ForkJoinTask 理解

Question

subtask.fork() 返回ForkJoinTask<Int> 对象后会发生什么？之后，compute() 方法被调用了很多次，但是从哪里以及如何调用？

我是 Java 新手，我正在尝试学习 fork/join 框架的概念。我在网上看到了下面的代码。我在阅读 Java API 后知道subtask.fork() 返回一个ForkJoinTask<V> 的对象，在这种情况下是ForkJoinTask<Int>。我无法理解的是之后会发生什么？输出表明 compute() 方法在此之后已被多次调用，但从何处以及如何调用？

public class MyRecursiveAction extends RecursiveAction {

    public static void main(String[] args) {
        MyRecursiveAction mra1 = new MyRecursiveAction(100);
        ForkJoinPool fjp1 = new ForkJoinPool();
        fjp1.invoke(mra1);
    }

    private long workLoad = 0;

    public MyRecursiveAction(long workLoad) {
        this.workLoad = workLoad;
    }

    @Override
    protected void compute() {

        //if work is above threshold, break tasks up into smaller tasks
        if(this.workLoad > 16) {
            System.out.println("Splitting workLoad : " + this.workLoad);

            List<MyRecursiveAction> subtasks =
                new ArrayList<MyRecursiveAction>();

            subtasks.addAll(createSubtasks());

            for(RecursiveAction subtask : subtasks){
                subtask.fork();
            }

        } else {
            System.out.println("Doing workLoad myself: " + this.workLoad);
        }
    }

    private List<MyRecursiveAction> createSubtasks() {
        List<MyRecursiveAction> subtasks =
            new ArrayList<MyRecursiveAction>();

        MyRecursiveAction subtask1 = new MyRecursiveAction(this.workLoad / 2);
        MyRecursiveAction subtask2 = new MyRecursiveAction(this.workLoad / 2);

        subtasks.add(subtask1);
        subtasks.add(subtask2);

        return subtasks;
    }

}

输出：

Splitting workLoad : 100
Splitting workLoad : 50
Splitting workLoad : 50
Splitting workLoad : 25
Doing workLoad myself: 12
Doing workLoad myself: 12
Splitting workLoad : 25
Doing workLoad myself: 12
Doing workLoad myself: 12
Splitting workLoad : 25
Doing workLoad myself: 12
Splitting workLoad : 25
Doing workLoad myself: 12
Doing workLoad myself: 12
Doing workLoad myself: 12

Answer 1

您需要调用 invokeAll 以便将您的工作分成两部分而不是手动执行，然后将创建 2 个新线程来“计算”您的结果，然后如果它不够小，则 2 个新线程等等..，

您无法预测线程将以什么顺序执行，因此每次运行此代码时消息顺序都会有所不同。

public class MyRecursiveAction extends RecursiveAction {

    public static void main(String[] args) {
        MyRecursiveAction mra1 = new MyRecursiveAction(100);
        ForkJoinPool fjp1 = new ForkJoinPool();
        fjp1.invoke(mra1);
    }

    private long workLoad = 0;

    public MyRecursiveAction(long workLoad) {
        this.workLoad = workLoad;
    }

    @Override
    protected void compute() {

        //if work is above threshold, break tasks up into smaller tasks
        if(this.workLoad > 16) {
            System.out.println("Splitting workLoad : " + this.workLoad);

            invokeAll(new MyRecursiveAction(this.workLoad / 2), new MyRecursiveAction(this.workLoad / 2));

        } else {
            System.out.println("Doing workLoad myself: " + this.workLoad);
        }
    }

}

Java doc of ForkJoinTask