ExecutorCompletionService

ExecutorCompletionService | API reference | Android Developers

Summary: Ctors | Methods | Inherited Methods

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 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

`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

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

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`	`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

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

Throws
`InterruptedException`

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

Throws
`NullPointerException`
`RejectedExecutionException`

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

Throws
`NullPointerException`
`RejectedExecutionException`

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

Throws
`InterruptedException`