Java并发编程核心方法与框架-Fork-Join分治编程（一）

在JDK1.7版本中提供了Fork-Join并行执行任务框架，它的主要作用是把大任务分割成若干个小任务，再对每个小任务得到的结果进行汇总，这种开发方法也叫做分治编程，可以极大地利用CPU资源，提高任务执行的效率。

使用RecursiveAction分解任务

public class MyRecursiveAction extends RecursiveAction {

    private int beginValue;
    private int endValue;
    
    public MyRecursiveAction(int beginValue, int endValue) {
        super();
        this.beginValue = beginValue;
        this.endValue = endValue;
    }
    @Override
    protected void compute() {
        System.out.println("MyRecursiveAction.compute()----" + Thread.currentThread().getName());
        if (endValue - beginValue > 2) {
            int middleValue = (beginValue + endValue)/2;
            MyRecursiveAction leftAction = new MyRecursiveAction(beginValue, middleValue);
            MyRecursiveAction rightAction = new MyRecursiveAction(middleValue + 1, endValue);
            invokeAll(leftAction, rightAction);
        } else {
            System.out.println("组合result:" + beginValue + "--" + endValue);
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        pool.submit(new MyRecursiveAction(1, 10));
        Thread.sleep(5000);
    }
}

控制台输出结果如下：

MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
组合result:1--3
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
组合result:4--5
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
组合result:6--8
MyRecursiveAction.compute()----ForkJoinPool-1-worker-1
组合result:9--10

使用RecursiveTask取得返回值

public class MyRecursiveTask extends RecursiveTask {

    @Override
    protected Integer compute() {
        System.out.println("Time:" + System.currentTimeMillis());
        return 100;
    }
}

public class Test1 {
    public static void main(String[] args) {
        try {
            MyRecursiveTask task1 = new MyRecursiveTask();
            System.out.println(task1.hashCode());
            ForkJoinPool pool = new ForkJoinPool();
            ForkJoinTask task2 = pool.submit(task1);
            System.out.println(task2.hashCode() + " " + task2.get());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

程序输出结果如下：

1311053135
Time:1473599556598
1311053135 100

也可以使用join()方法取得返回值

public class Test2 {
    public static void main(String[] args) {
        try {
            MyRecursiveTask task1 = new MyRecursiveTask();
            System.out.println(task1.hashCode());
            ForkJoinPool pool = new ForkJoinPool();
            ForkJoinTask task2 = pool.submit(task1);
            System.out.println(task2.hashCode() + " " + task2.join());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

程序输出结果如下

1311053135
Time:1473599778476
1311053135 100

方法join()和方法get()虽然都能取得计算后的结果值，但对异常的处理存在区别。使用get()方法执行任务时，当子任务出现异常时，可以在main线程中进行捕获。使用join()方法出现异常时则直接抛出。

---