Java使用Fork/Join框架来并行执行任务

时间：2022-09-08 20:01:17

现代的计算机已经向多CPU方向发展，即使是普通的PC，甚至现在的智能手机、多核处理器已被广泛应用。在未来，处理器的核心数将会发展的越来越多。

虽然硬件上的多核CPU已经十分成熟，但是很多应用程序并未这种多核CPU做好准备，因此并不能很好地利用多核CPU的性能优势。

为了充分利用多CPU、多核CPU的性能优势，级软基软件系统应该可以充分“挖掘”每个CPU的计算能力，决不能让某个CPU处于“空闲”状态。为此，可以考虑把一个任务拆分成多个“小任务”,把多个"小任务"放到多个处理器核心上并行执行。当多个“小任务”执行完成之后，再将这些执行结果合并起来即可。

如下面的示意图所示：

Java使用Fork/Join框架来并行执行任务

第一步分割任务。首先我们需要有一个fork类来把大任务分割成子任务，有可能子任务还是很大，所以还需要不停的分割，直到分割出的子任务足够小。

第二步执行任务并合并结果。分割的子任务分别放在双端队列里，然后几个启动线程分别从双端队列里获取任务执行。子任务执行完的结果都统一放在一个队列里，启动一个线程从队列里拿数据，然后合并这些数据。


Java提供了ForkJoinPool来支持将一个任务拆分成多个“小任务”并行计算，再把多个“小任务”的结果合成总的计算结果。

ForkJoinPool是ExecutorService的实现类，因此是一种特殊的线程池。ForkJoinPool提供了如下两个常用的构造器。

  •  public ForkJoinPool(int parallelism)：创建一个包含parallelism个并行线程的ForkJoinPool
  •  public ForkJoinPool() ：以Runtime.getRuntime().availableProcessors()的返回值作为parallelism来创建ForkJoinPool

创建ForkJoinPool实例后，可以钓鱼ForkJoinPool的submit(ForkJoinTask task)或者invoke(ForkJoinTask task)来执行指定任务。其中ForkJoinTask代表一个可以并行、合并的任务。ForkJoinTask是一个抽象类，它有两个抽象子类：RecursiveAction和RecursiveTask。

  • RecursiveTask代表有返回值的任务
  • RecursiveAction代表没有返回值的任务。


一、RecursiveAction

下面以一个没有返回值的大任务为例，介绍一下RecursiveAction的用法。

大任务是：打印0-200的数值。

小任务是：每次只能打印50个数值。

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.TimeUnit;

//RecursiveAction为ForkJoinTask的抽象子类，没有返回值的任务
class PrintTask extends RecursiveAction {
// 每个"小任务"最多只打印50个数
private static final int MAX = 50;

private int start;
private int end;

PrintTask(int start, int end) {
this.start = start;
this.end = end;
}

@Override
protected void compute() {
// 当end-start的值小于MAX时候，开始打印
if ((end - start) < MAX) {
for (int i = start; i < end; i++) {
System.out.println(Thread.currentThread().getName() + "的i值:"
+ i);
}
} else {
// 将大任务分解成两个小任务
int middle = (start + end) / 2;
PrintTask left = new PrintTask(start, middle);
PrintTask right = new PrintTask(middle, end);
// 并行执行两个小任务
left.fork();
right.fork();
}
}
}

public class ForkJoinPoolTest {
/**
 * @param args
 * @throws Exception
 */
public static void main(String[] args) throws Exception {
// 创建包含Runtime.getRuntime().availableProcessors()返回值作为个数的并行线程的ForkJoinPool
ForkJoinPool forkJoinPool = new ForkJoinPool();
// 提交可分解的PrintTask任务
forkJoinPool.submit(new PrintTask(0, 200));
forkJoinPool.awaitTermination(2, TimeUnit.SECONDS);//阻塞当前线程直到 ForkJoinPool 中所有的任务都执行结束
// 关闭线程池
forkJoinPool.shutdown();
}

