I don't think the poster wants to go through the java source code. What he wants to do is not to go into the main libraries. Fortunately, eclipse allows you to do this using step filters (although why it is not allowed by default, the bad is never known, I would think that people who want to debug the main libraries will be an edge case ...). To do this, you can go to Window → Preferences → Java → Debugging → Filter a step , enable it and check the packages that you want to automatically cross over (I will check them all).
See this post for more information along with some great screenshots - http://www.eclipsezone.com/eclipse/forums/t83338.rhtml
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