Java使用Fork/Join框架来并行执行任务

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：

 

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<T> task)或者invoke(ForkJoinTask<T> 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。

 

[java]  view plain copy

 

 

1. import java.util.concurrent.ForkJoinPool;  
2. import java.util.concurrent.RecursiveAction;  
3. import java.util.concurrent.TimeUnit;  
4.   
5. //RecursiveAction为ForkJoinTask的抽象子类，没有返回值的任务  
6. class PrintTask extends RecursiveAction {  
7.     // 每个"小任务"最多只打印50个数  
8.     private static final int MAX = 50;  
9.   
10.     private int start;  
11.     private int end;  
12.   
13.     PrintTask(int start, int end) {  
14.         this.start = start;  
15.         this.end = end;  
16.     }  
17.   
18.     @Override  
19.     protected void compute() {  
20.         // 当end-start的值小于MAX时候，开始打印  
21.         if ((end - start) < MAX) {  
22.             for (int i = start; i < end; i++) {  
23.                 System.out.println(Thread.currentThread().getName() + "的i值:"  
24.                         + i);  
25.             }  
26.         } else {  
27.             // 将大任务分解成两个小任务  
28.             int middle = (start + end) / 2;  
29.             PrintTask left = new PrintTask(start, middle);  
30.             PrintTask right = new PrintTask(middle, end);  
31.             // 并行执行两个小任务  
32.             left.fork();  
33.             right.fork();  
34.         }  
35.     }  
36. }  
37.   
38. public class ForkJoinPoolTest {  
39.     /** 
40.      * @param args 
41.      * @throws Exception 
42.      */  
43.     public static void main(String[] args) throws Exception {  
44.         // 创建包含Runtime.getRuntime().availableProcessors()返回值作为个数的并行线程的ForkJoinPool  
45.         ForkJoinPool forkJoinPool = new ForkJoinPool();  
46.         // 提交可分解的PrintTask任务  
47.         forkJoinPool.submit(new PrintTask(0, 200));  
48.         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  
49.         // 关闭线程池  
50.         forkJoinPool.shutdown();  
51.     }  
52.   
53. }  

 

 

运行结果如下：

 

[java]  view plain copy

 

 