}

运行结果如下：

ForkJoinPool-1-worker-2的i值:75
ForkJoinPool-1-worker-2的i值:76
ForkJoinPool-1-worker-2的i值:77
ForkJoinPool-1-worker-2的i值:78
ForkJoinPool-1-worker-2的i值:79
ForkJoinPool-1-worker-2的i值:80
ForkJoinPool-1-worker-2的i值:81
ForkJoinPool-1-worker-2的i值:82
ForkJoinPool-1-worker-2的i值:83
ForkJoinPool-1-worker-2的i值:84
ForkJoinPool-1-worker-2的i值:85
ForkJoinPool-1-worker-2的i值:86
ForkJoinPool-1-worker-2的i值:87
ForkJoinPool-1-worker-2的i值:88
ForkJoinPool-1-worker-2的i值:89
ForkJoinPool-1-worker-2的i值:90
ForkJoinPool-1-worker-2的i值:91
ForkJoinPool-1-worker-2的i值:92
ForkJoinPool-1-worker-2的i值:93
ForkJoinPool-1-worker-2的i值:94
ForkJoinPool-1-worker-2的i值:95
ForkJoinPool-1-worker-2的i值:96
ForkJoinPool-1-worker-2的i值:97
ForkJoinPool-1-worker-2的i值:98
ForkJoinPool-1-worker-2的i值:99
ForkJoinPool-1-worker-2的i值:50
ForkJoinPool-1-worker-2的i值:51
ForkJoinPool-1-worker-2的i值:52
ForkJoinPool-1-worker-2的i值:53
ForkJoinPool-1-worker-2的i值:54
ForkJoinPool-1-worker-2的i值:55
ForkJoinPool-1-worker-2的i值:56
ForkJoinPool-1-worker-2的i值:57
ForkJoinPool-1-worker-2的i值:58
ForkJoinPool-1-worker-2的i值:59
ForkJoinPool-1-worker-2的i值:60
ForkJoinPool-1-worker-2的i值:61
ForkJoinPool-1-worker-2的i值:62
ForkJoinPool-1-worker-2的i值:63
ForkJoinPool-1-worker-2的i值:64
ForkJoinPool-1-worker-2的i值:65
ForkJoinPool-1-worker-2的i值:66
ForkJoinPool-1-worker-2的i值:67
ForkJoinPool-1-worker-2的i值:68
ForkJoinPool-1-worker-2的i值:69
ForkJoinPool-1-worker-1的i值:175
ForkJoinPool-1-worker-1的i值:176
ForkJoinPool-1-worker-1的i值:177
ForkJoinPool-1-worker-1的i值:178
ForkJoinPool-1-worker-1的i值:179
ForkJoinPool-1-worker-1的i值:180
ForkJoinPool-1-worker-1的i值:181
ForkJoinPool-1-worker-1的i值:182
ForkJoinPool-1-worker-1的i值:183
ForkJoinPool-1-worker-1的i值:184
ForkJoinPool-1-worker-1的i值:185
ForkJoinPool-1-worker-1的i值:186
ForkJoinPool-1-worker-1的i值:187
ForkJoinPool-1-worker-1的i值:188
ForkJoinPool-1-worker-1的i值:189
ForkJoinPool-1-worker-1的i值:190
ForkJoinPool-1-worker-1的i值:191
ForkJoinPool-1-worker-1的i值:192
ForkJoinPool-1-worker-1的i值:193
ForkJoinPool-1-worker-1的i值:194
ForkJoinPool-1-worker-1的i值:195
ForkJoinPool-1-worker-1的i值:196
ForkJoinPool-1-worker-1的i值:197
ForkJoinPool-1-worker-1的i值:198
ForkJoinPool-1-worker-1的i值:199
ForkJoinPool-1-worker-1的i值:150
ForkJoinPool-1-worker-1的i值:151
ForkJoinPool-1-worker-1的i值:152
ForkJoinPool-1-worker-1的i值:153
ForkJoinPool-1-worker-1的i值:154
ForkJoinPool-1-worker-1的i值:155
ForkJoinPool-1-worker-1的i值:156
ForkJoinPool-1-worker-1的i值:157
ForkJoinPool-1-worker-1的i值:158
ForkJoinPool-1-worker-1的i值:159
ForkJoinPool-1-worker-1的i值:160
ForkJoinPool-1-worker-1的i值:161
ForkJoinPool-1-worker-1的i值:162
ForkJoinPool-1-worker-1的i值:163
ForkJoinPool-1-worker-1的i值:164
ForkJoinPool-1-worker-1的i值:165
ForkJoinPool-1-worker-1的i值:166
ForkJoinPool-1-worker-1的i值:167
ForkJoinPool-1-worker-1的i值:168
ForkJoinPool-1-worker-1的i值:169
ForkJoinPool-1-worker-1的i值:170
ForkJoinPool-1-worker-1的i值:171
ForkJoinPool-1-worker-1的i值:172
ForkJoinPool-1-worker-1的i值:173
ForkJoinPool-1-worker-1的i值:174
ForkJoinPool-1-worker-1的i值:125
ForkJoinPool-1-worker-1的i值:126
ForkJoinPool-1-worker-1的i值:127
ForkJoinPool-1-worker-1的i值:128
