【Tomcat源碼分析】生命周期機制 Lifecycle

作者：派大星 2024-08-30 10:40:12

開發前端

Lifecycle，實則是一臺精密的狀態機，以時間為軸，管理著組件從誕生到消亡的每個階段。當組件處于STARTING_PREP、STARTING或STARTED狀態時，調用 start()方法如同石沉大海，毫無反應。

前言

Tomcat 內部，如同精密齒輪般運轉著各種組件，彼此協作，構建起龐大的 web 服務器架構。在這錯綜復雜的體系中，Lifecycle（生命周期機制）猶如靈魂，貫穿始終，賦予組件生命，協調運作。

在深入探究各組件之前，我們不妨先解開 Lifecycle 的神秘面紗，領略它如何賦予組件生命，以及它背后的實現原理。

什么是 Lifecycle?

Lifecycle，實則是一臺精密的狀態機，以時間為軸，管理著組件從誕生到消亡的每個階段。

當組件處于STARTING_PREP、STARTING或STARTED狀態時，調用 start()方法如同石沉大海，毫無反應。

然而，當它處于 NEW 狀態時，調用start()方法則會觸發一系列連鎖反應：init()方法率先登場，緊隨其后的是 start()方法的執行。

反之，當組件處于STOPPING_PREP、STOPPING或STOPPED狀態時，調用 stop()方法亦是徒勞。

但在 NEW 狀態下，調用stop()方法則會將組件直接置于 STOPPED 狀態，這往往發生在組件啟動失敗，其子組件尚未啟動的情況下。值得注意的是，當一個組件停止時，它會竭盡全力嘗試停止其所有子組件，即使這些子組件尚未啟動。

Lifecycle 方法

讓我們一同揭開 Lifecycle 的面紗，探尋它擁有的方法，如下：

public interface Lifecycle {
    // 添加監聽器
    public void addLifecycleListener(LifecycleListener listener);
    // 獲取所以監聽器
    public LifecycleListener[] findLifecycleListeners();
    // 移除某個監聽器
    public void removeLifecycleListener(LifecycleListener listener);
    // 初始化方法
    public void init() throws LifecycleException;
    // 啟動方法
    public void start() throws LifecycleException;
    // 停止方法，和start對應
    public void stop() throws LifecycleException;
    // 銷毀方法，和init對應
    public void destroy() throws LifecycleException;
    // 獲取生命周期狀態
    public LifecycleState getState();
    // 獲取字符串類型的生命周期狀態
    public String getStateName();
}

LifecycleBase

LifecycleBase是Lifecycle的基石，承載著生命周期的基本實現。我們一起深入探索 LifecycleBase 的每個方法：

增加、刪除和獲取監聽器

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);
}

生命周期監聽器們，守護著組件的每個狀態轉變。它們被整齊地排列在一個線程安全的 CopyOnWriteArrayList 中。無論是添加還是移除監聽器，都直接調用 List 的相應方法，保證了操作的安全性。
而 findLifecycleListeners 方法則以數組的形式返回所有監聽器。為了確保線程安全，每次調用該方法時，都會生成一個新的數組，將監聽器們復制其中，避免了并發修改帶來的風險。

init()

@Override
public final synchronized void init() throws LifecycleException {
    // 非NEW狀態，不允許調用init()方法
    if (!state.equals(LifecycleState.NEW)) {
        invalidTransition(Lifecycle.BEFORE_INIT_EVENT);
    }

    try {
        // 初始化邏輯之前，先將狀態變更為`INITIALIZING`
        setStateInternal(LifecycleState.INITIALIZING, null, false);
        // 初始化，該方法為一個abstract方法，需要組件自行實現
        initInternal();
        // 初始化完成之后，狀態變更為`INITIALIZED`
        setStateInternal(LifecycleState.INITIALIZED, null, false);
    } catch (Throwable t) {
        // 初始化的過程中，可能會有異常拋出，這時需要捕獲異常，并將狀態變更為`FAILED`
        ExceptionUtils.handleThrowable(t);
        setStateInternal(LifecycleState.FAILED, null, false);
        throw new LifecycleException(
                sm.getString("lifecycleBase.initFail",toString()), t);
    }
}

setStateInternal 方法，負責維護組件的當前狀態。每一次狀態轉換，都會經過它的仔細審核，并在轉換成功后，它會向外界發出通知。

為了保證狀態的實時可見性，state 被聲明為 volatile 類型，確保任何線程對 state 的修改都能立即被其他線程感知，避免出現狀態滯后的情況。

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));
    }

    // 是否校驗狀態
    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不允許為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());
        }
    }

