protected void checkReadOnly() {
    if (system != null) {
      throw new IllegalStateException(
          "A DistributedSystemConfig object cannot be modified after it has been used to create an AdminDistributedSystem.");
    }
  }
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