如何使动态JPanel坚持到底?

I'm using GridBagLayout and JPanel 2 seems to always be in the center of the JPanel 1. JPanel 2 gets dinamically adjusted in height throughout the program, so I can't stick it to the top using gbc.insets.top = -15; (gbc = GridBagConstraints) for example. I've tried gbc.anchor = GridBagConstraints.PAGE_START; or gbc.gridheight = 3; but they don't seem to do what I'm looking for.

假设JPanel 1和JFrame保持相同的大小(JFrame不可调整大小),我如何才能使JPanel 2始终位于顶部,而又没有这个间隙呢?

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