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Tomcat源码分析 （三）----- 生命周期机制 Lifecycle

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 2023-03-29
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原文作者：chen_hao 原文地址：https://www.cnblogs.com/java-chen-hao/p/11316521.html

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，我们需要看看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

我们看看Lifecycle有哪些方法，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。我们逐一来看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);
    }
  1. 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。
  2. 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()

    @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);
        }
    }

我们看看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我们理解组件的生命周期有很大的帮助，也有助于我们对设计模式的回顾。

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