ForkJoinPool-1-worker-1的i值:129
ForkJoinPool-1-worker-1的i值:130
ForkJoinPool-1-worker-1的i值:131
ForkJoinPool-1-worker-1的i值:132
ForkJoinPool-1-worker-1的i值:133
ForkJoinPool-1-worker-1的i值:134
ForkJoinPool-1-worker-1的i值:135
ForkJoinPool-1-worker-1的i值:136
ForkJoinPool-1-worker-1的i值:137
ForkJoinPool-1-worker-1的i值:138
ForkJoinPool-1-worker-1的i值:139
ForkJoinPool-1-worker-1的i值:140
ForkJoinPool-1-worker-1的i值:141
ForkJoinPool-1-worker-1的i值:142
ForkJoinPool-1-worker-1的i值:143
ForkJoinPool-1-worker-1的i值:144
ForkJoinPool-1-worker-1的i值:145
ForkJoinPool-1-worker-1的i值:146
ForkJoinPool-1-worker-1的i值:147
ForkJoinPool-1-worker-1的i值:148
ForkJoinPool-1-worker-1的i值:149
ForkJoinPool-1-worker-1的i值:100
ForkJoinPool-1-worker-1的i值:101
ForkJoinPool-1-worker-1的i值:102
ForkJoinPool-1-worker-1的i值:103
ForkJoinPool-1-worker-1的i值:104
ForkJoinPool-1-worker-1的i值:105
ForkJoinPool-1-worker-1的i值:106
ForkJoinPool-1-worker-1的i值:107
ForkJoinPool-1-worker-1的i值:108
ForkJoinPool-1-worker-1的i值:109
ForkJoinPool-1-worker-1的i值:110
ForkJoinPool-1-worker-1的i值:111
ForkJoinPool-1-worker-1的i值:112
ForkJoinPool-1-worker-1的i值:113
ForkJoinPool-1-worker-1的i值:114
ForkJoinPool-1-worker-1的i值:115
ForkJoinPool-1-worker-1的i值:116
ForkJoinPool-1-worker-1的i值:117
ForkJoinPool-1-worker-1的i值:118
ForkJoinPool-1-worker-1的i值:119
ForkJoinPool-1-worker-1的i值:120
ForkJoinPool-1-worker-1的i值:121
ForkJoinPool-1-worker-1的i值:122
ForkJoinPool-1-worker-1的i值:123
ForkJoinPool-1-worker-1的i值:124
ForkJoinPool-1-worker-1的i值:25
ForkJoinPool-1-worker-1的i值:26
ForkJoinPool-1-worker-1的i值:27
ForkJoinPool-1-worker-1的i值:28
ForkJoinPool-1-worker-1的i值:29
ForkJoinPool-1-worker-1的i值:30
ForkJoinPool-1-worker-1的i值:31
ForkJoinPool-1-worker-1的i值:32
ForkJoinPool-1-worker-1的i值:33
ForkJoinPool-1-worker-1的i值:34
ForkJoinPool-1-worker-1的i值:35
ForkJoinPool-1-worker-1的i值:36
ForkJoinPool-1-worker-1的i值:37
ForkJoinPool-1-worker-1的i值:38
ForkJoinPool-1-worker-1的i值:39
ForkJoinPool-1-worker-1的i值:40
ForkJoinPool-1-worker-1的i值:41
ForkJoinPool-1-worker-1的i值:42
ForkJoinPool-1-worker-1的i值:43
ForkJoinPool-1-worker-1的i值:44
ForkJoinPool-1-worker-1的i值:45
ForkJoinPool-1-worker-1的i值:46
ForkJoinPool-1-worker-1的i值:47
ForkJoinPool-1-worker-1的i值:48
ForkJoinPool-1-worker-1的i值:49
ForkJoinPool-1-worker-1的i值:0
ForkJoinPool-1-worker-1的i值:1
ForkJoinPool-1-worker-1的i值:2
ForkJoinPool-1-worker-1的i值:3
ForkJoinPool-1-worker-1的i值:4
ForkJoinPool-1-worker-1的i值:5
ForkJoinPool-1-worker-1的i值:6
ForkJoinPool-1-worker-1的i值:7
ForkJoinPool-1-worker-1的i值:8
ForkJoinPool-1-worker-1的i值:9
ForkJoinPool-1-worker-1的i值:10
ForkJoinPool-1-worker-1的i值:11
ForkJoinPool-1-worker-1的i值:12
ForkJoinPool-1-worker-1的i值:13
ForkJoinPool-1-worker-1的i值:14
ForkJoinPool-1-worker-1的i值:15
ForkJoinPool-1-worker-1的i值:16
ForkJoinPool-1-worker-1的i值:17
ForkJoinPool-1-worker-1的i值:18
ForkJoinPool-1-worker-1的i值:19
ForkJoinPool-1-worker-1的i值:20
ForkJoinPool-1-worker-1的i值:21
ForkJoinPool-1-worker-1的i值:22
ForkJoinPool-1-worker-1的i值:23
ForkJoinPool-1-worker-1的i值:24
ForkJoinPool-1-worker-2的i值:70
ForkJoinPool-1-worker-2的i值:71
ForkJoinPool-1-worker-2的i值:72
ForkJoinPool-1-worker-2的i值:73
ForkJoinPool-1-worker-2的i值:74

