import sun.java2d.NullSurfaceData; //导入依赖的package包/类
public SurfaceData replaceSurfaceData() {
    if (getBounds().isEmpty()) {
        surfaceData = NullSurfaceData.theInstance;
        return surfaceData;
    }

    // the layer redirects all painting to the buffer's graphics
    // and blits the buffer to the layer surface (in drawInCGLContext callback)
    CGraphicsConfig gc = (CGraphicsConfig)getGraphicsConfiguration();
    surfaceData = gc.createSurfaceData(this);
    setScale(gc.getDevice().getScaleFactor());
    // the layer holds a reference to the buffer, which in
    // turn has a reference back to this layer
    if (surfaceData instanceof CGLSurfaceData) {
        validate((CGLSurfaceData)surfaceData);
    }

    return surfaceData;
}