When running a script for the first time using the GroovyScriptEngine, if two or more concurrent threads try to run it, it is possible that a concurrency issue happens when accessing updateCacheEntry at groovy.util.GroovyScriptEngine.updateCacheEntry(GroovyScriptEngine.java:285) I think this is due that the private Map "dependencies" at groovy.util.GroovyScriptEngine$ScriptCacheEntry.dependencies (GroovyScriptEngine.java:101) is not synchronized. It could be solved by: private Map dependencies= Collections.synchronizedMap(new HashMap()); I have attached my test case (to be run by executing the main method) and a sample groovy script.