Adding source code library projects

Typically, you would create a Browsing-Only Project for the JDK sources and then add this to your root project. How to go about this is described in the Tutorial.
The procedure is the same for any source code library.
Note that you can add the subproject to any project in the tree, symbol information will be available throughout.
See also Upgrading source code library projects (e.g. JDK).