Tomcat原始碼分析 （三）----- 生命週期機制 Lifecycle
阿新 • • 發佈：2019-08-12
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);
}- 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。 - findLifecycleListeners返回的是一個數組，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。