Class ExecutorCompletionService<V>
java.lang.Object
java.util.concurrent.ExecutorCompletionService<V>
- Type Parameters:
V
- the type of values the tasks of this service produce and consume
- All Implemented Interfaces:
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);
}
- Since:
- 1.5
-
Constructor Summary
ConstructorDescriptionExecutorCompletionService
(Executor executor) Creates an ExecutorCompletionService using the supplied executor for base task execution and aLinkedBlockingQueue
as a completion queue.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. -
Method Summary
Modifier and TypeMethodDescriptionpoll()
Retrieves and removes the Future representing the next completed task, ornull
if none are present.Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.Submits a Runnable task for execution and returns a Future representing that task.Submits a value-returning task for execution and returns a Future representing the pending results of the task.take()
Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
-
Constructor Details
-
ExecutorCompletionService
Creates an ExecutorCompletionService using the supplied executor for base task execution and aLinkedBlockingQueue
as a completion queue.- Parameters:
executor
- the executor to use- Throws:
NullPointerException
- if executor isnull
-
ExecutorCompletionService
Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue.- Parameters:
executor
- the executor to usecompletionQueue
- the queue to use as the completion queue normally one dedicated for use by this service. This queue is treated as unbounded -- failed attemptedQueue.add
operations for completed tasks cause them not to be retrievable.- Throws:
NullPointerException
- if executor or completionQueue arenull
-
-
Method Details
-
submit
Description copied from interface:CompletionService
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.- Specified by:
submit
in interfaceCompletionService<V>
- Parameters:
task
- the task to submit- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is null
-
submit
Description copied from interface:CompletionService
Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.- Specified by:
submit
in interfaceCompletionService<V>
- Parameters:
task
- the task to submitresult
- the result to return upon successful completion- Returns:
- a Future representing pending completion of the task,
and whose
get()
method will return the given result value upon completion - Throws:
RejectedExecutionException
- if the task cannot be scheduled for executionNullPointerException
- if the task is null
-
take
Description copied from interface:CompletionService
Retrieves and removes the Future representing the next completed task, waiting if none are yet present.- Specified by:
take
in interfaceCompletionService<V>
- Returns:
- the Future representing the next completed task
- Throws:
InterruptedException
- if interrupted while waiting
-
poll
Description copied from interface:CompletionService
Retrieves and removes the Future representing the next completed task, ornull
if none are present.- Specified by:
poll
in interfaceCompletionService<V>
- Returns:
- the Future representing the next completed task, or
null
if none are present
-
poll
Description copied from interface:CompletionService
Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.- Specified by:
poll
in interfaceCompletionService<V>
- Parameters:
timeout
- how long to wait before giving up, in units ofunit
unit
- aTimeUnit
determining how to interpret thetimeout
parameter- Returns:
- the Future representing the next completed task or
null
if the specified waiting time elapses before one is present - Throws:
InterruptedException
- if interrupted while waiting
-