这里有两件事情可以做:
- 阅读AWT和Swing绘图指南
- 使用调试器,在paintComponent方法中设置断点。然后向上遍历堆栈跟踪,查看谁提供了Graphics参数。
仅供参考,这是我从我在末尾发布的代码示例中得到的堆栈跟踪:
Thread [AWT-EventQueue-0] (Suspended (breakpoint at line 15 in TestPaint))
TestPaint.paintComponent(Graphics) line: 15
TestPaint(JComponent).paint(Graphics) line: 1054
JPanel(JComponent).paintChildren(Graphics) line: 887
JPanel(JComponent).paint(Graphics) line: 1063
JLayeredPane(JComponent).paintChildren(Graphics) line: 887
JLayeredPane(JComponent).paint(Graphics) line: 1063
JLayeredPane.paint(Graphics) line: 585
JRootPane(JComponent).paintChildren(Graphics) line: 887
JRootPane(JComponent).paintToOffscreen(Graphics, int, int, int, int, int, int) line: 5228
RepaintManager$PaintManager.paintDoubleBuffered(JComponent, Image, Graphics, int, int, int, int) line: 1482
RepaintManager$PaintManager.paint(JComponent, JComponent, Graphics, int, int, int, int) line: 1413
RepaintManager.paint(JComponent, JComponent, Graphics, int, int, int, int) line: 1206
JRootPane(JComponent).paint(Graphics) line: 1040
GraphicsCallback$PaintCallback.run(Component, Graphics) line: 39
GraphicsCallback$PaintCallback(SunGraphicsCallback).runOneComponent(Component, Rectangle, Graphics, Shape, int) line: 78
GraphicsCallback$PaintCallback(SunGraphicsCallback).runComponents(Component[], Graphics, int) line: 115
JFrame(Container).paint(Graphics) line: 1967
JFrame(Window).paint(Graphics) line: 3867
RepaintManager.paintDirtyRegions(Map<Component,Rectangle>) line: 781
RepaintManager.paintDirtyRegions() line: 728
RepaintManager.prePaintDirtyRegions() line: 677
RepaintManager.access$700(RepaintManager) line: 59
RepaintManager$ProcessingRunnable.run() line: 1621
InvocationEvent.dispatch() line: 251
EventQueue.dispatchEventImpl(AWTEvent, Object) line: 705
EventQueue.access$000(EventQueue, AWTEvent, Object) line: 101
EventQueue$3.run() line: 666
EventQueue$3.run() line: 664
AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: not available [native method]
ProtectionDomain$1.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 76
EventQueue.dispatchEvent(AWTEvent) line: 675
EventDispatchThread.pumpOneEventForFilters(int) line: 211
EventDispatchThread.pumpEventsForFilter(int, Conditional, EventFilter) line: 128
EventDispatchThread.pumpEventsForHierarchy(int, Conditional, Component) line: 117
EventDispatchThread.pumpEvents(int, Conditional) line: 113
EventDispatchThread.pumpEvents(Conditional) line: 105
EventDispatchThread.run() line: 90
Graphics参数来自此处:
RepaintManager.paintDirtyRegions(Map) line: 781
涉及到的代码段如下:
Graphics g = JComponent.safelyGetGraphics(
dirtyComponent, dirtyComponent);
if (g != null) {
g.setClip(rect.x, rect.y, rect.width, rect.height);
try {
dirtyComponent.paint(g);
} finally {
g.dispose();
}
}
如果您仔细看一下,您会发现它从JComponent本身检索图形(间接使用javax.swing.JComponent.safelyGetGraphics(Component,Component)),后者最终从其第一个“重量级父级”(裁剪到组件边界)中获取,该父级自己从其相应的本地资源中获取。
关于必须将Graphics强制转换为Graphics2D的事实,这只是发生在使用Window Toolkit时的情况,Graphics实际上扩展了Graphics2D,但您可以使用其他不必扩展Graphics2D的Graphics(这种情况并不经常发生,但AWT/Swing允许您这样做)。
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;
import javax.swing.JPanel;
class TestPaint extends JPanel {
public TestPaint() {
setBackground(Color.WHITE);
}
@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawOval(0, 0, getWidth(), getHeight());
}
public static void main(String[] args) {
JFrame jFrame = new JFrame();
jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
jFrame.setSize(300, 300);
jFrame.add(new TestPaint());
jFrame.setVisible(true);
}
}