DevNotes - Tomcat源码分析—生命周期机制Lifecycle - 《Java 笔记》

什么是Lifecycle?
Lifecycle方法
LifecycleBase
总结

JavaWeb Tomcat
Tomcat里面有各种各样的组件，每个组件各司其职，组件之间又相互协作共同完成web服务器这样的工程。在这些组件之上，Lifecycle（生命周期机制）至关重要！在学习各个组件之前，需要看看Lifecycle是什么以及能做什么？实现原理又是怎样的？

什么是Lifecycle?

Lifecycle，其实就是一个状态机，对组件的由生到死状态的管理。
当组件在STARTING_PREP、STARTING或STARTED时，调用start()方法没有任何效果
当组件在NEW状态时，调用start()方法会导致init()方法被立刻执行，随后start()方法被执行
当组件在STOPPING_PREP、STOPPING或STOPPED时，调用stop()方法没有任何效果
当一个组件在NEW状态时，调用stop()方法会将组件状态变更为STOPPED，比较典型的场景就是组件启动失败，其子组件还没有启动。当一个组件停止的时候，它将尝试停止它下面的所有子组件，即使子组件还没有启动。

Lifecycle方法

Lifecycle的方法，如下所示：

public interface Lifecycle {
    // 添加监听器
    public void addLifecycleListener(LifecycleListener listener);
    // 获取所以监听器
    public LifecycleListener[] findLifecycleListeners();
    // 移除某个监听器
    public void removeLifecycleListener(LifecycleListener listener);
    // 初始化方法
    public void init() throws LifecycleException;
    // 启动方法
    public void start() throws LifecycleException;
    // 停止方法，和start对应
    public void stop() throws LifecycleException;
    // 销毁方法，和init对应
    public void destroy() throws LifecycleException;
    // 获取生命周期状态
    public LifecycleState getState();
    // 获取字符串类型的生命周期状态
    public String getStateName();
}

LifecycleBase

LifecycleBase是Lifecycle的基本实现。逐一来看Lifecycle的各个方法。

增加、删除和获取监听器

private final List<LifecycleListener> lifecycleListeners = new CopyOnWriteArrayList<>();
@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);
}

生命周期监听器保存在一个线程安全的List中，CopyOnWriteArrayList。所以add和remove都是直接调用此List的相应方法。

findLifecycleListeners返回的是一个数组，为了线程安全，所以这儿会生成一个新数组。

`init()`方法

@Override
public final synchronized void init() throws LifecycleException {
 // 非NEW状态，不允许调用init()方法
 if (!state.equals(LifecycleState.NEW)) {
     invalidTransition(Lifecycle.BEFORE_INIT_EVENT);
 }
 try {
     // 初始化逻辑之前，先将状态变更为`INITIALIZING`
     setStateInternal(LifecycleState.INITIALIZING, null, false);
     // 初始化，该方法为一个abstract方法，需要组件自行实现
     initInternal();
     // 初始化完成之后，状态变更为`INITIALIZED`
     setStateInternal(LifecycleState.INITIALIZED, null, false);
 } catch (Throwable t) {
     // 初始化的过程中，可能会有异常抛出，这时需要捕获异常，并将状态变更为`FAILED`
     ExceptionUtils.handleThrowable(t);
     setStateInternal(LifecycleState.FAILED, null, false);
     throw new LifecycleException(
         sm.getString("lifecycleBase.initFail",toString()), t);
 }
}

setStateInternal方法用于维护状态，同时在状态转换成功之后触发事件。为了状态的可见性，所以state声明为volatile类型的。

private volatile LifecycleState state = LifecycleState.NEW;。

private synchronized void setStateInternal(LifecycleState state,
                                        Object data, boolean check) throws LifecycleException {
 if (log.isDebugEnabled()) {
     log.debug(sm.getString("lifecycleBase.setState", this, state));
 }
 // 是否校验状态
 if (check) {
     // Must have been triggered by one of the abstract methods (assume
     // code in this class is correct)
     // null is never a valid state
     // state不允许为null
     if (state == null) {
         invalidTransition("null");
         // Unreachable code - here to stop eclipse complaining about
         // a possible NPE further down the method
         return;
     }
     // Any method can transition to failed
     // startInternal() permits STARTING_PREP to STARTING
     // stopInternal() permits STOPPING_PREP to STOPPING and FAILED to
     // STOPPING
     if (!(state == LifecycleState.FAILED ||
           (this.state == LifecycleState.STARTING_PREP &&
            state == LifecycleState.STARTING) ||
           (this.state == LifecycleState.STOPPING_PREP &&
            state == LifecycleState.STOPPING) ||
           (this.state == LifecycleState.FAILED &&
            state == LifecycleState.STOPPING))) {
         // No other transition permitted
         invalidTransition(state.name());
     }
 }
 // 设置状态
 this.state = state;
 // 触发事件
 String lifecycleEvent = state.getLifecycleEvent();
 if (lifecycleEvent != null) {
     fireLifecycleEvent(lifecycleEvent, data);
 }
}

