@Override
  public void setEntityConfigXMLFile(String xmlFile) {
    checkReadOnly();
    entityConfigXMLFile = xmlFile;
    configChanged();
  }
Checks to see if this config object is "read only". If it is, then an
{@link IllegalStateException} is thrown.

@since GemFire 4.0
geode-core/src/main/java/org/apache/geode/admin/internal/DistributedSystemConfigImpl.java