在swt画布上绘制多个矩形

import org.eclipse.swt.SWT;
import org.eclipse.swt.events.PaintEvent;
import org.eclipse.swt.events.PaintListener;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Canvas;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class RectangleDrawingExample {
    public static void main(String[] args) {
        Display display = new Display();
        Shell shell = new Shell(display);
        shell.setSize(400, 300);
        shell.setText("Rectangle Drawing Example");
        shell.setLayout(new FillLayout());

        Composite composite = new Composite(shell, SWT.NONE);
        composite.setLayout(new FillLayout());

        Canvas canvas = new Canvas(composite, SWT.NONE);
        canvas.addPaintListener(new PaintListener() {
            @Override
            public void paintControl(PaintEvent e) {
                GC gc = e.gc;
                Color red = display.getSystemColor(SWT.COLOR_RED);
                Color blue = display.getSystemColor(SWT.COLOR_BLUE);

                // 绘制多个矩形
                Rectangle rect1 = new Rectangle(50, 50, 100, 100);
                gc.setBackground(red);
                gc.fillRectangle(rect1);

                Rectangle rect2 = new Rectangle(200, 50, 150, 80);
                gc.setBackground(blue);
                gc.fillRectangle(rect2);

                Rectangle rect3 = new Rectangle(100, 200, 120, 150);
                gc.setForeground(red);
                gc.drawRectangle(rect3);
            }
        });

        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
        display.dispose();
    }
}