1. ForkJoinPool-1-worker-2的i值:75  
2. ForkJoinPool-1-worker-2的i值:76  
3. ForkJoinPool-1-worker-2的i值:77  
4. ForkJoinPool-1-worker-2的i值:78  
5. ForkJoinPool-1-worker-2的i值:79  
6. ForkJoinPool-1-worker-2的i值:80  
7. ForkJoinPool-1-worker-2的i值:81  
8. ForkJoinPool-1-worker-2的i值:82  
9. ForkJoinPool-1-worker-2的i值:83  
10. ForkJoinPool-1-worker-2的i值:84  
11. ForkJoinPool-1-worker-2的i值:85  
12. ForkJoinPool-1-worker-2的i值:86  
13. ForkJoinPool-1-worker-2的i值:87  
14. ForkJoinPool-1-worker-2的i值:88  
15. ForkJoinPool-1-worker-2的i值:89  
16. ForkJoinPool-1-worker-2的i值:90  
17. ForkJoinPool-1-worker-2的i值:91  
18. ForkJoinPool-1-worker-2的i值:92  
19. ForkJoinPool-1-worker-2的i值:93  
20. ForkJoinPool-1-worker-2的i值:94  
21. ForkJoinPool-1-worker-2的i值:95  
22. ForkJoinPool-1-worker-2的i值:96  
23. ForkJoinPool-1-worker-2的i值:97  
24. ForkJoinPool-1-worker-2的i值:98  
25. ForkJoinPool-1-worker-2的i值:99  
26. ForkJoinPool-1-worker-2的i值:50  
27. ForkJoinPool-1-worker-2的i值:51  
28. ForkJoinPool-1-worker-2的i值:52  
29. ForkJoinPool-1-worker-2的i值:53  
30. ForkJoinPool-1-worker-2的i值:54  
31. ForkJoinPool-1-worker-2的i值:55  
32. ForkJoinPool-1-worker-2的i值:56  
33. ForkJoinPool-1-worker-2的i值:57  
34. ForkJoinPool-1-worker-2的i值:58  
35. ForkJoinPool-1-worker-2的i值:59  
36. ForkJoinPool-1-worker-2的i值:60  
37. ForkJoinPool-1-worker-2的i值:61  
38. ForkJoinPool-1-worker-2的i值:62  
39. ForkJoinPool-1-worker-2的i值:63  
40. ForkJoinPool-1-worker-2的i值:64  
41. ForkJoinPool-1-worker-2的i值:65  
42. ForkJoinPool-1-worker-2的i值:66  
43. ForkJoinPool-1-worker-2的i值:67  
44. ForkJoinPool-1-worker-2的i值:68  
45. ForkJoinPool-1-worker-2的i值:69  
46. ForkJoinPool-1-worker-1的i值:175  
47. ForkJoinPool-1-worker-1的i值:176  
48. ForkJoinPool-1-worker-1的i值:177  
49. ForkJoinPool-1-worker-1的i值:178  
50. ForkJoinPool-1-worker-1的i值:179  
51. ForkJoinPool-1-worker-1的i值:180  
52. ForkJoinPool-1-worker-1的i值:181  
53. ForkJoinPool-1-worker-1的i值:182  
54. ForkJoinPool-1-worker-1的i值:183  
55. ForkJoinPool-1-worker-1的i值:184  
56. ForkJoinPool-1-worker-1的i值:185  
57. ForkJoinPool-1-worker-1的i值:186  
58. ForkJoinPool-1-worker-1的i值:187  
59. ForkJoinPool-1-worker-1的i值:188  
60. ForkJoinPool-1-worker-1的i值:189  
61. ForkJoinPool-1-worker-1的i值:190  
62. ForkJoinPool-1-worker-1的i值:191  
63. ForkJoinPool-1-worker-1的i值:192  
64. ForkJoinPool-1-worker-1的i值:193  
65. ForkJoinPool-1-worker-1的i值:194  
66. ForkJoinPool-1-worker-1的i值:195  
67. ForkJoinPool-1-worker-1的i值:196  
68. ForkJoinPool-1-worker-1的i值:197  
69. ForkJoinPool-1-worker-1的i值:198  
70. ForkJoinPool-1-worker-1的i值:199  
71. ForkJoinPool-1-worker-1的i值:150  
72. ForkJoinPool-1-worker-1的i值:151  
73. ForkJoinPool-1-worker-1的i值:152  
74. ForkJoinPool-1-worker-1的i值:153  
75. ForkJoinPool-1-worker-1的i值:154  
76. ForkJoinPool-1-worker-1的i值:155  
77. ForkJoinPool-1-worker-1的i值:156  
78. ForkJoinPool-1-worker-1的i值:157  
79. ForkJoinPool-1-worker-1的i值:158  
80. ForkJoinPool-1-worker-1的i值:159  
81. ForkJoinPool-1-worker-1的i值:160  
82. ForkJoinPool-1-worker-1的i值:161  
83. ForkJoinPool-1-worker-1的i值:162  
84. ForkJoinPool-1-worker-1的i值:163  
85. ForkJoinPool-1-worker-1的i值:164  
86. ForkJoinPool-1-worker-1的i值:165  
87. ForkJoinPool-1-worker-1的i值:166  
88. ForkJoinPool-1-worker-1的i值:167  
89. ForkJoinPool-1-worker-1的i值:168  
90. ForkJoinPool-1-worker-1的i值:169  
91. ForkJoinPool-1-worker-1的i值:170  
92. ForkJoinPool-1-worker-1的i值:171  
93. ForkJoinPool-1-worker-1的i值:172  
94. ForkJoinPool-1-worker-1的i值:173  
95. ForkJoinPool-1-worker-1的i值:174  
96. ForkJoinPool-1-worker-1的i值:125  
97. ForkJoinPool-1-worker-1的i值:126  
98. ForkJoinPool-1-worker-1的i值:127  
99. ForkJoinPool-1-worker-1的i值:128  
100. ForkJoinPool-1-worker-1的i值:129  
101. ForkJoinPool-1-worker-1的i值:130  
