Fork/Join

1. 概念
   Fork/Join 是 JDK 1.7 加入的新的线程池实现，它体现的是一种分治思想，适用于能够进行任务拆分的 cpu 密集型运算.所谓的任务拆分，是将一个大任务拆分为算法上相同的小任务，直至不能拆分可以直接求解。跟递归相关的一些计算，如归并排序、斐波那契数列、都可以用分治思想进行求解.Fork/Join 在分治的基础上加入了多线程，可以把每个任务的分解和合并交给不同的线程来完成，进一步提升了运算效率Fork/Join 默认会创建与 cpu 核心数大小相同的线程池
2. 使用
   提交给 Fork/Join 线程池的任务需要继承 RecursiveTask（有返回值）或 RecursiveAction（没有返回值），例如下面定义了一个对 1~n 之间的整数求和的任务

@Slf4j(topic = "c.AddTask")
class AddTask1 extends RecursiveTask<Integer> {
        int n;
        public AddTask1(int n) {
            this.n = n;
        }
        @Override
        public String toString() {
            return "{" + n + '}';
        }
        @Override
        protected Integer compute() {
            // 如果 n 已经为 1，可以求得结果了
            if (n == 1) {
                log.debug("join() {}", n);
                return n;
            }

            // 将任务进行拆分(fork)
            AddTask1 t1 = new AddTask1(n - 1);
            t1.fork();
            log.debug("fork() {} + {}", n, t1);

            // 合并(join)结果
            int result = n + t1.join();
            log.debug("join() {} + {} = {}", n, t1, result);
            return result;
        }
    }

然后提交给 ForkJoinPool 来执行

public static void main(String[] args) {
 ForkJoinPool pool = new ForkJoinPool(4);
 System.out.println(pool.invoke(new AddTask1(5)));
}

结果

[ForkJoinPool-1-worker-0] - fork() 2 + {1} 
[ForkJoinPool-1-worker-1] - fork() 5 + {4} 
[ForkJoinPool-1-worker-0] - join() 1 
[ForkJoinPool-1-worker-0] - join() 2 + {1} = 3 
[ForkJoinPool-1-worker-2] - fork() 4 + {3} 
[ForkJoinPool-1-worker-3] - fork() 3 + {2} 
[ForkJoinPool-1-worker-3] - join() 3 + {2} = 6 
[ForkJoinPool-1-worker-2] - join() 4 + {3} = 10 
[ForkJoinPool-1-worker-1] - join() 5 + {4} = 15 
15 

用图来表示

改进

class AddTask3 extends RecursiveTask<Integer> {

    int begin;
    int end;
    public AddTask3(int begin, int end) {
        this.begin = begin;
        this.end = end;
    }
    @Override
    public String toString() {
        return "{" + begin + "," + end + '}';
    }
    @Override
    protected Integer compute() {
        // 5, 5
        if (begin == end) {
            log.debug("join() {}", begin);
            return begin;
        }
        // 4, 5
        if (end - begin == 1) {
            log.debug("join() {} + {} = {}", begin, end, end + begin);
            return end + begin;
        }

        // 1 5
        int mid = (end + begin) / 2; // 3
        AddTask3 t1 = new AddTask3(begin, mid); // 1,3
        t1.fork();
        AddTask3 t2 = new AddTask3(mid + 1, end); // 4,5
        t2.fork();
        log.debug("fork() {} + {} = ?", t1, t2);
        int result = t1.join() + t2.join();
        log.debug("join() {} + {} = {}", t1, t2, result);
        return result;
    }
}

然后提交给 ForkJoinPool 来执行

public static void main(String[] args) {
 ForkJoinPool pool = new ForkJoinPool(4);
 System.out.println(pool.invoke(new AddTask3(1, 10)));
}

[ForkJoinPool-1-worker-0] - join() 1 + 2 = 3 
[ForkJoinPool-1-worker-3] - join() 4 + 5 = 9 
[ForkJoinPool-1-worker-0] - join() 3 
[ForkJoinPool-1-worker-1] - fork() {1,3} + {4,5} = ? 
[ForkJoinPool-1-worker-2] - fork() {1,2} + {3,3} = ? 
[ForkJoinPool-1-worker-2] - join() {1,2} + {3,3} = 6 
[ForkJoinPool-1-worker-1] - join() {1,3} + {4,5} = 15 
15 

用图来表示