Java fork/join 框架说明示例

fork/join 框架设计为将并行任务递归拆分为较小的任务, 然后将每个子项目的结果合并以产生总体结果。它是 ExecutorService 接口的一个实现, 它将这些子任务分配给线程池中的工作线程, 称为 ForkJoinPool。让我们开始探索如何定义任务和子任务。
需要继承RecursiveTask这个类，并实现唯一的一个抽象方法
protected Long compute() {
return null;
}

此方法定义了将手头的任务拆分为子任务的逻辑和算法, 以便在不再可能或不方便地进一步划分时, 生成一个单独的工作项的结果。为此方法的实施经常类似以下伪代码:

if (task is small enough or no longer divisible(任务足够小或不再可分)) {compute task sequentially(顺序计算该任务)} else {split task in two subtasks(将任务拆分2个子任务)call this method recursively possibly further splitting each subtask(递归调用此方法可能进一步拆分每个子任务)wait for the completion of all subtasks(等待所有子任务的完成)combine the results of each subtask(合并每个子任务的结果)}

用fork/join框架并行求和示例代码：

import java.util.concurrent.ForkJoinPool;import java.util.concurrent.ForkJoinTask;import java.util.concurrent.RecursiveTask;import java.util.function.Function;import java.util.stream.LongStream;@SuppressWarnings("serial")public class ForkJoinRecursiveTasks extends RecursiveTask<Long>{    public static final ForkJoinPool FORK_JOIN_POOL = new ForkJoinPool();    public static final long THRESHOLD = 10_000;    private final long[] numbers;    private final int start;    private final int end;    private ForkJoinRecursiveTasks(long[] numbers, int start, int end) {        this.numbers = numbers;        this.start = start;        this.end = end;    }    @Override    protected Long compute() {        int length = end - start;        if (length <= THRESHOLD) {            return computeSequentially();        }        ForkJoinRecursiveTasks leftTask = new ForkJoinRecursiveTasks(numbers, start, start + length/2);        leftTask.fork();        ForkJoinRecursiveTasks rightTask = new ForkJoinRecursiveTasks(numbers, start + length/2, end);        Long rightResult = rightTask.compute();        Long leftResult = leftTask.join();        return leftResult + rightResult;    }    private Long computeSequentially() {        long sum = 0;        for (int i = start; i < end; i++) {            sum += numbers[i];        }        return sum;    }    public static long forkJoinSum(long n) {        long[] numbers = LongStream.rangeClosed(1, n).toArray();        ForkJoinTask<Long> task = new ForkJoinSumCalculator(numbers);        return FORK_JOIN_POOL.invoke(task);    }    public static <T, R> long measurePerf(Function<T, R> f, T input) {        long fastest = Long.MAX_VALUE;        for (int i = 0; i < 10; i++) {            long start = System.nanoTime();            R result = f.apply(input);            long duration = (System.nanoTime() - start) / 1_000_000;            System.out.println("Result: " + result);            if (duration < fastest) fastest = duration;        }        return fastest;    }    public static void main(String[] args) {        System.out.println("Range forkJoinSum done in: " + measurePerf(ForkJoinRecursiveTasks::forkJoinSum, 100_000_000L) + " msecs");    }}