102. ForkJoinPool-1-worker-1的i值:131  
103. ForkJoinPool-1-worker-1的i值:132  
104. ForkJoinPool-1-worker-1的i值:133  
105. ForkJoinPool-1-worker-1的i值:134  
106. ForkJoinPool-1-worker-1的i值:135  
107. ForkJoinPool-1-worker-1的i值:136  
108. ForkJoinPool-1-worker-1的i值:137  
109. ForkJoinPool-1-worker-1的i值:138  
110. ForkJoinPool-1-worker-1的i值:139  
111. ForkJoinPool-1-worker-1的i值:140  
112. ForkJoinPool-1-worker-1的i值:141  
113. ForkJoinPool-1-worker-1的i值:142  
114. ForkJoinPool-1-worker-1的i值:143  
115. ForkJoinPool-1-worker-1的i值:144  
116. ForkJoinPool-1-worker-1的i值:145  
117. ForkJoinPool-1-worker-1的i值:146  
118. ForkJoinPool-1-worker-1的i值:147  
119. ForkJoinPool-1-worker-1的i值:148  
120. ForkJoinPool-1-worker-1的i值:149  
121. ForkJoinPool-1-worker-1的i值:100  
122. ForkJoinPool-1-worker-1的i值:101  
123. ForkJoinPool-1-worker-1的i值:102  
124. ForkJoinPool-1-worker-1的i值:103  
125. ForkJoinPool-1-worker-1的i值:104  
126. ForkJoinPool-1-worker-1的i值:105  
127. ForkJoinPool-1-worker-1的i值:106  
128. ForkJoinPool-1-worker-1的i值:107  
129. ForkJoinPool-1-worker-1的i值:108  
130. ForkJoinPool-1-worker-1的i值:109  
131. ForkJoinPool-1-worker-1的i值:110  
132. ForkJoinPool-1-worker-1的i值:111  
133. ForkJoinPool-1-worker-1的i值:112  
134. ForkJoinPool-1-worker-1的i值:113  
135. ForkJoinPool-1-worker-1的i值:114  
136. ForkJoinPool-1-worker-1的i值:115  
137. ForkJoinPool-1-worker-1的i值:116  
138. ForkJoinPool-1-worker-1的i值:117  
139. ForkJoinPool-1-worker-1的i值:118  
140. ForkJoinPool-1-worker-1的i值:119  
141. ForkJoinPool-1-worker-1的i值:120  
142. ForkJoinPool-1-worker-1的i值:121  
143. ForkJoinPool-1-worker-1的i值:122  
144. ForkJoinPool-1-worker-1的i值:123  
145. ForkJoinPool-1-worker-1的i值:124  
146. ForkJoinPool-1-worker-1的i值:25  
147. ForkJoinPool-1-worker-1的i值:26  
148. ForkJoinPool-1-worker-1的i值:27  
149. ForkJoinPool-1-worker-1的i值:28  
150. ForkJoinPool-1-worker-1的i值:29  
151. ForkJoinPool-1-worker-1的i值:30  
152. ForkJoinPool-1-worker-1的i值:31  
153. ForkJoinPool-1-worker-1的i值:32  
154. ForkJoinPool-1-worker-1的i值:33  
155. ForkJoinPool-1-worker-1的i值:34  
156. ForkJoinPool-1-worker-1的i值:35  
157. ForkJoinPool-1-worker-1的i值:36  
158. ForkJoinPool-1-worker-1的i值:37  
159. ForkJoinPool-1-worker-1的i值:38  
160. ForkJoinPool-1-worker-1的i值:39  
161. ForkJoinPool-1-worker-1的i值:40  
162. ForkJoinPool-1-worker-1的i值:41  
163. ForkJoinPool-1-worker-1的i值:42  
164. ForkJoinPool-1-worker-1的i值:43  
165. ForkJoinPool-1-worker-1的i值:44  
166. ForkJoinPool-1-worker-1的i值:45  
167. ForkJoinPool-1-worker-1的i值:46  
168. ForkJoinPool-1-worker-1的i值:47  
169. ForkJoinPool-1-worker-1的i值:48  
170. ForkJoinPool-1-worker-1的i值:49  
171. ForkJoinPool-1-worker-1的i值:0  
172. ForkJoinPool-1-worker-1的i值:1  
173. ForkJoinPool-1-worker-1的i值:2  
174. ForkJoinPool-1-worker-1的i值:3  
175. ForkJoinPool-1-worker-1的i值:4  
176. ForkJoinPool-1-worker-1的i值:5  
177. ForkJoinPool-1-worker-1的i值:6  
178. ForkJoinPool-1-worker-1的i值:7  
179. ForkJoinPool-1-worker-1的i值:8  
180. ForkJoinPool-1-worker-1的i值:9  
181. ForkJoinPool-1-worker-1的i值:10  
182. ForkJoinPool-1-worker-1的i值:11  
183. ForkJoinPool-1-worker-1的i值:12  
184. ForkJoinPool-1-worker-1的i值:13  
185. ForkJoinPool-1-worker-1的i值:14  
186. ForkJoinPool-1-worker-1的i值:15  
187. ForkJoinPool-1-worker-1的i值:16  
188. ForkJoinPool-1-worker-1的i值:17  
189. ForkJoinPool-1-worker-1的i值:18  
190. ForkJoinPool-1-worker-1的i值:19  
191. ForkJoinPool-1-worker-1的i值:20  
192. ForkJoinPool-1-worker-1的i值:21  
193. ForkJoinPool-1-worker-1的i值:22  
194. ForkJoinPool-1-worker-1的i值:23  
195. ForkJoinPool-1-worker-1的i值:24  
196. ForkJoinPool-1-worker-2的i值:70  
197. ForkJoinPool-1-worker-2的i值:71  
198. ForkJoinPool-1-worker-2的i值:72  
199. ForkJoinPool-1-worker-2的i值:73  
200. 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。

