Tomcat原始碼分析 （三）----- 生命週期機制 Lifecycle

chen_hao發表於2019-08-12
Tomcat原始碼

Tomcat裡面有各種各樣的元件，每個元件各司其職，元件之間又相互協作共同完成web伺服器這樣的工程。在這些元件之上，Lifecycle（生命週期機制）至關重要！在學習各個元件之前，我們需要看看Lifecycle是什麼以及能做什麼？實現原理又是怎樣的？

什麼是Lifecycle?

Lifecycle，其實就是一個狀態機，對元件的由生到死狀態的管理。

  • 當元件在STARTING_PREP、STARTING或STARTED時，呼叫start()方法沒有任何效果
  • 當元件在NEW狀態時，呼叫start()方法會導致init()方法被立刻執行，隨後start()方法被執行
  • 當元件在STOPPING_PREP、STOPPING或STOPPED時，呼叫stop()方法沒有任何效果
  • 當一個元件在NEW狀態時，呼叫stop()方法會將元件狀態變更為STOPPED，比較典型的場景就是元件啟動失敗，其子元件還沒有啟動。當一個元件停止的時候，它將嘗試停止它下面的所有子元件，即使子元件還沒有啟動。

Lifecycle方法

我們看看Lifecycle有哪些方法，如下所示：

public interface Lifecycle {
    // 新增監聽器
    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;
    // 銷燬方法，和init對應
    public void destroy() throws LifecycleException;
    // 獲取生命週期狀態
    public LifecycleState getState();
    // 獲取字串型別的生命週期狀態
    public String getStateName();
}

LifecycleBase

LifecycleBase是Lifecycle的基本實現。我們逐一來看Lifecycle的各個方法。

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);
}
  1. 生命週期監聽器儲存在一個執行緒安全的List中，CopyOnWriteArrayList。所以add和remove都是直接呼叫此List的相應方法。
  2. findLifecycleListeners返回的是一個陣列，為了執行緒安全，所以這兒會生成一個新陣列。

init()

@Override
public final synchronized void init() throws LifecycleException {
    // 非NEW狀態，不允許呼叫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，需要元件自行實現
        initInternal();
        // 初始化完成之後，狀態變更為`INITIALIZED`
        setStateInternal(LifecycleState.INITIALIZED, null, false);
    } catch (Throwable t) {
        // 初始化的過程中，可能會有異常丟擲，這時需要捕獲異常，並將狀態變更為`FAILED`
        ExceptionUtils.handleThrowable(t);
        setStateInternal(LifecycleState.FAILED, null, false);
        throw new LifecycleException(
                sm.getString("lifecycleBase.initFail",toString()), t);
    }
}

setStateInternal方法用於維護狀態，同時在狀態轉換成功之後觸發事件。為了狀態的可見性，所以state宣告為volatile型別的。

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());
        }
    }

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

我們看看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;// 監聽器陣列 關注 事件(啟動或者關閉事件)
    // 迴圈通知所有生命週期時間偵聽器
    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, 建立一個事件物件, 然通知所有的監聽器發生了該事件.並做響應.

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前的狀態設定
        setStateInternal(LifecycleState.STARTING_PREP, null, false);
        // start邏輯，抽象方法，由元件自行實現
        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 {
            // 設定狀態為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 {
            // 設定狀態為STOPPING_PREP
            setStateInternal(LifecycleState.STOPPING_PREP, null, false);
        }

        // stop邏輯，抽象方法，元件自行實現
        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);
        }
        // 設定狀態為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前狀態設定
        setStateInternal(LifecycleState.DESTROYING, null, false);
       // 抽象方法，元件自行實現
        destroyInternal();
        // destroy後狀態設定
        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

從上述原始碼看得出來，LifecycleBase是使用了狀態機+模板模式來實現的。模板方法有下面這幾個：

// 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;
// 銷燬方法
protected abstract void destroyInternal() throws LifecycleException;

總結

Lifecycle其實非常簡單，程式碼也不復雜，但是剖析其實現對於我們理解元件的生命週期有很大的幫助，也有助於我們對設計模式的回顧。

 

 

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