Class ExecutorCompletionService<V>

java.lang.Object
- 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> 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

Constructors
Constructor	Description
`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

All Methods Instance Methods Concrete Methods
Modifier and Type	Method	Description
`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.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods declared in interface java.util.concurrent.CompletionService

poll, poll, take

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. 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

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
Throws:: RejectedExecutionException - if the task cannot be scheduled for execution; NullPointerException - if the task is null

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
Throws:: RejectedExecutionException - if the task cannot be scheduled for execution; NullPointerException - if the task is null