看看fireLifecycleEvent方法，

public void fireLifecycleEvent(String type, Object data) {
 // 事件监听,观察者模式的另一种方式
 LifecycleEvent event = new LifecycleEvent(lifecycle, type, data);
 LifecycleListener interested[] = listeners;// 监听器数组 关注 事件(启动或者关闭事件)
 // 循环通知所有生命周期时间侦听器
 for (int i = 0; i < interested.length; i++)
     // 每个监听器都有自己的逻辑
     interested[i].lifecycleEvent(event);
}

首先，创建一个事件对象，然通知所有的监听器发生了该事件，并做响应。

`start()`方法

@Override
public final synchronized void start() throws LifecycleException {
// `STARTING_PREP`、`STARTING`和`STARTED时，将忽略start()逻辑
if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||
        LifecycleState.STARTED.equals(state)) {
    if (log.isDebugEnabled()) {
        Exception e = new LifecycleException();
        log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e);
    } else if (log.isInfoEnabled()) {
        log.info(sm.getString("lifecycleBase.alreadyStarted", toString()));
    }
    return;
}
// `NEW`状态时，执行init()方法
if (state.equals(LifecycleState.NEW)) {
    init();
}
// `FAILED`状态时，执行stop()方法
else if (state.equals(LifecycleState.FAILED)) {
    stop();
}
// 不是`INITIALIZED`和`STOPPED`时，则说明是非法的操作
else if (!state.equals(LifecycleState.INITIALIZED) &&
        !state.equals(LifecycleState.STOPPED)) {
    invalidTransition(Lifecycle.BEFORE_START_EVENT);
}
try {
    // start前的状态设置
    setStateInternal(LifecycleState.STARTING_PREP, null, false);
    // start逻辑，抽象方法，由组件自行实现
    startInternal();
    // start过程中，可能因为某些原因失败，这时需要stop操作
    if (state.equals(LifecycleState.FAILED)) {
        // This is a 'controlled' failure. The component put itself into the
        // FAILED state so call stop() to complete the clean-up.
        stop();
    } else if (!state.equals(LifecycleState.STARTING)) {
        // Shouldn't be necessary but acts as a check that sub-classes are
        // doing what they are supposed to.
        invalidTransition(Lifecycle.AFTER_START_EVENT);
    } else {
        // 设置状态为STARTED
        setStateInternal(LifecycleState.STARTED, null, false);
    }
} catch (Throwable t) {
    // This is an 'uncontrolled' failure so put the component into the
    // FAILED state and throw an exception.
    ExceptionUtils.handleThrowable(t);
    setStateInternal(LifecycleState.FAILED, null, false);
    throw new LifecycleException(sm.getString("lifecycleBase.startFail", toString()), t);
}
}

