If I have global java.util.List <com.blah.CountedValue <> > countsList; then Drools Builder never finishes. If I have global java.util.List <com.blah.CountedValue > countsList; Then the problem does not occur. So it's the diamond brackets <> causing the hang. I'm not claiming this is the code I want - I was part way through changing some stuff between Drools and Java and ended up with the <> present whilst I was thinking. Either auto-save or my fingers doing the equivalent a bit later triggered the hang. The impact is massive, unfortunately, as Eclipse tries to build the workspace (by default) on starting, and you need to have finished workspace building before you can get the the dlg to disable the automatic workspace building or the use of Drools Builder. This happens in 6.1.0.201407130611 - but I was using 6.1.0.201407061813 when it went wrong. Is there anything else I can add to be helpful? Borris