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Added in API level 1

ExecutorCompletionService

public class ExecutorCompletionService
extends Object implements CompletionService<V>

java.lang.Object
   ↳ java.util.concurrent.ExecutorCompletionService<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> ecs
       = new ExecutorCompletionService<Result>(e);
   for (Callable<Result> s : solvers)
     ecs.submit(s);
   int n = solvers.size();
   for (int i = 0; i < n; ++i) {
     Result r = ecs.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> ecs
       = new ExecutorCompletionService<Result>(e);
   int n = solvers.size();
   List<Future<Result>> futures = new ArrayList<>(n);
   Result result = null;
   try {
     for (Callable<Result> s : solvers)
       futures.add(ecs.submit(s));
     for (int i = 0; i < n; ++i) {
       try {
         Result r = ecs.take().get();
         if (r != null) {
           result = r;
           break;
         }
       } catch (ExecutionException ignore) {}
     }
   }
   finally {
     for (Future<Result> f : futures)
       f.cancel(true);
   }

   if (result != null)
     use(result);
 }

Summary

Public constructors

ExecutorCompletionService(Executor executor)

Creates an ExecutorCompletionService using the supplied executor for base task execution and a LinkedBlockingQueue 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.

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
From interface java.util.concurrent.CompletionService

Public constructors

ExecutorCompletionService

Added in API level 1
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
Throws
NullPointerException if executor is null

ExecutorCompletionService

Added in API level 1
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.
Throws
NullPointerException if executor or completionQueue are null

Public methods

poll

Added in API level 1
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
Throws
InterruptedException

poll

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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

Added in API level 1
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
Throws
InterruptedException
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