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。

 

[java]  view plain copy

 

 

1. import java.util.Random;  
2. import java.util.concurrent.ForkJoinPool;  
3. import java.util.concurrent.Future;  
4. import java.util.concurrent.RecursiveTask;  
5.   
6. //RecursiveTask为ForkJoinTask的抽象子类，有返回值的任务  
7. class SumTask extends RecursiveTask<Integer> {  
8.     // 每个"小任务"最多只打印50个数  
9.     private static final int MAX = 20;  
10.     private int arr[];  
11.     private int start;  
12.     private int end;  
13.   
14.     SumTask(int arr[], int start, int end) {  
15.         this.arr = arr;  
16.         this.start = start;  
17.         this.end = end;  
18.     }  
19.   
20.     @Override  
21.     protected Integer compute() {  
22.         int sum = 0;  
23.         // 当end-start的值小于MAX时候，开始打印  
24.         if ((end - start) < MAX) {  
25.             for (int i = start; i < end; i++) {  
26.                 sum += arr[i];  
27.             }  
28.             return sum;  
29.         } else {  
30.             System.err.println("=====任务分解======");  
31.             // 将大任务分解成两个小任务  
32.             int middle = (start + end) / 2;  
33.             SumTask left = new SumTask(arr, start, middle);  
34.             SumTask right = new SumTask(arr, middle, end);  
35.             // 并行执行两个小任务  
36.             left.fork();  
37.             right.fork();  
38.             // 把两个小任务累加的结果合并起来  
39.             return left.join() + right.join();  
40.         }  
41.     }  
42.   
43. }  
44.   
45. public class ForkJoinPoolTest2 {  
46.     /** 
47.      * @param args 
48.      * @throws Exception 
49.      */  
50.     public static void main(String[] args) throws Exception {  
51.         int arr[] = new int[100];  
52.         Random random = new Random();  
53.         int total = 0;  
54.         // 初始化100个数字元素  
55.         for (int i = 0; i < arr.length; i++) {  
56.             int temp = random.nextInt(100);  
57.             // 对数组元素赋值,并将数组元素的值添加到total总和中  
58.             total += (arr[i] = temp);  
59.         }  
60.         System.out.println("初始化时的总和=" + total);  
61.         // 创建包含Runtime.getRuntime().availableProcessors()返回值作为个数的并行线程的ForkJoinPool  
62.         ForkJoinPool forkJoinPool = new ForkJoinPool();  
63.         // 提交可分解的PrintTask任务  
64.         Future<Integer> future = forkJoinPool.submit(new SumTask(arr, 0,  
65.                 arr.length));  
66.         System.out.println("计算出来的总和=" + future.get());  
67.         // 关闭线程池  
68.         forkJoinPool.shutdown();  
69.     }  
70.   
71. }  

计算结果如下：

[java]  view plain copy

 

 

1. 初始化时的总和=4283  
2. =====任务分解======  
3. =====任务分解======  
4. =====任务分解======  
5. =====任务分解======  
6. =====任务分解======  
7. =====任务分解======  
8. =====任务分解======  
9. 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：