code: public static synchronized boolean shutdown() {
          if (instance != null) {
              instance.markShuttingDown();
              instance.interrupt();
              connectionManagers.clear();
              instance = null;
              return true;
          }
          return false;
      }
docs: Shuts down the thread, allowing the class and instance to be collected.
  <p/>
  Since this is a daemon thread, its running will not prevent JVM shutdown.
  It will, however, prevent this class from being unloaded or garbage
  collected, in the context of a long-running application, until it is
  interrupted. This method will stop the thread's execution and clear its
  state. Any use of a service client will cause the thread to be restarted.
  
  @return true if an actual shutdown has been made; false otherwise.