ExecutorCompletionService

AI-generated Key Takeaways

• ExecutorCompletionService executes tasks using a provided Executor and places completed tasks on a queue for retrieval.

• It allows you to process the results of completed tasks as they become available, using methods like take() and poll().

• This class is particularly useful when handling multiple tasks concurrently and you need to retrieve their results in the order of completion.

• You can submit tasks using submit() and retrieve their results as Future objects.

• ExecutorCompletionService facilitates scenarios like processing the first non-null result from a set of tasks or iterating through results as they become available.

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> 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);
 }

Public 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.

Public Method Summary

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 Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

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	the executor to use

Throws

NullPointerException	if executor is null

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

Public Methods

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	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

public Future<V> poll ()

Retrieves and removes the Future representing the next completed task, or null if none are present.

Returns

• the Future representing the next completed task, or null if none are present

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	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

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	the task to submit

Returns

• a Future representing pending completion of the task

public Future<V> take ()

Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Returns

• the Future representing the next completed task

Throws

InterruptedException