Java 中的 ForkJoin 任務

Java 中的 fork / join 框架非常適合於可以分成更小部分並且並行求解的問題。fork / join 問題的基本步驟是：

• 將問題分成多個部分
• 將每個部件彼此平行地求解
• 將每個子解決方案組合成一個整體解決方案

一個 ForkJoinTask 是定義這樣的問題的介面。通常期望你將其一個抽象實現（通常是 RecursiveTask ） 子類化，而不是直接實現介面。

在這個例子中，我們將總結一個整數的集合，直到我們得到不超過 10 的批量大小。

import java.util.List;
import java.util.concurrent.RecursiveTask;

public class SummingTask extends RecursiveTask<Integer> {
    private static final int MAX_BATCH_SIZE = 10;

    private final List<Integer> numbers;
    private final int minInclusive, maxExclusive;

    public SummingTask(List<Integer> numbers) {
        this(numbers, 0, numbers.size());
    }

    // This constructor is only used internally as part of the dividing process
    private SummingTask(List<Integer> numbers, int minInclusive, int maxExclusive) {
        this.numbers = numbers;
        this.minInclusive = minInclusive;
        this.maxExclusive = maxExclusive;
    }

    @Override
    public Integer compute() {
        if (maxExclusive - minInclusive > MAX_BATCH_SIZE) {
            // This is too big for a single batch, so we shall divide into two tasks
            int mid = (minInclusive + maxExclusive) / 2;
            SummingTask leftTask = new SummingTask(numbers, minInclusive, mid);
            SummingTask rightTask = new SummingTask(numbers, mid, maxExclusive);

            // Submit the left hand task as a new task to the same ForkJoinPool
            leftTask.fork();

            // Run the right hand task on the same thread and get the result
            int rightResult = rightTask.compute();

            // Wait for the left hand task to complete and get its result
            int leftResult = leftTask.join();

            // And combine the result
            return leftResult + rightResult;
        } else {
            // This is fine for a single batch, so we will run it here and now
            int sum = 0;
            for (int i = minInclusive; i < maxExclusive; i++) {
                sum += numbers.get(i);
            }
            return sum;
        }
    }
}

現在可以將此任務的例項傳遞給 ForkJoinPool 的例項。

// Because I am not specifying the number of threads
// it will create a thread for each available processor
ForkJoinPool pool = new ForkJoinPool();

// Submit the task to the pool, and get what is effectively the Future
ForkJoinTask<Integer> task = pool.submit(new SummingTask(numbers));

// Wait for the result
int result = task.join();