ForkBlur.compute() 方法在哪里以及如何被调用？

Question

我从 Java 教程 oracle 中看到了下面的代码。我的问题是，ForkBlur.compute() 方法是如何以及在何处调用的？我在任何地方都看不到对 ForkBlur.compute() 的任何调用，但该方法确实已执行。有什么我想念的吗？

import java.awt.image.BufferedImage;
import java.io.File;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import javax.imageio.ImageIO;

/**
 * ForkBlur implements a simple horizontal image blur. It averages pixels in the
 * source array and writes them to a destination array. The sThreshold value
 * determines whether the blurring will be performed directly or split into two
 * tasks.
 *
 * This is not the recommended way to blur images; it is only intended to
 * illustrate the use of the Fork/Join framework.
 */
public class ForkBlur extends RecursiveAction {

    private int[] mSource;
    private int mStart;
    private int mLength;
    private int[] mDestination;
    private int mBlurWidth = 15; // Processing window size, should be odd.

    public ForkBlur(int[] src, int start, int length, int[] dst) {
        mSource = src;
        mStart = start;
        mLength = length;
        mDestination = dst;
    }

    // Average pixels from source, write results into destination.
    protected void computeDirectly() {
        int sidePixels = (mBlurWidth - 1) / 2;
        for (int index = mStart; index < mStart + mLength; index++) {
            // Calculate average.
            float rt = 0, gt = 0, bt = 0;
            for (int mi = -sidePixels; mi <= sidePixels; mi++) {
                int mindex = Math.min(Math.max(mi + index, 0), mSource.length - 1);
                int pixel = mSource[mindex];
                rt += (float) ((pixel & 0x00ff0000) >> 16) / mBlurWidth;
                gt += (float) ((pixel & 0x0000ff00) >> 8) / mBlurWidth;
                bt += (float) ((pixel & 0x000000ff) >> 0) / mBlurWidth;
            }

            // Re-assemble destination pixel.
            int dpixel = (0xff000000)
                    | (((int) rt) << 16)
                    | (((int) gt) << 8)
                    | (((int) bt) << 0);
            mDestination[index] = dpixel;
        }
    }
    protected static int sThreshold = 10000;

    @Override
    protected void compute() {
        if (mLength < sThreshold) {
            computeDirectly();
            return;
        }

        int split = mLength / 2;

        invokeAll(new ForkBlur(mSource, mStart, split, mDestination),
                new ForkBlur(mSource, mStart + split, mLength - split, 
                mDestination));
    }

    // Plumbing follows.
    public static void main(String[] args) throws Exception {
        String srcName = "/Users/justin/NetBeansProjects/JavaTutorialOracle/src/JTOConcurrency/Screen Shot 2015-12-28 at 10.45.31 PM.jpg";
        File srcFile = new File(srcName);
        BufferedImage image = ImageIO.read(srcFile);

        System.out.println("Source image: " + srcName);

        BufferedImage blurredImage = blur(image);

        String dstName = "blurred-tulips.jpg";
        File dstFile = new File(dstName);
        ImageIO.write(blurredImage, "jpg", dstFile);

        System.out.println("Output image: " + dstName);

    }

    public static BufferedImage blur(BufferedImage srcImage) {
        int w = srcImage.getWidth();
        System.out.println("w: " + w);
        int h = srcImage.getHeight();
        System.out.println("h: " + h);

        int[] src = srcImage.getRGB(0, 0, w, h, null, 0, w);

        System.out.println("Array size is " + src.length);
        System.out.println("Threshold is " + sThreshold);

        int processors = Runtime.getRuntime().availableProcessors();
        System.out.println(Integer.toString(processors) + " processor"
                + (processors != 1 ? "s are " : " is ")
                + "available");

        ForkBlur fb = new ForkBlur(src, 0, src.length, dst);

        ForkJoinPool pool = new ForkJoinPool();

        long startTime = System.currentTimeMillis();
        pool.invoke(fb);

        long endTime = System.currentTimeMillis();

        System.out.println("Image blur took " + (endTime - startTime) + 
                " milliseconds.");

        BufferedImage dstImage = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB); 
        dstImage.setRGB(0, 0, w, h, dst, 0, w);

        return dstImage;
    }
}

Answer 1

Forkblur 正在扩展 java.util.concurrent.RecursiveAction，其中包含方法 compute() 并将其描述为 'The main computation performed by this task.' 基本上发生的情况如下：

当您调用 ForkJoinPool.invoke() 并指定要执行的任务时（在本例中 ForkBlur 是一个 ForkJoinTask 实现），JAVA SDK 实际上会启动多个线程，每个线程一个以并行方式执行指示任务。要执行的实际操作在 compute() 方法中定义。因此，此方法始终由 JAVA SDK 调用，而不会由程序员调用。

ForkBlur.compute() 方法在哪里以及如何被调用？

Where and how was the ForkBlur.compute() method invoked?

java

multithreading

fork-join