Java使用Fork/Join框架來並行執行任務

現代的計算機已經向多CPU方向發展，即使是普通的PC，甚至現在的智能手機、多核處理器已被廣泛應用。在未來，處理器的核心數將會發展的越來越多。

雖然硬件上的多核CPU已經十分成熟，但是很多應用程序並未這種多核CPU做好準備，因此並不能很好地利用多核CPU的性能優勢。

爲了充分利用多CPU、多核CPU的性能優勢，級軟基軟件系統應該可以充分“挖掘”每個CPU的計算能力，決不能讓某個CPU處於“空閒”狀態。爲此，可以考慮把一個任務拆分成多個“小任務”,把多個"小任務"放到多個處理器核心上並行執行。當多個“小任務”執行完成之後，再將這些執行結果合併起來即可。

如下面的示意圖所示：

第一步分割任務。首先我們需要有一個fork類來把大任務分割成子任務，有可能子任務還是很大，所以還需要不停的分割，直到分割出的子任務足夠小。

第二步執行任務併合並結果。分割的子任務分別放在雙端隊列裏，然後幾個啓動線程分別從雙端隊列裏獲取任務執行。子任務執行完的結果都統一放在一個隊列裏，啓動一個線程從隊列裏拿數據，然後合併這些數據。

Java提供了ForkJoinPool來支持將一個任務拆分成多個“小任務”並行計算，再把多個“小任務”的結果合成總的計算結果。

ForkJoinPool是ExecutorService的實現類，因此是一種特殊的線程池。ForkJoinPool提供瞭如下兩個常用的構造器。

•  public ForkJoinPool(int parallelism)：創建一個包含parallelism個並行線程的ForkJoinPool
•  public ForkJoinPool() ：以Runtime.getRuntime().availableProcessors()的返回值作爲parallelism來創建ForkJoinPool

創建ForkJoinPool實例後，可以釣魚ForkJoinPool的submit(ForkJoinTask<T> task)或者invoke(ForkJoinTask<T> task)來執行指定任務。其中ForkJoinTask代表一個可以並行、合併的任務。ForkJoinTask是一個抽象類，它有兩個抽象子類：RecursiveAction和RecursiveTask。

• RecursiveTask代表有返回值的任務
• RecursiveAction代表沒有返回值的任務。

一、RecursiveAction

下面以一個沒有返回值的大任務爲例，介紹一下RecursiveAction的用法。

大任務是：打印0-200的數值。

小任務是：每次只能打印50個數值。

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

//RecursiveAction爲ForkJoinTask的抽象子類，沒有返回值的任務
class PrintTask extends RecursiveAction {
    // 每個"小任務"最多隻打印50個數
    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() {
        // 當end-start的值小於MAX時候，開始打印
        if ((end - start) < MAX) {
            for (int i = start; i < end; i++) {
                System.out.println(Thread.currentThread().getName() + "的i值:"
                        + i);
            }
        } else {
            // 將大任務分解成兩個小任務
            int middle = (start + end) / 2;
            PrintTask left = new PrintTask(start, middle);
            PrintTask right = new PrintTask(middle, end);
            // 並行執行兩個小任務
            left.fork();
            right.fork();
        }
    }
}

public class ForkJoinPoolTest {
    /**
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        // 創建包含Runtime.getRuntime().availableProcessors()返回值作爲個數的並行線程的ForkJoinPool
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        // 提交可分解的PrintTask任務
        forkJoinPool.submit(new PrintTask(0, 200));
        forkJoinPool.awaitTermination(2, TimeUnit.SECONDS);//阻塞當前線程直到 ForkJoinPool 中所有的任務都執行結束
        // 關閉線程池
        forkJoinPool.shutdown();
    }

}

運行結果如下：

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

從上面結果來看，ForkJoinPool啓動了兩個線程來執行這個打印任務，這是因爲筆者的計算機的CPU是雙核的。不僅如此，讀者可以看到程序雖然打印了0-199這兩百個數字，但是並不是連續打印的，這是因爲程序將這個打印任務進行了分解，分解後的任務會並行執行，所以不會按順序從0打印 到199。

二、RecursiveTask

下面以一個有返回值的大任務爲例，介紹一下RecursiveTask的用法。

大任務是：計算隨機的100個數字的和。

小任務是：每次只能20個數值的和。

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

//RecursiveTask爲ForkJoinTask的抽象子類，有返回值的任務
class SumTask extends RecursiveTask<Integer> {
    // 每個"小任務"最多隻打印50個數
    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;
        // 當end-start的值小於MAX時候，開始打印
        if ((end - start) < MAX) {
            for (int i = start; i < end; i++) {
                sum += arr[i];
            }
            return sum;
        } else {
            System.err.println("=====任務分解======");
            // 將大任務分解成兩個小任務
            int middle = (start + end) / 2;
            SumTask left = new SumTask(arr, start, middle);
            SumTask right = new SumTask(arr, middle, end);
            // 並行執行兩個小任務
            left.fork();
            right.fork();
            // 把兩個小任務累加的結果合併起來
            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個數字元素
        for (int i = 0; i < arr.length; i++) {
            int temp = random.nextInt(100);
            // 對數組元素賦值,並將數組元素的值添加到total總和中
            total += (arr[i] = temp);
        }
        System.out.println("初始化時的總和=" + total);
        // 創建包含Runtime.getRuntime().availableProcessors()返回值作爲個數的並行線程的ForkJoinPool
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        // 提交可分解的PrintTask任務
        Future<Integer> future = forkJoinPool.submit(new SumTask(arr, 0,
                arr.length));
        System.out.println("計算出來的總和=" + future.get());
        // 關閉線程池
        forkJoinPool.shutdown();
    }

}

計算結果如下：

初始化時的總和=4283
=====任務分解======
=====任務分解======
=====任務分解======
=====任務分解======
=====任務分解======
=====任務分解======
=====任務分解======
計算出來的總和=4283

從上面結果來看，ForkJoinPool將任務分解了7次，程序通過SumTask計算出來的結果，和初始化數組時統計出來的總和是相等的，這表明計算結果一切正常。

讀者還參考以下文章加深對ForkJoinPool的理解：

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

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