javadoc location for eclipse offline
Of course, I had already installed a JDK with a source. On the machine I found a JRE7 and JDK1.7xxx, and the projects use the JRE7.
In Window - Preferences - Java - Installed JRE I found a link to the JRE7, which does not contain sources or a Javadoc.
So I pressed "Edit" and browsed the directory to the JDK1.7, then pressed "Restore Default". This will change the JRE system libraries to the JDK. The Javadoc locations for rt.jar and the other jars is now at Oracle, but nevertheless the installed src.zip is used, if there is no internet connection.
Alternatively, it is indeed possible to attach the src.zip file (file!) to the rt.jar of JRE7.
You can download the Java 7 javadoc from
http://www.oracle.com/technetwork/java/javase/documentation/java-se-7-doc-download-435117.html