Tomcat8.5.43源码分析-(3)Tomcat生命周期与监听器

在继续Tomcat启动过程源码分析之前，我们先回顾一下Tomcat的生命周期，查看LifeCycle.java类的注释：

/**
 * Common interface for component life cycle methods.  Catalina components
 * may implement this interface (as well as the appropriate interface(s) for
 * the functionality they support) in order to provide a consistent mechanism
 * to start and stop the component.
 * <br>
 * The valid state transitions for components that support {@link Lifecycle}
 * are:
 * <pre>
 *            start()
 *  -----------------------------
 *  |                           |
 *  | init()                    |
 * NEW -»-- INITIALIZING        |
 * | |           |              |     ------------------«-----------------------
 * | |           |auto          |     |                                        |
 * | |          \|/    start() \|/   \|/     auto          auto         stop() |
 * | |      INITIALIZED --»-- STARTING_PREP --»- STARTING --»- STARTED --»---  |
 * | |         |                                                            |  |
 * | |destroy()|                                                            |  |
 * | --»-----«--    ------------------------«--------------------------------  ^
 * |     |          |                                                          |
 * |     |         \|/          auto                 auto              start() |
 * |     |     STOPPING_PREP ----»---- STOPPING ------»----- STOPPED -----»-----
 * |    \|/                               ^                     |  ^
 * |     |               stop()           |                     |  |
 * |     |       --------------------------                     |  |
 * |     |       |                                              |  |
 * |     |       |    destroy()                       destroy() |  |
 * |     |    FAILED ----»------ DESTROYING ---«-----------------  |
 * |     |                        ^     |                          |
 * |     |     destroy()          |     |auto                      |
 * |     --------»-----------------    \|/                         |
 * |                                 DESTROYED                     |
 * |                                                               |
 * |                            stop()                             |
 * ----»-----------------------------»------------------------------
 *
 * Any state can transition to FAILED.
 *
 * Calling start() while a component is in states STARTING_PREP, STARTING or
 * STARTED has no effect.
 *
 * Calling start() while a component is in state NEW will cause init() to be
 * called immediately after the start() method is entered.
 *
 * Calling stop() while a component is in states STOPPING_PREP, STOPPING or
 * STOPPED has no effect.
 *
 * Calling stop() while a component is in state NEW transitions the component
 * to STOPPED. This is typically encountered when a component fails to start and
 * does not start all its sub-components. When the component is stopped, it will
 * try to stop all sub-components - even those it didn't start.
 *
 * Attempting any other transition will throw {@link LifecycleException}.
 *
 * </pre>
 * The {@link LifecycleEvent}s fired during state changes are defined in the
 * methods that trigger the changed. No {@link LifecycleEvent}s are fired if the
 * attempted transition is not valid.
 *
 * @author Craig R. McClanahan
 */

在上一篇文章中，我们提到LifecycleBase抽象类中的生命周期的各个阶段，会把生命周期事件通知给注册的监听器，在LifeCycleBase.fireLifecycleEvent中体现。那么监听器又是如何接受事件的呢？其实早在Catalina.createStartDigester()中，各个容器的生命周期被注册了最基本的监听器，即实现了LifecycleListener接口的监听器。

默认在Server.xml中注册监听器列表如下：

  <Listener className="org.apache.catalina.startup.VersionLoggerListener" />
  <!-- Security listener. Documentation at /docs/config/listeners.html
  <Listener className="org.apache.catalina.security.SecurityListener" />
  -->
  <!--APR library loader. Documentation at /docs/apr.html -->
  <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
  <!-- Prevent memory leaks due to use of particular java/javax APIs-->
  <Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
  <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
  <Listener className="org.apache.catalina.core.ThreadLocalLeakPreventionListener" />

这里整理createStartDigester()中除去Server.xml注册基本的生命周期监听器之外，还额外注册若干重要监听器：

• StandardEngine：EngineConfig 用于打印Engine启动和停止日志；
• StandardHost：HostConfig 重要监听器，后详；
• StandardContext：ContextConfig 重要监听器，后详；

在创建的过程中，各层级的容器也会显示调用LifecycleBase.addLifecycleListener(LifecycleListener listener)方法。在发生了生命周期事件后，遍历所有注册的监听器，调用生命周期监听器的接口方法lifecycleEvent()。

以EngineHost为例：

    /**
     * Process the START event for an associated Engine.
     *
     * @param event The lifecycle event that has occurred
     */
    @Override
    public void lifecycleEvent(LifecycleEvent event) {

        // Identify the engine we are associated with
        try {
            engine = (Engine) event.getLifecycle();
        } catch (ClassCastException e) {
            log.error(sm.getString("engineConfig.cce", event.getLifecycle()), e);
            return;
        }

        // Process the event that has occurred
        if (event.getType().equals(Lifecycle.START_EVENT))
            start();
        else if (event.getType().equals(Lifecycle.STOP_EVENT))
            stop();

    }

发生了Lifecycle.START_EVENT事件，调用自身start()方法；发生了Lifecycle.STOP_EVENT事件，调用自身的stop()方法。

也就是说，各容器在发出生命周期事件的消息时，其注册的每个监听器会对这个事件进行轮询响应。