concurrency5ForkJoinDemo

1package pkg7concurrency;2 3/*4 * concurrency5ForkJoinDemo.java5 * -----------------6 * The Fork/Join framework (Java 7) for divide-and-conquer parallelism using a7 * work-stealing pool. RecursiveTask returns a result; RecursiveAction doesn't.8 *9 * IDEA: split the problem (fork), solve subparts in parallel, combine (join).10 * Threshold avoids over-splitting tiny chunks (overhead > benefit).11 */12import java.util.concurrent.*;13 14public class concurrency5ForkJoinDemo {15 16    static class SumTask extends RecursiveTask<Long> {17        private static final int THRESHOLD = 10_000;18        private final long[] arr;19        private final int lo, hi;20 21        SumTask(long[] arr, int lo, int hi) { this.arr = arr; this.lo = lo; this.hi = hi; }22 23        @Override protected Long compute() {24            if (hi - lo <= THRESHOLD) {            // small enough: compute directly25                long sum = 0;26                for (int i = lo; i < hi; i++) sum += arr[i];27                return sum;28            }29            int mid = (lo + hi) >>> 1;30            SumTask left = new SumTask(arr, lo, mid);31            SumTask right = new SumTask(arr, mid, hi);32            left.fork();                            // run left asynchronously33            long rightResult = right.compute();     // compute right on this thread34            long leftResult = left.join();          // wait for left35            return leftResult + rightResult;36        }37    }38 39    public static void main(String[] args) {40        long[] data = new long[1_000_000];41        for (int i = 0; i < data.length; i++) data[i] = i + 1;     // 1..1,000,00042 43        ForkJoinPool pool = ForkJoinPool.commonPool();44        long parallelSum = pool.invoke(new SumTask(data, 0, data.length));45 46        long expected = (long) data.length * (data.length + 1) / 2;  // n(n+1)/247        System.out.println("parallelism: " + pool.getParallelism());48        System.out.println("fork/join sum = " + parallelSum);49        System.out.println("expected      = " + expected);50        System.out.println("match: " + (parallelSum == expected));51    }52}