java - 并行编程。计算()方法，java

Question

我有一个扩展RecursiveAction的类。同一个类包含以数组为参数的构造函数和compute()方法。在计算方法中，它接下来说:如果array长度大于500，则将该数组分成两半，并通过MergeSort.merge()方法对它们进行排序。如果数组长度小于 500，则只需对数组进行排序。

private static class SortTask extends RecursiveAction {
   private final int THRESHOLD = 500;
     private int[] list;
      SortTask(int[] list) {
       this.list = list;
         }
        @Override
protected void compute() {
if (list.length < THRESHOLD)
java.util.Arrays.sort(list);
else {
    // Obtain the first half
int[] firstHalf = new int[list.length / 2];
System.arraycopy(list, 0, firstHalf, 0, list.length / 2);


// Obtain the second half
int secondHalfLength = list.length - list.length / 2;
int[] secondHalf = new int[secondHalfLength];
System.arraycopy(list, list.length / 2,
secondHalf, 0, secondHalfLength);
// Recursively sort the two halves
invokeAll(new SortTask(firstHalf),
new SortTask(secondHalf));
// Merge firstHalf with secondHalf into list
MergeSort.merge(firstHalf, secondHalf, list);
}
}
}
}

它说“递归地对两半进行排序”，它是通过以下方式实现的:

invokeAll(new SortTask(firstHalf), new SortTask(secondHalf));

这是否意味着每次创建 new SortTask 对象时，都会调用 compute() 方法？

Answer 1

答案是否定的。Compute 不是 Task 构造函数的一部分。最简单的情况是池 shutdownNow() - 此调用将尝试终止/中断所有提交的任务。

每次新的SortTask创建ForkJoinPool时都会 fork 此任务:

public static void invokeAll(ForkJoinTask<?>... tasks) {
    Throwable ex = null;
    int last = tasks.length - 1;
    for (int i = last; i >= 0; --i) {
        ForkJoinTask<?> t = tasks[i];
        if (t == null) {
            if (ex == null)
                ex = new NullPointerException();
        }
        else if (i != 0)
            t.fork();
    ...
}

compute 方法是您的任务负载 - ForkJoinPool 将按其计划执行此操作。

来自ForkJoinTask javadoc :

The primary coordination mechanisms are fork(), that arranges asynchronous execution, and join(), that doesn't proceed until the task's result has been computed.

RecursiveTask 实现抽象方法:

protected final boolean exec() {
    result = compute();
    return true;
}

ForkJoinWorkerThread 有方法:

final void More execTask(ForkJoinTask<?> t) {
    currentSteal = t;
    for (;;) {
       if (t != null)
           t.doExec();
    ...
}

并且在doExec调用exec

final void doExec() {
    if (status >= 0) {
        boolean completed;
        try {
            completed = exec();
    ...
}