CompletableFuture get result without blocking

Question

private static boolean validateSMTP(final ArrayList mxList, String address) throws ExecutionException, InterruptedException {
  ExecutorService pool = Executors.newFixedThreadPool(mxList.size());
  List<CompletableFuture<Boolean>> allVerifiers = new ArrayList<>();
    for (int mx = 0; mx < mxList.size(); mx++) {
        CompletableFuture<Boolean> verifier = createAsyncVerifier((String) mxList.get(mx), address, pool);
        verifier.thenApply(isvalid -> isvalid);
        verifier.get();
    }
  return false;
}

In the above code I want to create mxList.size() CompletableFuture, execute each. If the result of any of them is true I want to break the loop, when I used the get() method it blocks and I waste the benefit of concurrency, any idea about how to do that?

Answer 1

Here's an implementation that submits all tasks and then gets results, returning on the first true result:

private static boolean validateSMTP(final ArrayList<String> mxList, String address)
        throws ExecutionException, InterruptedException {

    ExecutorService pool = Executors.newFixedThreadPool(mxList.size());

    return mxList.stream()
            .map(mx -> createAsyncVerifier(mx, address, pool))
            .collect(Collectors.toList())
            .stream()
            .map(CompletableFuture<Boolean>::join)
            .filter(b -> b)
            .findFirst()
            .orElse(Boolean.FALSE);
}

The .collect(Collectors.toList()) ensures that all tasks are submitted. In the second stream, join is called, but this doesn't cause unnecessary waits as all tasks are already submitted.

findFirst will return as soon as the first element passes the filter.