【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
    }
}