private void checkClosed() {
    if (isClosed) {
      throw new IllegalStateException(
          "Cannot access a closed GemFireHealth instance.");
    }
  }
Throws an {@link IllegalStateException} if this <code>GemFireHealthImpl</code> is closed.
geode-core/src/main/java/org/apache/geode/admin/internal/GemFireHealthImpl.java