code: public void setPrefix(String prefix) {
      if (!Strings.isNullOrEmpty(getProject().getUserProperty(SYSPROP_PREFIX()))) {
        log("Ignoring prefix attribute because it is overridden by user properties.", Project.MSG_WARN);
      } else {
        SysGlobals.initializeWith(prefix);
      }
    }
docs: Initializes custom prefix for all junit4 properties. This must be consistent
  across all junit4 invocations if done from the same classpath. Use only when REALLY needed.