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);
}
- 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。 - 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。