While using certain libraries on App Engine, you may encounter the following exception notice:
javax.servlet.ServletContext log: Exception while dispatching incoming RPC call threw an unexpected exception: java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThreadGroup)
If you see this exception message, your application or (more frequently the case) a library is attempting to create a new thread. App Engine doesn’t allow frontend instances to spawn threads, so any attempt to start up a thread will result in AccessControlExceptions.
However, App Engine does allow backend threads: threads which run within backend instances. If your application absolutely needs to run threads, run the threading component within a backend or a backend module.