帮你查了一下API,其中有段话可能你没注意看：
The button mask returned by InputEvent.getModifiers() reflects only the button that changed state, not the current state of all buttons. (Note: Due to overlap in the values of ALT_MASK/BUTTON2_MASK and META_MASK/BUTTON3_MASK, this is not always true for mouse events involving modifier keys). To get the state of all buttons and modifier keys, use InputEvent.getModifiersEx().

InputEvent.getModifiers()只能反射单个键的更改状态，如果想获得组合键的状态，必须用InputEvent.getModifiersEx().

下面的代码我测试过，可以判断：
gText.addMouseListener(new MouseAdapter()
{
public void mousePressed(MouseEvent e)
{

if(e.getButton()==e.BUTTON1 )
gLabel.setText("button1"); //判断左键

if( e.getButton()==e.BUTTON3)
gLabel.setText("button3");//判断右键

if(e.getModifiersEx()==(e.BUTTON3_DOWN_MASK + e.BUTTON1_DOWN_MASK))
gLabel.setText("bt1+bt3!"); //判断左右键同时按下

if(e.getModifiersEx()==(e.SHIFT_DOWN_MASK + e.BUTTON1_DOWN_MASK))
gLabel.setText("shift+bt1");// shift+左键
//...............其他雷同，修改常量值就可以
}

}
);