如何使用多个线程来完成一项工作？

Question

下面的代码用于分形曼德布罗。它工作得很好，但现在我想在它上面使用线程的概念。结果应该是一样的，但工作必须由多个线程+10来完成。

这是我的代码：

import java.awt.Graphics;
import java.awt.image.BufferedImage;
import javax.swing.JFrame;

public class test extends JFrame {

    private final int MAX_ITER = 570;
    private final double ZOOM = 150;
    private BufferedImage I;
    private double zx, zy, cX, cY, tmp;
    private static int x,y;

    public test() throws InterruptedException {
        super("Mandelbrot Set");
        setBounds(100, 100, 800, 600);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        I = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);
        for ( y = 0; y < getHeight(); y++) {

            for ( x = 0; x < getWidth(); x++) {
                Thread T = new Thread() {//*******
                    public void run() {

                          zx = zy = 0;
                          cX = (x - 400) / ZOOM;
                          cY = (y - 300) / ZOOM;
                          int iter = MAX_ITER;
                          while (zx * zx + zy * zy < 4 && iter > 0) {
                              tmp = zx * zx - zy * zy + cX;
                              zy = 2.0 * zx * zy + cY;
                              zx = tmp;
                              iter--;
                          }
                          I.setRGB(x, y, iter | (iter << 8));
                          System.out.println(Thread.currentThread().getId());
                    }

                };//*******

                T.start();//********
                T.join();//**********

            }
        }
    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(I, 0, 0, this);
    }

    public static void main(String[] args) throws InterruptedException {
        new test().setVisible(true);
    }
}

我试图在循环中实例化线程，但我没有得到我想要的结果 有什么建议吗？

Answer 1

您在启动线程后立即调用T.join。 Thread#join 阻塞当前线程，直到该线程完成。这意味着计算将在另一个线程上进行，但您不会获得并行性的任何好处，因为在另一个线程完成之前您不会启动另一个线程。

您可以使用CountDownLatch 之类的方式启动所有线程并等待它们一起完成，或者尝试使用fork/join framework

Answer 2

您要做的第一件事，就是完成EDT 的漫长过程。为此，请使用SwingWorker：

import java.awt.Graphics;
import java.awt.image.BufferedImage;
import javax.swing.JFrame;
import javax.swing.SwingWorker;

public class Test extends JFrame {//see https://www.javatpoint.com/java-naming-conventions

    private final int MAX_ITER = 570;
    private final double ZOOM = 150;
    private BufferedImage bImage;

    public Test() throws InterruptedException {
        super("Mandelbrot Set");
        setBounds(100, 100, 800, 600);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        bImage = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);
        new Draw().execute();
    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(bImage, 0, 0, this);
    }

    public static void main(String[] args) throws InterruptedException {
        new Test().setVisible(true);
    }

    class Draw extends SwingWorker<Void, Void>{

        private double zx, zy, cX, cY, tmp;
        private int x,y;

        @Override
        protected Void doInBackground() throws Exception {

            for ( y = 0; y < getHeight(); y++) {
                for ( x = 0; x < getWidth(); x++) {
                    zx = zy = 0;
                    cX = (x - 400) / ZOOM;
                    cY = (y - 300) / ZOOM;
                    int iter = MAX_ITER;
                    while ((((zx * zx) + (zy * zy)) < 4) && (iter > 0)) {
                        tmp = ((zx * zx) - (zy * zy)) + cX;
                        zy = (2.0 * zx * zy) + cY;
                        zx = tmp;
                        iter--;
                    }

                    bImage.setRGB(x, y, iter | (iter << 8));
                }
                Thread.sleep(50); //added to slow down and demonstrate painting
                repaint();
            }
            return null;
        }
    }
}

完成后，您可以创建多个SwingWorkers 来完成这项工作。例如：

public class Test extends JFrame {//see https://www.javatpoint.com/java-naming-conventions

    private final int MAX_ITER = 570;
    private final double ZOOM = 150;
    private volatile BufferedImage bImage; 

    public Test() throws InterruptedException {
        super("Mandelbrot Set");
        setBounds(100, 100, 800, 600);
        setResizable(false);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        bImage = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_RGB);

        for (int y = 0; y < getHeight(); y++) {
            new Draw(y).execute();
        }
    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(bImage, 0, 0, this);
    }

    public static void main(String[] args) throws InterruptedException {
        new Test().setVisible(true);
    }

    class Draw extends SwingWorker<Void, Void>{

        private double zx, zy, cX, cY, tmp;
        private int x,y;

        Draw(int y){
            this.y = y;
        }

        @Override
        protected Void doInBackground() throws Exception {

            for ( x = 0; x < getWidth(); x++) {
                zx = zy = 0;
                cX = (x - 400) / ZOOM;
                cY = (y - 300) / ZOOM;
                int iter = MAX_ITER;
                while ((((zx * zx) + (zy * zy)) < 4) && (iter > 0)) {
                    tmp = ((zx * zx) - (zy * zy)) + cX;
                    zy = (2.0 * zx * zy) + cY;
                    zx = tmp;
                    iter--;
                }
                bImage.setRGB(x, y, iter | (iter << 8));
            }
            Thread.sleep(50); //added to slow down and demonstrate painting
            repaint();

            return null;
        }
    }
}

（我不确定BufferedImage bImage是否需要volatile。希望有人能对此发表评论）。

Answer 3

您的代码看起来会计算正确的值，但您需要在所有线程完成后绘制图像才能真正看到结果。现在只需在线程结束时调用 repaint 以使屏幕刷新 WxH 次。为避免不必要的渲染，您可以保留一个全局计数器，该计数器在进入线程时递增，退出时递减。当计数器变为 0 时重新绘制。