Introduction

By default, ForkJoinPool creates a thread for each processor, and parallelStream will use this shared thread pool to submit tasks if not specified.

So how do we handle a specific situation where we want to use a custom ForkJoinPool?

Common operations

If we want to do an addition from 1 to 1000, we can use parallel stream like this.

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List<Integer> integerList= IntStream.range(1,1000).boxed().collect(Collectors.toList());
        ForkJoinPool customThreadPool = new ForkJoinPool(4);

        Integer total= integerList.parallelStream().reduce(0, Integer::sum);
        log.info("{}",total);

Output result.

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INFO com.javaisland.CustThreadPool - 499500

Using a custom ForkJoinPool

The above example uses a shared thread pool. Let’s look at how to use a custom thread pool to commit a parallel stream:

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List<Integer> integerList= IntStream.range(1,1000).boxed().collect(Collectors.toList());

ForkJoinPool customThreadPool = new ForkJoinPool(4);
        Integer actualTotal = customThreadPool.submit(
                () -> integerList.parallelStream().reduce(0, Integer::sum)).get();
        log.info("{}",actualTotal);

In the above example, we defined a ForkJoinPool with 4 threads and used it to submit this parallelStream.

Output result.

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INFO com.javaisland.CustThreadPool - 499500

Summary

If you don’t want to use a public thread pool, you can use a custom ForkJoinPool to submit it.