This is how I did.
1. Build jdk9/dev to an exploded-image, so you have build/<platform>/jdk.
2. mkdir jdk/classes to trick IntelliJ IDEA this is a JDK.
3. Add this jdk directory as an SDK. In classpath and sourcepath, add
the modules you are interested in (classpath to
<build>/jdk/modules/mod_name, sourcepath to
<src>/jdk/src/mod_name/share/classes).
4. Create an empty project, add whatever jdk src directory you like into
its Modules/Source, or add the whole jdk there, and "Mark Directory as"
sources for each sub-directory. You can manually edit <content> and
<sourceFolder> in the iml file.
5. Done.
--Max
