在SWT java中如何在画布内绘制带有边框的透明合成

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 TransparentCanvasExample {
    public static void main(String[] args) {
        Display display = new Display();
        Shell shell = new Shell(display, SWT.NO_TRIM);
        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;
                Rectangle clientArea = canvas.getClientArea();

                // 设置透明度
                gc.setAlpha(128);

                // 绘制边框
                gc.setForeground(display.getSystemColor(SWT.COLOR_RED));
                gc.drawRectangle(clientArea.x, clientArea.y, clientArea.width - 1, clientArea.height - 1);

                // 设置不透明度
                gc.setAlpha(255);

                // 绘制其他内容
                gc.setBackground(display.getSystemColor(SWT.COLOR_BLUE));
                gc.fillRectangle(clientArea.x + 10, clientArea.y + 10, clientArea.width - 20, clientArea.height - 20);
            }
        });

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