`stop()`方法

@Override
public final synchronized void stop() throws LifecycleException {
 // `STOPPING_PREP`、`STOPPING`和STOPPED时，将忽略stop()的执行
 if (LifecycleState.STOPPING_PREP.equals(state) || LifecycleState.STOPPING.equals(state) ||
         LifecycleState.STOPPED.equals(state)) {
     if (log.isDebugEnabled()) {
         Exception e = new LifecycleException();
         log.debug(sm.getString("lifecycleBase.alreadyStopped", toString()), e);
     } else if (log.isInfoEnabled()) {
         log.info(sm.getString("lifecycleBase.alreadyStopped", toString()));
     }
     return;
 }
 // `NEW`状态时，直接将状态变更为`STOPPED`
 if (state.equals(LifecycleState.NEW)) {
     state = LifecycleState.STOPPED;
     return;
 }
 // stop()的执行，必须要是`STARTED`和`FAILED`
 if (!state.equals(LifecycleState.STARTED) && !state.equals(LifecycleState.FAILED)) {
     invalidTransition(Lifecycle.BEFORE_STOP_EVENT);
 }
 try {
     // `FAILED`时，直接触发BEFORE_STOP_EVENT事件
     if (state.equals(LifecycleState.FAILED)) {
         // Don't transition to STOPPING_PREP as that would briefly mark the
         // component as available but do ensure the BEFORE_STOP_EVENT is
         // fired
         fireLifecycleEvent(BEFORE_STOP_EVENT, null);
     } else {
         // 设置状态为STOPPING_PREP
         setStateInternal(LifecycleState.STOPPING_PREP, null, false);
     }
     // stop逻辑，抽象方法，组件自行实现
     stopInternal();
     // Shouldn't be necessary but acts as a check that sub-classes are
     // doing what they are supposed to.
     if (!state.equals(LifecycleState.STOPPING) && !state.equals(LifecycleState.FAILED)) {
         invalidTransition(Lifecycle.AFTER_STOP_EVENT);
     }
     // 设置状态为STOPPED
     setStateInternal(LifecycleState.STOPPED, null, false);
 } catch (Throwable t) {
     ExceptionUtils.handleThrowable(t);
     setStateInternal(LifecycleState.FAILED, null, false);
     throw new LifecycleException(sm.getString("lifecycleBase.stopFail",toString()), t);
 } finally {
     if (this instanceof Lifecycle.SingleUse) {
         // Complete stop process first
         setStateInternal(LifecycleState.STOPPED, null, false);
         destroy();
     }
 }
}

`destroy()`方法

@Override
public final synchronized void destroy() throws LifecycleException {
 // `FAILED`状态时，直接触发stop()逻辑
 if (LifecycleState.FAILED.equals(state)) {
     try {
         // Triggers clean-up
         stop();
     } catch (LifecycleException e) {
         // Just log. Still want to destroy.
         log.warn(sm.getString(
             "lifecycleBase.destroyStopFail", toString()), e);
     }
 }
 // `DESTROYING`和`DESTROYED`时，忽略destroy的执行
 if (LifecycleState.DESTROYING.equals(state) ||
     LifecycleState.DESTROYED.equals(state)) {
     if (log.isDebugEnabled()) {
         Exception e = new LifecycleException();
         log.debug(sm.getString("lifecycleBase.alreadyDestroyed", toString()), e);
     } else if (log.isInfoEnabled() && !(this instanceof Lifecycle.SingleUse)) {
         // Rather than have every component that might need to call
         // destroy() check for SingleUse, don't log an info message if
         // multiple calls are made to destroy()
         log.info(sm.getString("lifecycleBase.alreadyDestroyed", toString()));
     }
     return;
 }
 // 非法状态判断
 if (!state.equals(LifecycleState.STOPPED) &&
     !state.equals(LifecycleState.FAILED) &&
     !state.equals(LifecycleState.NEW) &&
     !state.equals(LifecycleState.INITIALIZED)) {
     invalidTransition(Lifecycle.BEFORE_DESTROY_EVENT);
 }
 try {
     // destroy前状态设置
     setStateInternal(LifecycleState.DESTROYING, null, false);
     // 抽象方法，组件自行实现
     destroyInternal();
     // destroy后状态设置
     setStateInternal(LifecycleState.DESTROYED, null, false);
 } catch (Throwable t) {
     ExceptionUtils.handleThrowable(t);
     setStateInternal(LifecycleState.FAILED, null, false);
     throw new LifecycleException(
         sm.getString("lifecycleBase.destroyFail",toString()), t);
 }
}

模板方法

从上述源码看得出来，LifecycleBase是使用了状态机+模板模式来实现的。模板方法有下面这几个：

// 初始化方法
protected abstract void initInternal() throws LifecycleException;
// 启动方法
protected abstract void startInternal() throws LifecycleException;
// 停止方法
protected abstract void stopInternal() throws LifecycleException;
// 销毁方法
protected abstract void destroyInternal() throws LifecycleException;

总结

Lifecycle其实非常简单，代码也不复杂，但是剖析其实现对于理解组件的生命周期有很大的帮助，也有助于对设计模式的回顾。

Tomcat源码分析—生命周期机制Lifecycle

什么是Lifecycle?

Lifecycle方法

LifecycleBase

增加、删除和获取监听器

init()方法

start()方法

stop()方法

destroy()方法

模板方法

总结

`init()`方法

`start()`方法

`stop()`方法

`destroy()`方法