Hadoop Common源码分析之服务Service
Service是定义Hadoop中服务生命周期的一个接口。Service内部定义了服务的状态及生命周期，在服务被构造后，其一个生命周期内的状态为NOTINITED未初始化--INITED已初始化--已启动STARTED--已停止STOPPED，而这一生命周期内服务状态的变化，是随着如下方法链的调用而变化的：init()--start()--stop()，服务构造后整体方法调用及状态转移如下图所示： 下面，我们来看下Service的源码分析，先看下其内部定义的服务状态，如下： /** * Service states * 服务状态 */ public enum STATE { /** Constructed but not initialized */ // 服务实例已经构造但是还没有初始化 NOTINITED(0, "NOTINITED"), /** Initialized but not started or stopped */ // 服务实例已经初始化但还没有启动或停止 INITED(1, "INITED"), /** started and not stopped */ // 服务实例已经启动但没有停止 STARTED(2, "STARTED"), /** stopped. No further state transitions are permitted */ // 服务实例已经停止，不允许进一步的状态转换 STOPPED(3, "STOPPED"); /** * An integer value for use in array lookup and JMX interfaces. * Although {@link Enum#ordinal()} could do this, explicitly * identify the numbers gives more stability guarantees over time. */ private final int value; /** * A name of the state that can be used in messages */ private final String statename; private STATE(int value, String name) { this.value = value; this.statename = name; } /** * Get the integer value of a state * @return the numeric value of the state */ public int getValue() { return value; } /** * Get the name of a state * @return the state's name */ @Override public String toString() { return statename; } } STATE是一个枚举类，代表了服务生命周期内各阶段的状态，分别为： 1、NOTINITED--0：服务实例已经构造但是还没有初始化； 2、INITED--1：服务实例已经初始化但还没有启动或停止； 3、STARTED--2：服务实例已经启动但没有停止； 4、STOPPED--3：服务实例已经停止，不允许进一步的状态转换。 接下来，我们再看下导致服务状态变更的各个方法，如下： 1、初始化服务init()： NOTINITED-->INITED/STOPPED /** * Initialize the service. * 初始化服务 * * The transition MUST be from {@link STATE#NOTINITED} to {@link STATE#INITED} * unless the operation failed and an exception was raised, in which case * {@link #stop()} MUST be invoked and the service enter the state * {@link STATE#STOPPED}. * * 初始化后，服务状态由NOTINITED转换到INITED，除非异常发生导致服务初始化操作失败， * 在这种情况下，stop()方法应该被调用，并且服务状态进入STOPPED。 * * @param config the configuration of the service * @throws RuntimeException on any failure during the operation 初始化后，服务状态由NOTINITED转换到INITED，除非异常发生导致服务初始化操作失败，在这种情况下，stop()方法应该被调用，并且服务状态进入STOPPED。 2、启动服务start()：INITED-->STARTED/STOPPED /** * Start the service. * 启动服务 * * The transition MUST be from {@link STATE#INITED} to {@link STATE#STARTED} * unless the operation failed and an exception was raised, in which case * {@link #stop()} MUST be invoked and the service enter the state * {@link STATE#STOPPED}. * * 服务启动后，服务状态由INITED转换到STARTED，除非异常发生导致服务启动操作失败， * 在这种情况下，stop()方法应该被调用，并且服务状态进入STOPPED。 * * @throws RuntimeException on any failure during the operation */ 服务启动后，服务状态由INITED转换到STARTED，除非异常发生导致服务启动操作失败，在这种情况下，stop()方法应该被调用，并且服务状态进入STOPPED。 3、停止服务stop()：NOTINITED/INITED/STARTED-->STOPPED /** * Stop the service. This MUST be a no-op if the service is already * in the {@link STATE#STOPPED} state. It SHOULD be a best-effort attempt * to stop all parts of the service. * 停止服务。如果服务已经处于STOPPED状态，这必须是一个空操作。它应该尽力试图停止服务的所有部分。 * * The implementation must be designed to complete regardless of the service * state, including the initialized/uninitialized state of all its internal * fields. * 实现者必须被设计为完成这个操作，而不管服务的状态，包括其内部所有字段初始化/未初始化的状态。 * * @throws RuntimeException on any failure during the stop operation */ void stop(); 停止服务。如果服务已经处于STOPPED状态，这必须是一个空操作。它应该尽力试图停止服务的所有部分。实现者必须被设计为完成这个操作，而不管服务的状态，包括其内部所有字段初始化/未初始化的状态。 为了兼顾Java7闭包条款，Service接口还提供了close()方法，实际上就是调用stop()方法，代码如下： /** * A version of stop() that is designed to be usable in Java7 closure * clauses. * 适用于Java7闭包条款的stop()版本，实际上就是调用stop()方法。 * * Implementation classes MUST relay this directly to {@link #stop()} * 实现类必须直接传递给stop()方法。 * * @throws IOException never * @throws RuntimeException on any failure during the stop operation */ void close() throws IOException; Service的另外一大块内容，就是对于服务状态变更时间的监听，并提供了注册监听器与注销监听器的方法，如下： 1、注册监听器registerServiceListener() /** * Register a listener to the service state change events. * If the supplied listener is already listening to this service, * this method is a no-op. * 注册一个监听器到服务状态变更事件。 * 如果提供的监听器已经监听此服务，这种方法是一个空操作。 * @param listener a new listener */ void registerServiceListener(ServiceStateChangeListener listener); 2、注销监听器unregisterServiceListener() /** * Unregister a previously registered listener of the service state * change events. No-op if the listener is already unregistered. * 注销之前的一个注册到服务状态变更事件的监听器。 * 如果监听器已经注销，这种方法是一个空操作。 * @param listener the listener to unregister. */ void unregisterServiceListener(ServiceStateChangeListener listener); 还有，Service还提供了获取服务失败时发生的第一个异常和当时状态的方法，如下： /** * Get the first exception raised during the service failure. If null, * no exception was logged * 获取服务失败时发生的第一个异常 * @return the failure logged during a transition to the stopped state */ Throwable getFailureCause(); /** * Get the state in which the failure in {@link #getFailureCause()} occurred. * 获取服务失败发生的第一个异常时的服务状态 * @return the state or null if there was no failure */ STATE getFailureState(); 既然是一个服务，Service还提供了阻塞等待服务停止的waitForServiceToStop()方法，如下： /** * Block waiting for the service to stop; uses the termination notification * object to do so. * 阻塞，等待服务停止；通过终止通知对象实现。 * * This method will only return after all the service stop actions * have been executed (to success or failure), or the timeout elapsed * This method can be called before the service is inited or started; this is * to eliminate any race condition with the service stopping before * this event occurs. * @param timeout timeout in milliseconds. A value of zero means "forever" * @return true iff the service stopped in the time period */ boolean waitForServiceToStop(long timeout); 其它诸如获取服务名、获取当前服务状态、获取服务启动时间、查询是否服务是在一个特定的状态、获取生命周期历史的一个快照、获取远程依赖服务的正在停止服务的阻塞者等方法不再一一介绍，读者可自行查询，下面只把代码贴出： /** * Get the name of this service. * 获取服务名 * @return the service name */ String getName(); /** * Get the configuration of this service. * 获取服务的配置信息 * This is normally not a clone and may be manipulated, though there are no * guarantees as to what the consequences of such actions may be * @return the current configuration, unless a specific implentation chooses * otherwise. */ Configuration getConfig(); /** * Get the current service state * 获取当前服务状态 * @return the state of the service */ STATE getServiceState(); /** * Get the service start time * 获取服务启动时间 * @return the start time of the service. This will be zero if the service * has not yet been started. */ long getStartTime(); /** * Query to see if the service is in a specific state. * 查询是否服务是在一个特定的状态 * In a multi-threaded system, the state may not hold for very long. * 在多线程系统中，服务状态可能不会保持很长时间。 * @param state the expected state * @return true if, at the time of invocation, the service was in that state. */ boolean isInState(STATE state); /** * Get a snapshot of the lifecycle history; it is a static list * 获取生命周期历史的一个快照，它是一个静态列表 * @return a possibly empty but never null list of lifecycle events. */ public List<LifecycleEvent> getLifecycleHistory(); /** * Get the blockers on a service -remote dependencies * that are stopping the service from being <i>live</i>. * * 获取远程依赖服务的正在停止服务的阻塞者 * * @return a (snapshotted) map of blocker name->description values */ public Map<String, String> getBlockers();
