extensions/service/src/com/google/inject/service/Service.java

/*
 * Copyright (C) 2010 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject.service;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;

/**
 * An object with an operational state, asynchronous {@link #start()} and {@link #stop()} lifecycle
 * methods to transition in and out of this state. Example services include http servers, RPC
 * systems and timer tasks.
 *
 * @author dhanji@gmail.com (Dhanji R. Prasanna)
 */
public interface Service {
  /**
   * If the service has already been started, this method returns immediately without taking action.
   * A stopped service may not be restarted.
   *
   * @return a future for the startup result, regardless of whether this call initiated startup.
   *     Calling {@link Future#get} will block until the service has finished starting, and returns
   *     the resultant state. If the service fails to start, {@link Future#get} will throw an {@link
   *     ExecutionException}. If it has already finished starting, {@link Future#get} returns
   *     immediately.
   */
  Future<State> start();

  /**
   * If the service is {@link State#STARTED} initiates service shutdown and returns immediately. If
   * the service has already been stopped, this method returns immediately without taking action.
   *
   * @return a future for the shutdown result, regardless of whether this call initiated shutdown.
   *     Calling {@link Future#get} will block until the service has finished shutting down, and
   *     either returns {@link State#STOPPED} or throws an {@link ExecutionException}. If it has
   *     already finished stopping, {@link Future#get} returns immediately.
   */
  Future<State> stop();

  /**
   * Returns the current state of this service. One of {@link State} possible values, or null if
   * this is a brand new object, i.e., has not been put into any state yet.
   */
  State state();

  /** The lifecycle states of a service. */
  enum State {
    STARTED,
    STOPPED,
    FAILED
  }
}