【Tomcat源码分析】生命周期机制 Lifecycle-51CTO.COM

前言

Tomcat 内部，如同精密齿轮般运转着各种组件，彼此协作，构建起庞大的 web 服务器架构。在这错综复杂的体系中，Lifecycle（生命周期机制）犹如灵魂，贯穿始终，赋予组件生命，协调运作。

在深入探究各组件之前，我们不妨先解开 Lifecycle 的神秘面纱，领略它如何赋予组件生命，以及它背后的实现原理。

什么是 Lifecycle?

Lifecycle，实则是一台精密的状态机，以时间为轴，管理着组件从诞生到消亡的每个阶段。

当组件处于STARTING_PREP、STARTING或STARTED状态时，调用 start()方法如同石沉大海，毫无反应。

然而，当它处于 NEW 状态时，调用start()方法则会触发一系列连锁反应：init()方法率先登场，紧随其后的是 start()方法的执行。

反之，当组件处于STOPPING_PREP、STOPPING或STOPPED状态时，调用 stop()方法亦是徒劳。

但在 NEW 状态下，调用stop()方法则会将组件直接置于 STOPPED 状态，这往往发生在组件启动失败，其子组件尚未启动的情况下。值得注意的是，当一个组件停止时，它会竭尽全力尝试停止其所有子组件，即使这些子组件尚未启动。

Lifecycle 方法

让我们一同揭开 Lifecycle 的面纱，探寻它拥有的方法，如下：

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的基石，承载着生命周期的基本实现。我们一起深入探索 LifecycleBase 的每个方法：

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);
}

生命周期监听器们，守护着组件的每个状态转变。它们被整齐地排列在一个线程安全的 CopyOnWriteArrayList 中。无论是添加还是移除监听器，都直接调用 List 的相应方法，保证了操作的安全性。
而 findLifecycleListeners 方法则以数组的形式返回所有监听器。为了确保线程安全，每次调用该方法时，都会生成一个新的数组，将监听器们复制其中，避免了并发修改带来的风险。

init()

@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，负责维护组件的当前状态。每一次状态转换，都会经过它的仔细审核，并在转换成功后，它会向外界发出通知。

为了保证状态的实时可见性，state 被声明为 volatile 类型，确保任何线程对 state 的修改都能立即被其他线程感知，避免出现状态滞后的情况。

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()

@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()

@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()

@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 类巧妙地运用了状态机与模板模式的结合，以构筑其功能。

// 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，看似简单，代码简洁，却又设计精妙，展现出设计模式的优雅与力量。

深入剖析 Lifecycle 的实现，不仅让我们对组件的生命周期有了更深刻的理解，更让我们从中可以领悟到设计模式的精髓，为我们未来设计更优雅、更强大的系统提供了宝贵的参考。