Stopping on breakpoint can cause X window system to freeze

I was experiencing this problem when debugging a Java Swing app on my Ubuntu system. When by debugger hit a breakpoint, sometimes the whole X window system would freeze. My only option was to drop to the console (ctrl-alt-f1) and kill the java process. This fix seems to have fixed it:

Add the java VM options:

-Dsun.awt.disablegrab=true

for more details see this