深入浅出Tomcat/3 - Tomcat生命周期

Posted 2021-02-07 江边闲话集

在上面的部分，其实我们已经接触到Tomcat的生命周期了，接下来我们将仔细讨论和学习Tomcat的生命周期的具体实现。

LifeCycle接口

这个LifeCycle接口上面在讲解Server和Service时其实已经有所接触。Tomcat通过org.apache.catalina.LifeCycle接口统一接管Tomcat的生命周期，所有继承自它的组件（即Component）都需要实现这个接口。

我们先俯视看一看LifeCycle接口的定义。

根据上图，我们清晰看到上图包含了4个生命周期的方法：
1. init
2. start
3. stop
4. destroy
这4个方法不用多解释，很直观，我们前面也有所接触。

同时，每个方法又定义了几个常量字符串，用于LifeCycleEvent事件的type属性，用来描述各个状态的变化。举个例子，对于init周期，它包含before_init和 after_init两个字符串，其他类似。

它同时也定义了三个管理监听器的方法，分别如下：
• addLifeCycleListener
• findLifeCycleListeners
• removeLifeCycleListener
显而易见，它们用来增加、查找和删除生命周期的监听器。

最后定义了获取当前状态信息，用2个方法实现：
• getState
• getStateName
getState返回的是LifeCycleState，而且LifeCycleState是一个枚举类型。如下

public enum LifecycleState {
    NEW(false, null),
    INITIALIZING(false, Lifecycle.BEFORE_INIT_EVENT),
    INITIALIZED(false, Lifecycle.AFTER_INIT_EVENT),
    STARTING_PREP(false, Lifecycle.BEFORE_START_EVENT),
    STARTING(true, Lifecycle.START_EVENT),
    STARTED(true, Lifecycle.AFTER_START_EVENT),
    STOPPING_PREP(true, Lifecycle.BEFORE_STOP_EVENT),
    STOPPING(false, Lifecycle.STOP_EVENT),
    STOPPED(false, Lifecycle.AFTER_STOP_EVENT),
    DESTROYING(false, Lifecycle.BEFORE_DESTROY_EVENT),
    DESTROYED(false, Lifecycle.AFTER_DESTROY_EVENT),
    FAILED(false, null);

我们看到它列举了Tomcat各个生命周期。

LifeCycleBase类

LifeCycleBase是实现了LifeCycle的抽象类，Tomcat的几个生命周期的管理都交给它，所以弄清楚这个类的实现，基本上也就明白了Tomcat的生命周期了。

在LifeCycle接口里，我们提到的4个周期的方法以及3个监听器的管理方法，最后还有2个获取状态信息的方法，我们接下来逐一分析和了解。

4个生命周期方法

方法init

public final synchronized void init() throws LifecycleException {
if (!state.equals(LifecycleState.NEW)) {
invalidTransition(Lifecycle.BEFORE_INIT_EVENT);
}

try {
setStateInternal(LifecycleState.INITIALIZING, null, false);
initInternal();
setStateInternal(LifecycleState.INITIALIZED, null, false);
} catch (Throwable t) {
handleSubClassException(t, "lifecycleBase.initFail", toString());
}
}

先判断当前状态是不是NEW，如果不是抛出异常。因为init时，代表的是Tomcat的初始化，所以其状态需要为NEW的。接下来就是设置状态为LifecycleState.INITIALIZING,也就是正在初始化，然后调用initInternal开始初始化了，初始化完之后设置为LifecycleState.INITIALIZED

这里需要多说一下invalidTransition这个方法，因为后面很多地方都会用到它。

private void invalidTransition(String type) throws LifecycleException {
String msg = sm.getString("lifecycleBase.invalidTransition", type, toString(), state);
throw new LifecycleException(msg);
}

我们看到invalidTransition方法其实没做其他事情，就是抛出了一个异常而已。
同时，也介绍一下setStateInternal这个方法，后面也用的较多。

private synchronized void setStateInternal(LifecycleState state, Object data, boolean check)
throws LifecycleException {

if (log.isDebugEnabled()) {
log.debug(sm.getString("lifecycleBase.setState", this, state));
}

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
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());
}
}

this.state = state;
String lifecycleEvent = state.getLifecycleEvent();
if (lifecycleEvent != null) {
fireLifecycleEvent(lifecycleEvent, data);
}
}

变量check是传进来的，是否要检查。至于后面的逻辑注释写的比较清楚，不符合逻辑的状态转化都会报错，不多解释。

后面的fireLifeCycleEvent的代码如下：

protected void fireLifecycleEvent(String type, Object data) {
LifecycleEvent event = new LifecycleEvent(this, type, data);
for (LifecycleListener listener : lifecycleListeners) {
listener.lifecycleEvent(event);
}
}

也是很简单，就是让LifeCycle的事件监听者触发事件，至于如何去使用这些event，则是每个监听者自己的业务逻辑了。这也符合一贯的代码实现方法，如果有多个监听器，然后逐一触发这些监听器，这其实是事件监听最基本的实现方法。

