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<Integer> {

	@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<Integer> 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<Integer> 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。

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