int i = solvers.size(); i > 0; i--) {
Result r = cs.take().get();
if (r != null)
use(r);
}
}
Suppose instead that you would like to use the first non-null result
of the set of tasks, ignoring any that encounter exceptions,
and cancelling all other tasks when the first one is ready:
void solve(Executor e,
Collection<Callable<Result>> solvers)
throws InterruptedException {
CompletionService<Result> cs
= new ExecutorCompletionService<>(e);
int n = solvers.size();
List<Future<Result>> futures = new ArrayList<>(n);
Result result = null;
try {
solvers.forEach(solver -> futures.add(cs.submit(solver)));
for (int i = n; i > 0; i--) {
try {
Result r = cs.take().get();
if (r != null) {
result = r;
break;
}
} catch (ExecutionException ignore) {}
}
} finally {
futures.forEach(future -> future.cancel(true));
}
if (result != null)
use(result);
}
Summary
Public methods |
Future<V>
|
poll(long timeout, TimeUnit unit)
Retrieves and removes the Future representing the next
completed task, waiting if necessary up to the specified wait
time if none are yet present.
|
Future<V>
|
poll()
Retrieves and removes the Future representing the next
completed task, or null if none are present.
|
Future<V>
|
submit(Runnable task, V result)
Submits a Runnable task for execution and returns a Future
representing that task.
|
Future<V>
|
submit(Callable<V> task)
Submits a value-returning task for execution and returns a Future
representing the pending results of the task.
|
Future<V>
|
take()
Retrieves and removes the Future representing the next
completed task, waiting if none are yet present.
|
Inherited methods |
From class
java.lang.Object
Object
|
clone()
Creates and returns a copy of this object.
|
boolean
|
equals(Object obj)
Indicates whether some other object is "equal to" this one.
|
void
|
finalize()
Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object.
|
final
Class<?>
|
getClass()
Returns the runtime class of this Object .
|
int
|
hashCode()
Returns a hash code value for the object.
|
final
void
|
notify()
Wakes up a single thread that is waiting on this object's
monitor.
|
final
void
|
notifyAll()
Wakes up all threads that are waiting on this object's monitor.
|
String
|
toString()
Returns a string representation of the object.
|
final
void
|
wait(long timeoutMillis, int nanos)
Causes the current thread to wait until it is awakened, typically
by being notified or interrupted, or until a
certain amount of real time has elapsed.
|
final
void
|
wait(long timeoutMillis)
Causes the current thread to wait until it is awakened, typically
by being notified or interrupted, or until a
certain amount of real time has elapsed.
|
final
void
|
wait()
Causes the current thread to wait until it is awakened, typically
by being notified or interrupted.
|
|
From interface
java.util.concurrent.CompletionService
abstract
Future<V>
|
poll(long timeout, TimeUnit unit)
Retrieves and removes the Future representing the next
completed task, waiting if necessary up to the specified wait
time if none are yet present.
|
abstract
Future<V>
|
poll()
Retrieves and removes the Future representing the next
completed task, or null if none are present.
|
abstract
Future<V>
|
submit(Runnable task, V result)
Submits a Runnable task for execution and returns a Future
representing that task.
|
abstract
Future<V>
|
submit(Callable<V> task)
Submits a value-returning task for execution and returns a Future
representing the pending results of the task.
|
abstract
Future<V>
|
take()
Retrieves and removes the Future representing the next
completed task, waiting if none are yet present.
|
|
Public constructors
ExecutorCompletionService
public ExecutorCompletionService (Executor executor)
Creates an ExecutorCompletionService using the supplied
executor for base task execution and a
LinkedBlockingQueue
as a completion queue.
Parameters |
executor |
Executor : the executor to use |
ExecutorCompletionService
public ExecutorCompletionService (Executor executor,
BlockingQueue<Future<V>> completionQueue)
Creates an ExecutorCompletionService using the supplied
executor for base task execution and the supplied queue as its
completion queue.
Parameters |
executor |
Executor : the executor to use |
completionQueue |
BlockingQueue : the queue to use as the completion queue
normally one dedicated for use by this service. This
queue is treated as unbounded -- failed attempted
Queue.add operations for completed tasks cause
them not to be retrievable. |
Public methods
poll
public Future<V> poll (long timeout,
TimeUnit unit)
Retrieves and removes the Future representing the next
completed task, waiting if necessary up to the specified wait
time if none are yet present.
Parameters |
timeout |
long : how long to wait before giving up, in units of
unit |
unit |
TimeUnit : a TimeUnit determining how to interpret the
timeout parameter |
Returns |
Future<V> |
the Future representing the next completed task or
null if the specified waiting time elapses
before one is present |
poll
public Future<V> poll ()
Retrieves and removes the Future representing the next
completed task, or null
if none are present.
Returns |
Future<V> |
the Future representing the next completed task, or
null if none are present |
submit
public Future<V> submit (Runnable task,
V result)
Submits a Runnable task for execution and returns a Future
representing that task. Upon completion, this task may be
taken or polled.
Parameters |
task |
Runnable : the task to submit |
result |
V : the result to return upon successful completion |
Returns |
Future<V> |
a Future representing pending completion of the task,
and whose get() method will return the given
result value upon completion |
submit
public Future<V> submit (Callable<V> task)
Submits a value-returning task for execution and returns a Future
representing the pending results of the task. Upon completion,
this task may be taken or polled.
Parameters |
task |
Callable : the task to submit |
Returns |
Future<V> |
a Future representing pending completion of the task |
take
public Future<V> take ()
Retrieves and removes the Future representing the next
completed task, waiting if none are yet present.
Returns |
Future<V> |
the Future representing the next completed task |