import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.awt.image.BufferedImage; /** * This demo program uses a thread to compute an image "in the background". * As rows of pixels in the image are computed, they are copied to the * screen. (The image is a small piece of the famous Mandelbrot set, which * is used just because it takes some time to compute. There is no need * to understand what the image means.) The user starts the computation by * clicking a "Start" button. A separate thread is created and is run at * a lower priority, which will make sure that the GUI thread will get a * chance to run to repaint the display as necessary. */ public class BackgroundComputationDemo extends JPanel { /** * This main routine just shows a panel of type BackgroundComputationDemo. */ public static void main(String[] args) { JFrame window = new JFrame("Demo: Background Computation in a Thread"); BackgroundComputationDemo content = new BackgroundComputationDemo(); window.setContentPane(content); window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); window.pack(); window.setResizable(false); Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize(); window.setLocation( (screenSize.width - window.getWidth()) / 2, (screenSize.height - window.getHeight()) / 2 ); window.setVisible(true); } private Runner runner; // the thread that computes the image private volatile boolean running; // used to signal the thread to abort private JButton startButton; // button the user can click to start or abort the thread private BufferedImage image; // contains the image that is computed by this program /** * The display is a JPanel that shows the image. The part of the image that has * not yet been computed is gray. If the image has not yet been created, the * entire display is filled with gray. */ private JPanel display = new JPanel() { protected void paintComponent(Graphics g) { if (image == null) super.paintComponent(g); // fill with background color, gray else { /* Copy the image onto the display. This is synchronized because * there are two threads that compete for access to the image: * the thread that computes the image and the thread that does the * painting. The two threads both synchronize on the image object, * although any object could be used. */ synchronized(image) { g.drawImage(image,0,0,null); } } } }; /** * Constructor creates a panel to hold the display, with a "Start" button below it. */ public BackgroundComputationDemo() { display.setPreferredSize(new Dimension(800,600)); display.setBackground(Color.LIGHT_GRAY); setBorder(BorderFactory.createLineBorder(Color.BLACK, 1)); setLayout(new BorderLayout()); add(display, BorderLayout.CENTER); startButton = new JButton("Start"); JPanel bottom = new JPanel(); bottom.add(startButton); bottom.setBackground(Color.WHITE); add(bottom,BorderLayout.SOUTH); startButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { if (running) stop(); else start(); } }); } /** * This method is called when the user clicks the Start button, * while no thread is running. It starts a new thread and * sets the signaling variable, running, to true; Also changes * the text on the Start button to "Abort". * Note that the priority of the thread is set to be one less * than the priority of the thread that calls this method, that * is of Swing's event-handling thread. This means that the event-handling * thread is run in preference to the computation thread. When there is an * event to be handled, such as painting the display or reacting to a * button click, the event-handling thread will wake up to handle the * event. */ private void start() { startButton.setEnabled(false); int width = display.getWidth() + 2; int height = display.getHeight() + 2; if (image == null) image = new BufferedImage(width,height,BufferedImage.TYPE_INT_ARGB); Graphics g = image.getGraphics(); // fill image with gray g.setColor(Color.LIGHT_GRAY); g.fillRect(0,0,width,height); g.dispose(); display.repaint(); startButton.setText("Abort"); runner = new Runner(); try { runner.setPriority( Thread.currentThread().getPriority() - 1 ); } catch (Exception e) { } running = true; // Set the signal before starting the thread! runner.start(); } /** * This method is called when the user clicks the button while * a thread is running. A signal is sent to the thread to terminate, * by setting the value of the signaling variable, running, to false. */ private void stop() { startButton.setEnabled(false); running = false; runner = null; } /** * This class defines the thread that does the computation. The * run method computes the image one pixel at a time. After computing * the colors for each row of pixels, the colors are copied into the * image, and the part of the displays that shows that row is repainted. * (Since the thread runs in the background, at lower priority than * the event-handling thread, the event-handling thread wakes up * immediately to repaint the display.) */ private class Runner extends Thread { double xmin, xmax, ymin, ymax; int maxIterations; int[] rgb; int[] palette; int width, height; Runner() { width = image.getWidth(); height = image.getHeight(); rgb = new int[width]; palette = new int[256]; for (int i = 0; i < 256; i++) palette[i] = Color.getHSBColor(i/255F, 1, 1).getRGB(); xmin = -1.6744096740931858; xmax = -1.674409674093473; ymin = 4.716540768697223E-5; ymax = 4.716540790246652E-5; maxIterations = 10000; } public void run() { try { startButton.setEnabled(true); double x, y; double dx, dy; dx = (xmax-xmin)/(width-1); dy = (ymax-ymin)/(height-1); for (int row = 0; row < height; row++) { // Compute one row of pixels. y = ymax - dy*row; for (int col = 0; col < width; col++) { x = xmin + dx*col; int count = 0; double xx = x; double yy = y; while (count < maxIterations && (xx*xx + yy*yy) < 4) { count++; double newxx = xx*xx - yy*yy + x; yy = 2*xx*yy + y; xx = newxx; } if (count == maxIterations) rgb[col] = 0; else rgb[col] = palette[count%palette.length]; } if (! running) { // Check for the signal to abort the computation. return; } synchronized(image) { /* Add the newly computed row of pixel colors to the image. This is * synchronized because this thread and the thread that paints the * display might both try to access the image simultaneously. */ image.setRGB(0,row, width, 1, rgb, 0, width); } display.repaint(0,row,width,1); // Repaint just the newly computed row. } } finally { startButton.setText("Start Again"); startButton.setEnabled(true); running = false; // Make sure running is false after the thread ends. } } } }