How can CompletionService be used to optimize waiting for a list of futures and handle exceptions efficiently?

Optimizing Future List Waiting with CompletionService

When working with a list of futures, it's essential to efficiently wait for their completion or handle exceptions to prevent unnecessary delays.

Suppose you have the following method that returns a list of futures:

List<Future<O>> futures = getFutures();

To wait for completion or catch exceptions, a naive approach might be:

wait() {
  for(Future f : futures) {
    try {
      f.get();
    } catch(Exception e) {
       //Specific exception handling
       //Exception in a future, stop waiting
       return;
    }
  }
}

However, this approach waits for each future regardless of exceptions in previous futures.

A solution is to employ a CompletionService to receive futures as they become available. If an exception occurs, you can cancel other tasks:

Executor executor = Executors.newFixedThreadPool(4);
CompletionService<SomeResult> completionService = new ExecutorCompletionService<SomeResult>(executor);

//Submit tasks
for(int i = 0; i < 4; i++) {
  completionService.submit(() -> {
    ...
    return result;
  });
}

int received = 0;
boolean errors = false;

while(received < 4 && !errors) {
  Future<SomeResult> resultFuture = completionService.take(); //Blocks if none available
  try {
    SomeResult result = resultFuture.get();
    received++;
    ... //Process result
  } catch(Exception e) {
    //Log and set error flag
    errors = true;
  }
}

In this approach, tasks are submitted to the executor, and completed tasks are received through the completion service. If an exception occurs in a received task, the while loop terminates, and you can cancel any remaining tasks using the executor's shutdownNow() method.

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