Name: krC82822 Date: 12/10/2000


orig synopsis: "Opaque JLayeredPane can throw NullPointerException from print() and paint()"

10 Dec 2000, eval1127@eng -- same behavior in merlin(1.4) beta44
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0beta-b44)
Java HotSpot(TM) Client VM (build 1.4beta-B44, mixed mode)
java version "1.3.0beta_refresh"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0beta_refresh-b09)
Java HotSpot(TM) Client VM (build 1.3.0beta-b07, mixed mode)
Also JDK1.3 for Windows and JDK1.3FCS for Linux

When an opaque JLayeredPane is passed a Graphics object which hasn't had a
clipping rectangle set, the JLayeredPane will throw a NullPointerException from
either the print(Graphics g) or paint(Graphics g) methods.


/* Following code will throw a NullPointerException. */
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import javax.swing.JFrame;
import javax.swing.JLayeredPane;

public class JLayeredPaneBug {

    // Prints a JLayeredPane to an image.
    public static void main(String[] args) {

        // Create frame to hold JLayeredPane.
        JFrame frame = new JFrame();

        // Create an opaque JLayeredPane.
        JLayeredPane pane = new JLayeredPane();
        Dimension size = new Dimension(100,100);
        pane.setPreferredSize(size);
        pane.setOpaque(true);

        // Add JLayeredPane to JFrame and make the frame visible.
        frame.setContentPane(pane);
        frame.pack();
        frame.setVisible(true);

        // Create a compatible image from the JLayeredPane, get the
        // graphics context for this image, and print the JLayeredPane
        // into the image.
        Image image = pane.createImage(size.width,size.height);
        Graphics g = image.getGraphics();
        pane.print(g);
    }
}

The exact trace for the error message is:
Exception in thread "main" java.lang.NullPointerException
at javax.swing.JLayeredPane.paint(JLayeredPane.java:544)
at JLayeredPaneBug.main(JLayeredPaneBug.java:30)

The problem is in the paintComponent method of JLayeredPane.

/* Code from JLayeredPane lines 532-547.*/

    /**
     * Paints this JLayeredPane within the specified graphics context.
     *
     * @param g the Graphics context within which to paint
     */
    public void paint(Graphics g) {
        if(isOpaque()) {
            Rectangle r = g.getClipBounds(); // CAN RETURN NULL HERE!
            Color c = getBackground();
            if(c == null)
                c = Color.lightGray;
            g.setColor(c);
            g.fillRect(r.x, r.y, r.width, r.height); // ERROR IF r==NULL!
        }
        super.paint(g);
    }

Specifically, NULL is a valid return from the getClipBounds method. If NULL is
actually returned the fillRect method will generate a NullPointerException. One
possibility is to replace the getClipBounds() by getBounds().
(Review ID: 110821)
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