A basic repaint efficiency problem

Question

When the repaint() method of a swing component is called by some thread, but the whole swing application window is full covered by other windows, then, will the paintComponent(Graphics g) method of the component actually be called?
And why? Who takes care of this?

Answer 1

The paint() method is called.
Inside JComponent.paint(), the clip rectangle (the rectangle that covers all that needs to be repainted) is compared to the bounds of the component. If the bounds are such that some of the component is within the repaint area, then the component's paintComponent() method is called.
The paintChildren() is called, and each child which is within the clip rectangle is painted (with some extra work so that painting all of something deep in the Z-stack doesn't overpaint something higher-up outside the clip area).
JComponent has the responsibility of deciding which of its children to paint.
The OS's window manager has the responsibility of providing the bounds of the area that needs to be repainted.
Whether or not the repaint method is called when a window is obscured is OS dependent- on OS X, the image of any inactive window is cached, so the paint method of window 'A' won't get called if you drag window 'B' over the top of 'A', minimise and restore 'A', or move 'A' around without resizing (unless you add in extra window event handlers for those cases). On Windows NT, repaints will occur for most of these events, as it has a much less advanced windowing system.
Pete