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 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。

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

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运行结果如下：

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

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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。

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

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计算结果如下：

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

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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。