从上面结果来看，ForkJoinPool启动了两个线程来执行这个打印任务，这是因为笔者的计算机的CPU是双核的。不仅如此，读者可以看到程序虽然打印了0-199这两百个数字，但是并不是连续打印的，这是因为程序将这个打印任务进行了分解，分解后的任务会并行执行，所以不会按顺序从0打印 到199。

Java使用Fork/Join框架来并行执行任务

二、RecursiveTask

下面以一个有返回值的大任务为例，介绍一下RecursiveTask的用法。

大任务是：计算随机的100个数字的和。

小任务是：每次只能20个数值的和。

import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.Future;
import java.util.concurrent.RecursiveTask;

//RecursiveTask为ForkJoinTask的抽象子类，有返回值的任务
class SumTask extends RecursiveTask<integer> {
// 每个"小任务"最多只打印50个数
private static final int MAX = 20;
private int arr[];
private int start;
private int end;

SumTask(int arr[], int start, int end) {
this.arr = arr;
this.start = start;
this.end = end;
}

@Override
protected Integer compute() {
int sum = 0;
// 当end-start的值小于MAX时候，开始打印
if ((end - start) < MAX) {
for (int i = start; i < end; i++) {
sum += arr[i];
}
return sum;
} else {
System.err.println("=====任务分解======");
// 将大任务分解成两个小任务
int middle = (start + end) / 2;
SumTask left = new SumTask(arr, start, middle);
SumTask right = new SumTask(arr, middle, end);
// 并行执行两个小任务
left.fork();
right.fork();
// 把两个小任务累加的结果合并起来
return left.join() + right.join();
}
}

}

public class ForkJoinPoolTest2 {
/**
 * @param args
 * @throws Exception
 */
public static void main(String[] args) throws Exception {
int arr[] = new int[100];
Random random = new Random();
int total = 0;
// 初始化100个数字元素
for (int i = 0; i < arr.length; i++) {
int temp = random.nextInt(100);
// 对数组元素赋值,并将数组元素的值添加到total总和中
total += (arr[i] = temp);
}
System.out.println("初始化时的总和=" + total);
// 创建包含Runtime.getRuntime().availableProcessors()返回值作为个数的并行线程的ForkJoinPool
ForkJoinPool forkJoinPool = new ForkJoinPool();
// 提交可分解的PrintTask任务
Future<integer> future = forkJoinPool.submit(new SumTask(arr, 0,
arr.length));
System.out.println("计算出来的总和=" + future.get());
// 关闭线程池
forkJoinPool.shutdown();
}

}
</integer></integer>

计算结果如下： 
初始化时的总和=4283
=====任务分解======
=====任务分解======
=====任务分解======
=====任务分解======
=====任务分解======
=====任务分解======
=====任务分解======
计算出来的总和=4283

从上面结果来看，ForkJoinPool将任务分解了7次，程序通过SumTask计算出来的结果，和初始化数组时统计出来的总和是相等的，这表明计算结果一切正常。



读者还参考以下文章加深对ForkJoinPool的理解：

http://www.infoq.com/cn/articles/fork-join-introduction/

http://www.ibm.com/developerworks/cn/java/j-lo-forkjoin/

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