以上就是init的方法实现。

方法start
直接上代码

public final synchronized void start() throws LifecycleException {

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;
}

if (state.equals(LifecycleState.NEW)) {
init();
} else if (state.equals(LifecycleState.FAILED)) {
stop();
} else if (!state.equals(LifecycleState.INITIALIZED) &&
!state.equals(LifecycleState.STOPPED)) {
invalidTransition(Lifecycle.BEFORE_START_EVENT);
}

try {
setStateInternal(LifecycleState.STARTING_PREP, null, false);
startInternal();
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 {
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.
handleSubClassException(t, "lifecycleBase.startFail", toString());
}
}

第一步就是检查状态的合理性，如果已经在准备或者开始了，直接抛出已经开始的exception。如果是NEW的话，说明init这一步没有做，那就初始化一下。
正式开始启动了，首先需要将状态设置为LifecycleState.STARTING_PREP，接下调用startInternal开始启动。执行完startInternal后验证state，如果状态不对，要么停止，要么抛出异常。

方法destroy
destroy其实调用的是stop方法。

方法stop
Stop方法

public final synchronized void stop() throws LifecycleException {

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;
}

if (state.equals(LifecycleState.NEW)) {
state = LifecycleState.STOPPED;
return;
}

if (!state.equals(LifecycleState.STARTED) && !state.equals(LifecycleState.FAILED)) {
invalidTransition(Lifecycle.BEFORE_STOP_EVENT);
}

try {
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 {
setStateInternal(LifecycleState.STOPPING_PREP, null, false);
}

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);
}

setStateInternal(LifecycleState.STOPPED, null, false);
} catch (Throwable t) {
handleSubClassException(t, "lifecycleBase.stopFail", toString());
} finally {
if (this instanceof Lifecycle.SingleUse) {
// Complete stop process first
setStateInternal(LifecycleState.STOPPED, null, false);
destroy();
}
}
}

首先，检查一下当前状态，如果当前状态为与STOP相关的几个状态，则抛出已经停止的异常。如果为NEW，说明还没有初始化，直接将STOPPED的状态赋值一下即可。如果状态不是STARTED，是不可以停止的，这个时候直接抛出异常。将触发的event给监听器和前面类似，不多做解释。

3个关于监听器的方法

前面介绍到LifeCycleBase有三个方法来管理监听器：
• addLifeCycleListener
• findLifeCycleListeners
• removeLifeCycleListener
我们看看它们的实现。

@Override
public void addLifecycleListener(LifecycleListener listener) {
lifecycleListeners.add(listener);
}


/**
* {@inheritDoc}
*/
@Override
public LifecycleListener[] findLifecycleListeners() {
return lifecycleListeners.toArray(new LifecycleListener[0]);
}


/**
* {@inheritDoc}
*/
@Override
public void removeLifecycleListener(LifecycleListener listener) {
lifecycleListeners.remove(listener);
}

代码比较简单了，但是需要注意的是lifecycleListeners是一个CopyOnWriteArrayList。

private final List<LifecycleListener> lifecycleListeners = new CopyOnWriteArrayList<>();

关于CopyOnWriteArrayList，它继承了ArrayList。我们都知道ArrayList不是线程安全的，但CopyOnWriteArrayList则是线程安全的。

/**
* Creates a list containing the elements of the specified
* collection, in the order they are returned by the collection\'s
* iterator.
*
* @param c the collection of initially held elements
* @throws NullPointerException if the specified collection is null
*/
public CopyOnWriteArrayList(Collection<? extends E> c) {
Object[] elements;
if (c.getClass() == CopyOnWriteArrayList.class)
elements = ((CopyOnWriteArrayList<?>)c).getArray();
else {
elements = c.toArray();
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elements.getClass() != Object[].class)
elements = Arrays.copyOf(elements, elements.length, Object[].class);
}
setArray(elements);
}

我们可以看到代码行

elements = c.toArray();

它其实是Copy一份c，这种开销是很大的。尽管开销很大，但是当遍历操作的数量大大超过可变操作的数量时，这种方法可能比其他替代方法更有效。
同时，在add和remove时，我们看到使用到ReentrantLock来保证线程安全。
CopyOnWriteArray比较适用于读多修改少的情景。在Tomcat这里，一般来说Listener都是在Server.xml，如果在想增加或删除Listener，必须重新启动Tomcat，在这种场景下，其实正好符合读多写少这种特征。

public void add(int index, E element) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
if (index > len || index < 0)
throw new IndexOutOfBoundsException("Index: "+index+
", Size: "+len);
Object[] newElements;
int numMoved = len - index;
if (numMoved == 0)
newElements = Arrays.copyOf(elements, len + 1);
else {
newElements = new Object[len + 1];
System.arraycopy(elements, 0, newElements, 0, index);
System.arraycopy(elements, index, newElements, index + 1,
numMoved);
}
newElements[index] = element;
setArray(newElements);
} finally {
lock.unlock();
}
}

/**
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices). Returns the element that was removed from the list.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
E oldValue = get(elements, index);
int numMoved = len - index - 1;
if (numMoved == 0)
setArray(Arrays.copyOf(elements, len - 1));
else {
Object[] newElements = new Object[len - 1];
System.arraycopy(elements, 0, newElements, 0, index);
System.arraycopy(elements, index + 1, newElements, index,
numMoved);
setArray(newElements);
}
return oldValue;
} finally {
lock.unlock();
}
}

获取状态

@Override
public LifecycleState getState() {
return state;
}


/**
* {@inheritDoc}
*/
@Override
public String getStateName() {
return getState().toString();
}

代码非常简单，不介绍了。