Can Java 8 Parallel Streams Utilize a Custom Thread Pool?

Custom Thread Pool for Java 8 Parallel Streams

Java 8's parallel stream provides a convenient way to parallelize operations and improve performance. However, in certain scenarios, it may be desirable to use a custom thread pool to control thread allocation and compartmentalize different tasks within an application.

Can Parallel Streams Use a Custom Thread Pool?

Initially, there was no explicit way to assign a custom thread pool to a parallel stream. This could pose challenges when dealing with applications with multiple modules and the need to prevent slow tasks from blocking others.

The Fork-Join Pool Trick

However, there is a workaround using the Fork-Join Pool API. By submitting a parallel task within a separate fork-join pool, it is possible to isolate it from the common pool used by the stream operations. Here's an example:

final int parallelism = 4;
ForkJoinPool forkJoinPool = null;
try {
    forkJoinPool = new ForkJoinPool(parallelism);
    final List<Integer> primes = forkJoinPool.submit(() -> {
        // Parallel task here, for example
        IntStream.range(1, 1_000_000).parallel()
                .filter(PrimesPrint::isPrime)
                .boxed().collect(Collectors.toList())
    }).get();
    System.out.println(primes);
} catch (InterruptedException | ExecutionException e) {
    throw new RuntimeException(e);
} finally {
    if (forkJoinPool != null) {
        forkJoinPool.shutdown();
    }
}

This code creates a dedicated fork-join pool with a specified parallelism and submits the parallel task execution to it. This allows the specified tasks to operate within their own thread pool, separating them from the common pool.

Conclusion

The workaround using a fork-join pool provides a means to use a custom thread pool for parallel streams in Java 8. It allows for finer control over thread allocation and compartmentalization, enabling the parallel execution of tasks without cross-module blocking.

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