【杂谈】Tomcat 之 Lifecycle接口
前言
此篇随笔记录《How Tomcat works》中关于Lifecycle接口的相关总结
Lifecycle接口的主要目的
核心：统一。
已知Tomcat的卡特琳娜（Catalina）由许多组件构成。当Catalina启动的时候，这些组件也要跟着一起启动，并且当Catalina关闭的时候，这些组件也要同时关闭，并且要进行必要的清理操作，释放资源。例如，当容器关闭的时候，需要调用已加载的servlet对象的destroy方法，session对象也要持久化到secondary storage（二级存储，通常指的就是硬盘）。这就要求所有Component有一致的方法，可以统一处理。
如果没有统一会怎么样？
其实也不会怎么样，就是编程起来就比较麻烦。比如有的对象初始化方法叫initiate,有的又叫init。用都可以用，就是不太好用，即你要记住这个对象的初始化方法到底叫什么。而统一之后呢，可以放到一个List中，遍历调用。这不就方便多了嘛。
Lifecycle的事件与监听
Lifecycle，翻译过来就是“生命周期”。那生命周期变化，从一个生命周期进入另一个生命周期，自然就会有相应的事件来表示。比如，人从幼儿长大成人，这是一个事件“成长”，而从中年步入老年，又是另外一个事件“衰老”。那么既然有事件的触发，那就会有事件的监听处理，常见的有两大种。
Lifecycle事件监听器原理
其实就是观察者模式了，熟悉的人就不用看了。关联的类清单如下：
- LifecycleEvent（事件对象）
- Lifecycle 接口
- LifecycleListener 接口
- LifecycleBase => 用于存储并触发监听器，《How Tomcat works》里是比较旧的，对应的是LifecycleSupport类
（1）LifecycleEvent
LifecycleEvent继承于EventObject，EventObject是JDK提供的，它只有一个字段，即Object类型的source，用于表示事件的产生者。
public class EventObject implements java.io.Serializable { private static final long serialVersionUID = 5516075349620653480L; /** * The object on which the Event initially occurred. */ protected transient Object source; public EventObject(Object source) { if (source == null) throw new IllegalArgumentException("null source"); this.source = source; } public Object getSource() { return source; } public String toString() { return getClass().getName() + "[source=" + source + "]"; } }
public final class LifecycleEvent extends EventObject { private static final long serialVersionUID = 1L; /** * * * @param lifecycle 指明当前事件在哪个lifecycle组件上产生 * @param type Event type 事件类型 * @param data Event data 额外的数据 */ public LifecycleEvent(Lifecycle lifecycle, String type, Object data) { //把lifecycle赋给source super(lifecycle); this.type = type; this.data = data; } /** * 与此事件关联的事件数据 */ private final Object data; /** * 事件类型 */ private final String type; public Object getData() { return data; } public Lifecycle getLifecycle() { return (Lifecycle) getSource(); } public String getType() { return this.type; } }
（2）Lifecycle接口
public interface Lifecycle { //统一的事件类型 public static final String BEFORE_INIT_EVENT = "before_init"; public static final String AFTER_INIT_EVENT = "after_init"; public static final String START_EVENT = "start"; public static final String BEFORE_START_EVENT = "before_start"; public static final String AFTER_START_EVENT = "after_start"; public static final String STOP_EVENT = "stop"; public static final String BEFORE_STOP_EVENT = "before_stop"; public static final String AFTER_STOP_EVENT = "after_stop"; public static final String AFTER_DESTROY_EVENT = "after_destroy"; public static final String BEFORE_DESTROY_EVENT = "before_destroy"; public static final String PERIODIC_EVENT = "periodic"; public static final String CONFIGURE_START_EVENT = "configure_start"; public static final String CONFIGURE_STOP_EVENT = "configure_stop"; //lifecycle表示的是一个组件，故可以在组件上添加监听器 public void addLifecycleListener(LifecycleListener listener); //获取注册在此组件上的监听器 public LifecycleListener[] findLifecycleListeners(); //移除某个监听器 public void removeLifecycleListener(LifecycleListener listener); //统一的生命周期方法 public void init() throws LifecycleException; public void start() throws LifecycleException; public void stop() throws LifecycleException; public void destroy() throws LifecycleException; //获取组件状态 public LifecycleState getState(); public String getStateName(); public interface SingleUse { } }
（3）LifecycleListener接口
public interface LifecycleListener { //接收事件对象作为参数 //在方法体内，根据事件类型做自定义的响应 public void lifecycleEvent(LifecycleEvent event); }
（4）LifecycleBase 抽象类
一般组件都以此为父类。主要看存监听器的List和fireLifecycleEvent方法。以下把大部分代码省略了，感兴趣的可自行翻阅源码。
public abstract class LifecycleBase implements Lifecycle { private static final Log log = LogFactory.getLog(LifecycleBase.class); private static final StringManager sm = StringManager.getManager(LifecycleBase.class); //用于存储注册在此组件上的监听器 private final List<LifecycleListener> lifecycleListeners = new CopyOnWriteArrayList<>(); //组件状态 private volatile LifecycleState state = LifecycleState.NEW; private boolean throwOnFailure = true; public boolean getThrowOnFailure() { return throwOnFailure; } public void setThrowOnFailure(boolean throwOnFailure) { this.throwOnFailure = throwOnFailure; } @Override public void addLifecycleListener(LifecycleListener listener) { lifecycleListeners.add(listener); } @Override public LifecycleListener[] findLifecycleListeners() { return lifecycleListeners.toArray(new LifecycleListener[0]); } @Override public void removeLifecycleListener(LifecycleListener listener) { lifecycleListeners.remove(listener); } //触发事件，告知所有监听器。 protected void fireLifecycleEvent(String type, Object data) { LifecycleEvent event = new LifecycleEvent(this, type, data); for (LifecycleListener listener : lifecycleListeners) { listener.lifecycleEvent(event); } } @Override public final synchronized void init() throws LifecycleException { //...省略 } protected abstract void initInternal() throws LifecycleException; @Override public final synchronized void start() throws LifecycleException { //...省略 } protected abstract void startInternal() throws LifecycleException; @Override public final synchronized void stop() throws LifecycleException { //...省略 } protected abstract void stopInternal() throws LifecycleException; @Override public final synchronized void destroy() throws LifecycleException { //...省略 } protected abstract void destroyInternal() throws LifecycleException; @Override public LifecycleState getState() { return state; } @Override public String getStateName() { return getState().toString(); } protected synchronized void setState(LifecycleState state) throws LifecycleException { setStateInternal(state, null, true); } protected synchronized void setState(LifecycleState state, Object data) throws LifecycleException { setStateInternal(state, data, true); } private synchronized void setStateInternal(LifecycleState state, Object data, boolean check) throws LifecycleException { //...省略 } private void invalidTransition(String type) throws LifecycleException { //...省略 } private void handleSubClassException(Throwable t, String key, Object... args) throws LifecycleException { //...省略 } }
