java - ForkBlur.compute() 方法是在哪里以及如何调用的？

Question

我在Java教程oracle中看到了下面的代码。我的问题是，如何以及在何处调用 ForkBlur.compute() 方法？我在任何地方都没有看到任何对 ForkBlur.compute() 的调用，但该方法确实被执行了。我有什么遗漏的吗？

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“此任务执行的主要计算”。基本上发生的情况如下:

当你调用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实际上启动了一个编号线程，每个线程以并行方式执行指示任务。实际要执行的操作在compute()方法中定义。因此，该方法总是由JAVA SDK调用，而不是由程序员调用。