• 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;
    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;
    10. private int start;
    11. private int end;
    13. PrintTask(int start, int end) {
    14. this.start = start;
    15. this.end = end;
    16. }
    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. }
    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. }
    53. }
    复制代码


    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
    复制代码


    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;
    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;
    14. SumTask(int arr[], int start, int end) {
    15. this.arr = arr;
    16. this.start = start;
    17. this.end = end;
    18. }
    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. }
    43. }
    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. }
    71. }
    复制代码


    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
    复制代码

    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。
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  • 原文地址：https://www.cnblogs.com/anjijiji/p/6248669.html
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