fork join例子说明

public static class SelectMaxProblem {
        private final int[] numbers;
        private final int start;
        private final int end;
        public final int size;

        // constructors elided

        public SelectMaxProblem(int[] numbers, int start, int end) {
            this.numbers = numbers;
            this.start = start;
            this.end = end;
            this.size = end - start;
        }

        public int solveSequentially() {
            int max = Integer.MIN_VALUE;
            for (int i=start; i<end; i++) {
                int n = numbers[i];
                if (n > max)
                    max = n;
            }
            return max;
        }

        public SelectMaxProblem subproblem(int subStart, int subEnd) {
            return new SelectMaxProblem(numbers, start + subStart,
                    start + subEnd);
        }

        @Override
        public String toString() {
            return String.format("P{start:%2s, end:%2s}",start,end);
        }
    }

public static class MaxWithFJ extends RecursiveAction {
        private final int threshold;
        private final SelectMaxProblem problem;
        public int result;

        public MaxWithFJ(SelectMaxProblem problem, int threshold) {
            this.problem = problem;
            this.threshold = threshold;
        }

        protected void compute() {
            String pre = problem.toString();
            if (problem.size < threshold) {
                result = problem.solveSequentially();
                print(pre,"return result:"+result);
            }else {
                int midpoint = problem.size / 2;
                SelectMaxProblem leftProblem = problem.subproblem(0, midpoint);
                SelectMaxProblem rightProblem = problem.subproblem(midpoint + 1, problem.size);
                MaxWithFJ left = new MaxWithFJ(leftProblem, threshold);
                MaxWithFJ right = new MaxWithFJ(rightProblem, threshold);
                print(pre,leftProblem + "|" + rightProblem);
                left.fork();
                right.fork();
                print(pre,"fork");
                print(pre,"begin left join");
                left.join();
                print(pre,"after left join.begin rigth join");
                right.join();
                print(pre,"join");
                result = Math.max(left.result, right.result);
            }
        }

        public static void main(String[] args) {
            int size = 40;
            int[] numbers = new int[size];
            for (int i = 0; i < size; i++) {
                numbers[i] = i;
            }
            SelectMaxProblem problem = new SelectMaxProblem(numbers,0,numbers.length);
            int threshold = 10;
            int nThreads = 2;
            MaxWithFJ mfj = new MaxWithFJ(problem, threshold);
            ForkJoinPool fjPool = new ForkJoinPool(nThreads);
            fjPool.invoke(mfj);
            int result = mfj.result;
            print("main",result);
        }

        static void print(String pre,Object object) {
            Thread thread = Thread.currentThread();
            String msg = String.format("Thread[%s] %s > %s", thread.getId(), pre, object);

            System.out.println(msg);
        }
    }

输出结果:

Thread[11] P{start: 0, end:40} > P{start: 0, end:20}|P{start:21, end:40}
Thread[11] P{start: 0, end:40} > fork
Thread[11] P{start: 0, end:40} > begin left join
Thread[12] P{start: 0, end:20} > P{start: 0, end:10}|P{start:11, end:20}
Thread[12] P{start: 0, end:20} > fork
Thread[12] P{start: 0, end:20} > begin left join
Thread[13] P{start:21, end:40} > P{start:21, end:30}|P{start:31, end:40}
Thread[13] P{start:21, end:40} > fork
Thread[13] P{start:21, end:40} > begin left join
Thread[12] P{start: 0, end:10} > P{start: 0, end: 5}|P{start: 6, end:10}
Thread[13] P{start:21, end:30} > return result:29
Thread[12] P{start: 0, end:10} > fork
Thread[13] P{start:21, end:40} > after left join.begin rigth join
Thread[12] P{start: 0, end:10} > begin left join
Thread[12] P{start: 0, end: 5} > return result:4
Thread[13] P{start:31, end:40} > return result:39
Thread[12] P{start: 0, end:10} > after left join.begin rigth join
Thread[13] P{start:21, end:40} > join
Thread[12] P{start: 6, end:10} > return result:9
Thread[13] P{start:11, end:20} > return result:19
Thread[12] P{start: 0, end:10} > join
Thread[12] P{start: 0, end:20} > after left join.begin rigth join
Thread[12] P{start: 0, end:20} > join
Thread[11] P{start: 0, end:40} > after left join.begin rigth join
Thread[11] P{start: 0, end:40} > join
Thread[1] main > 39

说明:
1.RecursiveAction#fork代表用新的线程来执行
2.RecursiveAction#join会让当前线程开始等待.这里和普通的Thread#join的区别是,线程在调用join函数时,会去执行别的任务.可以从输出看到编号12和13的线程,join后再执行子任务.