How to Efficiently Wait for a List of Futures and Handle Exceptions?

Waiting Efficiently on a List of Futures

When dealing with a list of futures, the goal is to wait until completion or the occurrence of an exception in any future. A naive approach involving individual calls to f.get() can lead to unnecessary waiting if an exception occurs early on.

Avoiding Unnecessary Waiting with CompletionService

To address this issue, the CompletionService class comes into play. It allows asynchronous tasks to be executed and their results to be retrieved in a thread-safe manner as they become available. Here's how to use it:

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

// Submit tasks to the service
for (int i = 0; i < 4; i++) {
    completionService.submit(new Callable<SomeResult>() {
        @Override
        public SomeResult call() {
            // Task logic here
            return result;
        }
    });
}

int received = 0;
boolean errors = false;

// Loop until all tasks are complete or an error occurs
while (received < 4 && !errors) {
    Future<SomeResult> resultFuture = completionService.take(); // Blocks if nothing available
    try {
        SomeResult result = resultFuture.get();
        received++;
        // Process result here
    } catch (Exception e) {
        // Log error
        errors = true;
    }
}

// Consider canceling remaining tasks if an error occurred</code>

By using the CompletionService approach, you can monitor the completion of tasks in real-time and halt further processing if an error occurs, thus avoiding unnecessary waiting.

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