11. java线程池-ForkJoinPool

朱雀 2024-04-17 21:44 66阅读 0赞

> java 多线程系列文章列表, 请查看目录: [《java 多线程学习笔记》][java]

## 1. ForkJoinPool ##

java7 提供了ForkJoinPool 来支持将一个任务采用递归方式分成多个小任务计算, 然后再把小任务的计算结果合并为总的结果.

### 1.1 ForkJoinPool 的创建方式 ###

*  java7 提供了两个构造器:
    
     *  ForkJoinPool(int paralleism): 创建时指定并行线程数
     *  ForkJoinPool(): 以当前机器的cpu个数(Runtime.avaliableProcessors())作为并行参数
 *  java8 新增了静态方法创建通用池:
    
     *  ForkJoinPool.commomPool(): 返回一个通用池, 通用池状态不会受shutdown()和shutdownNow()方法影响.
     *  int getCommonPoolParallelism(): 返回通用池的并行线程数

## 1.2 子任务接口 ##

ForkJoin 子任务有两种, 一种是没有返回值的, 一种是有返回值的. java 分别提供了对应的接口:

*  RecursiveAction: 定义无返回值子任务应该实现的接口
 *  RecursiveTask: 定义有返回值子任务应该实现的接口

## 2. 无返回值ForkJoinPool 用法示例 ##

### 2.1 创建子任务类 ###

*  无返回值子任务需要集成RecursiveAction
 *  需要指定拆分标准THRESHOLD, 明确开始计算的临界值
 *  采用二分法进行子任务拆分

public class PrintTask extends RecursiveAction { 
    
    
        // 子任务拆分的标准
        private static final int THRESHOLD = 3;
    
        // 子任务处理的左临界值
        private int start;
    
        // 子任务处理的右临界值
        private int end;
    
        public PrintTask(int start, int end) { 
            this.start = start;
            this.end = end;
        }
    
        @Override
        protected void compute() { 
            // 如果起始值和结束值之间的差小于THRESHOLD, 则开始打印
            if (end - start < THRESHOLD) { 
                for (int i = start; i < end; i++) { 
                    System.out.println(Thread.currentThread().getName() + "--" + i);
                };
            }else { 
                // 二分法拆分子任务
                int middle = (start + end)/2;
    
                // 创建递归线程任务
                PrintTask left = new PrintTask(start, middle);
                PrintTask right = new PrintTask(middle, end);
    
                // 并行执行递归任务
                left.fork();
                right.fork();
            }
        }
    
        public static void main(String[] args) throws InterruptedException { 
    
            ForkJoinPool forkJoinPool = new ForkJoinPool(3);
    
            forkJoinPool.submit(new PrintTask(0, 30));
    
            forkJoinPool.awaitTermination(2, TimeUnit.SECONDS);
    
            forkJoinPool.shutdown();
        }
    }

### 2.2 测试输出 ###

ForkJoinPool-1-worker-2--13
    ForkJoinPool-1-worker-2--14
    ForkJoinPool-1-worker-2--11
    ForkJoinPool-1-worker-2--12
    ForkJoinPool-1-worker-2--9
    ForkJoinPool-1-worker-2--10
    ForkJoinPool-1-worker-2--7
    ForkJoinPool-1-worker-2--8
    ForkJoinPool-1-worker-2--20
    ForkJoinPool-1-worker-2--21
    ForkJoinPool-1-worker-2--18
    ForkJoinPool-1-worker-2--19
    ForkJoinPool-1-worker-2--16
    ForkJoinPool-1-worker-2--17
    ForkJoinPool-1-worker-2--15
    ForkJoinPool-1-worker-2--24
    ForkJoinPool-1-worker-2--25
    ForkJoinPool-1-worker-2--22
    ForkJoinPool-1-worker-2--23
    ForkJoinPool-1-worker-2--26
    ForkJoinPool-1-worker-2--27
    ForkJoinPool-1-worker-2--1
    ForkJoinPool-1-worker-2--2
    ForkJoinPool-1-worker-2--0
    ForkJoinPool-1-worker-2--3
    ForkJoinPool-1-worker-2--4
    ForkJoinPool-1-worker-1--28
    ForkJoinPool-1-worker-1--29
    ForkJoinPool-1-worker-3--5
    ForkJoinPool-1-worker-3--6

## 2. 有返回值ForkJoinPool 用法示例 ##

### 2.1 子任务 ###

*  有返回值子任务需要继承类 RecursiveTask
 *  需要指定拆分标准THRESHOLD, 明确开始计算的临界值
 *  使用join()方法等待子线程开始完成

public class CalTask extends RecursiveTask<Integer> { 
    
        // 子任务拆分的标准
        private static final int THRESHOLD = 10;
    
        // 子任务处理的左临界值
        private int start;
    
        // 子任务处理的右临界值
        private int end;
    
        // 待计算数组
        private List<Integer> array;
    
        public CalTask(List<Integer> list, int start, int end) { 
            this.start = start;
            this.end = end;
            this.array = list;
        }
    
        @Override
        protected Integer compute() { 
    
            int sum = 0;
    
            // 如果起始值和结束值之间的差小于THRESHOLD, 则开始打印
            if (end - start < THRESHOLD) { 
                for (int i = start; i < end; i++) { 
                    sum += array.get(i);
                };
                return sum;
            }else { 
                // 二分法拆分子任务
                int middle = (start + end)/2;
    
                // 创建递归线程任务
                CalTask left = new CalTask(array, start, middle);
                CalTask right = new CalTask(array, middle, end);
    
                // 并行执行递归任务
                left.fork();
                right.fork();
    
                // 合并子任务计算结果
                return left.join() + right.join();
            }
        }
    
        public static void main(String[] args) throws ExecutionException, InterruptedException { 
    
            // 创建数组
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i <= 100; i++) { 
                list.add(i);
            }
    
            // 创建ForkJoinPool 线程池
            ForkJoinPool forkJoinPool = ForkJoinPool.commonPool();
    
            // 提交多线程任务
            ForkJoinTask<Integer> task = forkJoinPool.submit(new CalTask(list, 0, list.size()));
    
            // 获取返回结果
            Integer result = task.get();
    
            System.out.println("1+2+3+...+" + list.get(list.size() -1) + "=" + result);
    
        }
    }

### 3.2 测试输出 ###

1+2+3+...+100=5050

[java]: https://blog.csdn.net/zongf0504/article/details/100186060