    // 設置狀態
    this.state = state;
    // 觸發事件
    String lifecycleEvent = state.getLifecycleEvent();
    if (lifecycleEvent != null) {
        fireLifecycleEvent(lifecycleEvent, data);
    }
}

看看fireLifecycleEvent方法，

public void fireLifecycleEvent(String type, Object data) {
    // 事件監聽,觀察者模式的另一種方式
    LifecycleEvent event = new LifecycleEvent(lifecycle, type, data);
    LifecycleListener interested[] = listeners;// 監聽器數組 關注 事件(啟動或者關閉事件)
    // 循環通知所有生命周期時間偵聽器
    for (int i = 0; i < interested.length; i++)
        // 每個監聽器都有自己的邏輯
        interested[i].lifecycleEvent(event);
}

首先, 創建一個事件對象, 然通知所有的監聽器發生了該事件.并做響應.

start()

@Override
public final synchronized void start() throws LifecycleException {
    // `STARTING_PREP`、`STARTING`和`STARTED時，將忽略start()邏輯
    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`狀態時，執行init()方法
    if (state.equals(LifecycleState.NEW)) {
        init();
    }

    // `FAILED`狀態時，執行stop()方法
    else if (state.equals(LifecycleState.FAILED)) {
        stop();
    }

    // 不是`INITIALIZED`和`STOPPED`時，則說明是非法的操作
    else if (!state.equals(LifecycleState.INITIALIZED) &&
            !state.equals(LifecycleState.STOPPED)) {
        invalidTransition(Lifecycle.BEFORE_START_EVENT);
    }

    try {
        // start前的狀態設置
        setStateInternal(LifecycleState.STARTING_PREP, null, false);
        // start邏輯，抽象方法，由組件自行實現
        startInternal();
        // start過程中，可能因為某些原因失敗，這時需要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 {
            // 設置狀態為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時，將忽略stop()的執行
    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`狀態時，直接將狀態變更為`STOPPED`
    if (state.equals(LifecycleState.NEW)) {
        state = LifecycleState.STOPPED;
        return;
    }

    // stop()的執行，必須要是`STARTED`和`FAILED`
    if (!state.equals(LifecycleState.STARTED) && !state.equals(LifecycleState.FAILED)) {
        invalidTransition(Lifecycle.BEFORE_STOP_EVENT);
    }

    try {
        // `FAILED`時，直接觸發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 {
            // 設置狀態為STOPPING_PREP
            setStateInternal(LifecycleState.STOPPING_PREP, null, false);
        }

        // stop邏輯，抽象方法，組件自行實現
        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);
        }
        // 設置狀態為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`狀態時，直接觸發stop()邏輯
    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`時，忽略destroy的執行
    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;
    }

    // 非法狀態判斷
    if (!state.equals(LifecycleState.STOPPED) &&
            !state.equals(LifecycleState.FAILED) &&
            !state.equals(LifecycleState.NEW) &&
            !state.equals(LifecycleState.INITIALIZED)) {
        invalidTransition(Lifecycle.BEFORE_DESTROY_EVENT);
    }

    try {
        // destroy前狀態設置
        setStateInternal(LifecycleState.DESTROYING, null, false);
       // 抽象方法，組件自行實現
        destroyInternal();
        // destroy后狀態設置
        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

從上述源碼看得出來，LifecycleBase 類巧妙地運用了狀態機與模板模式的結合，以構筑其功能。

// 初始化方法
protected abstract void initInternal() throws LifecycleException;
// 啟動方法
protected abstract void startInternal() throws LifecycleException;
// 停止方法
protected abstract void stopInternal() throws LifecycleException;
// 銷毀方法
protected abstract void destroyInternal() throws LifecycleException;

總結

Lifecycle，看似簡單，代碼簡潔，卻又設計精妙，展現出設計模式的優雅與力量。

深入剖析 Lifecycle 的實現，不僅讓我們對組件的生命周期有了更深刻的理解，更讓我們從中可以領悟到設計模式的精髓，為我們未來設計更優雅、更強大的系統提供了寶貴的參考。

責任編輯：武曉燕來源：碼上遇見你

Lifecycle 狀態機管理

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