ExecutorCompletionService (Java Platform SE 8 ) (original) (raw)

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> solvers) throws InterruptedException { CompletionService ecs = new ExecutorCompletionService(e); int n = solvers.size(); List<Future> futures = new ArrayList<Future>(n); Result result = null; try { for (Callable 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 f : futures) f.cancel(true); }

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

}`