ExecutorCompletionService

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> solvers)
throws InterruptedException, ExecutionException {
CompletionService 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> solvers)
throws InterruptedException {
CompletionService cs
= new ExecutorCompletionService<>(e);
int n = solvers.size();
List> 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);
}`

Parameters