A manager for monitoring and controlling a set of services. This class provides methods for starting, stopping and inspecting a collection of services. Additionally, users can monitor state transitions with the listener mechanism.

While it is recommended that service lifecycles be managed via this class, state transitions initiated via other mechanisms do not impact the correctness of its methods. For example, if the services are started by some mechanism besides startAsync, the listeners will be invoked when appropriate and awaitHealthy will still work as expected.

Here is a simple example of how to use a ServiceManager to start a server.

class Server {
  public static void main(String[] args) {
    Set<Service> services = ...;
    ServiceManager manager = new ServiceManager(services);
    manager.addListener(new Listener() {
        public void stopped() {}
        public void healthy() {
          // Services have been initialized and are healthy, start accepting requests...
        }
        public void failure(Service service) {
          // Something failed, at this point we could log it, notify a load balancer, or take
          // some other action.  For now we will just exit.
          System.exit(1);
        }
      },
      MoreExecutors.directExecutor());

    Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {
        // Give the services 5 seconds to stop to ensure that we are responsive to shutdown
        // requests.
        try {
          manager.stopAsync().awaitStopped(5, TimeUnit.SECONDS);
        } catch (TimeoutException timeout) {
          // stopping timed out
        }
      }
    });
    manager.startAsync();  // start all the services asynchronously
  }
}

This class uses the ServiceManager's methods to start all of its services, to respond to service failure and to ensure that when the JVM is shutting down all the services are stopped.