Java™ Platform
Standard Ed. 6

java.util.concurrent
Class ExecutorCompletionService<V>

java.lang.Object
  extended by java.util.concurrent.ExecutorCompletionService<V>
All Implemented Interfaces:
CompletionService<V>

public class ExecutorCompletionService<V>
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> 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<Future<Result>>(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);
   }
 


Constructor Summary
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.
 
Method Summary
 Future<V> poll()
          Retrieves and removes the Future representing the next completed task or null if none are present.
 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> submit(Callable<V> task)
          Submits a value-returning task for execution and returns a Future representing the pending results of the task.
 Future<V> submit(Runnable task, V result)
          Submits a Runnable task for execution and returns a Future representing that task.
 Future<V> take()
          Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

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 - the executor to use
Throws:
NullPointerException - if executor is null

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 - the executor to use
completionQueue - the queue to use as the completion queue normally one dedicated for use by this service
Throws:
NullPointerException - if executor or completionQueue are null
Method Detail

submit

public Future<V> submit(Callable<V> task)
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 interface CompletionService<V>
Parameters:
task - the task to submit
Returns:
a Future representing pending completion of the task

submit

public Future<V> submit(Runnable task,
                        V result)
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 interface CompletionService<V>
Parameters:
task - the task to submit
result - 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

take

public Future<V> take()
               throws InterruptedException
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 interface CompletionService<V>
Returns:
the Future representing the next completed task
Throws:
InterruptedException - if interrupted while waiting

poll

public Future<V> poll()
Description copied from interface: CompletionService
Retrieves and removes the Future representing the next completed task or null if none are present.

Specified by:
poll in interface CompletionService<V>
Returns:
the Future representing the next completed task, or null if none are present

poll

public Future<V> poll(long timeout,
                      TimeUnit unit)
               throws InterruptedException
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 interface CompletionService<V>
Parameters:
timeout - how long to wait before giving up, in units of unit
unit - a TimeUnit determining how to interpret the timeout 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

Java™ Platform
Standard Ed. 6

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For further API reference and developer documentation, see Java SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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