tomcat 8.0.36
知识点：
- 动态监听器的好处可以根据环境条件进行选择性添加。
- 静态监听器有七类。
- ServletContextAttributeListener
- ServletRequestListener
- ServletRequestAttributeListener
- HttpSessionIdListener
- HttpSessionAttributeListener
- HttpSessionListener
- ServletContextListener
- 动态添加的ServletContextListener，其事件源ServletContext缺失插入性等功能。
- 在触发ServletContext时，不能再动态添加ServletContextListener，但可以动态添加其它类型的监听器。
获取通过web.xml，web-fragment.xml或者注解等方式配置的监听器集，统称他们为静态监听器集。
为什么称为静态监听器集，因为他们是固定必须添加进来的，相对于动态来说，动态是可以根据所需或者不同的条件，例如servlet api的版本等因素，进行选择性添加。
String listeners[] = findApplicationListeners();
接下来就是定义一个结果集，这个结果集是用于接收监听器实例化后的实例。而实例化的过程也是挺丰富多彩的，例如JNDI注入，POST构造方法调用等，这里就不多叙述了。
Object results[] = new Object[listeners.length];
for (int i = 0; i < results.length; i++) {
String listener = listeners[i];
results[i] = getInstanceManager().newInstance(listener);
}
然后把监听器集的实例区分开来，为什么要区分开来？可能是因为事件类型的触发频率要比生命周期的频率高吧，反正能触发就行。
ArrayList<Object> eventListeners = new ArrayList<>();
ArrayList<Object> lifecycleListeners = new ArrayList<>();
for (int i = 0; i < results.length; i++) {
if ((results[i] instanceof ServletContextAttributeListener)
|| (results[i] instanceof ServletRequestAttributeListener)
|| (results[i] instanceof ServletRequestListener)
|| (results[i] instanceof HttpSessionIdListener)
|| (results[i] instanceof HttpSessionAttributeListener)) {
eventListeners.add(results[i]);
}
if ((results[i] instanceof ServletContextListener)
|| (results[i] instanceof HttpSessionListener)) {
lifecycleListeners.add(results[i]);
}
}
这里就是合并了，当然是与动态监听器集合并，两者合并后就是最终的监听器集。
在合并生命周期类型的监听器集时，会把动态的ServletContextListener存入noPluggabilityListeners，而noPluggabilityListeners的意思是无插入性监听器集，其用途就在后面。
for (Object eventListener : getApplicationEventListeners()) {
eventListeners.add(eventListener);
}
setApplicationEventListeners(eventListeners.toArray());
for (Object lifecycleListener : getApplicationLifecycleListeners()) {
lifecycleListeners.add(lifecycleListener);
if (lifecycleListener instanceof ServletContextListener) {
noPluggabilityListeners.add(lifecycleListener);
}
}
setApplicationLifecycleListeners(lifecycleListeners.toArray());
接着，就是关闭一个选项功能，防止程序在触发ServletContextListener事件时，获取ServletContext再次动态添加ServletContextListener。
context.setNewServletContextListenerAllowed(false);
再接着，创建了两个事件，这两个事件所用到的ServletContext不一样，一个是功能齐全的ServletContext，另一个是noPluggabilityServletContext，意思是无插入功能的ServletContext，当然就是比功能齐全的ServletContext缺失大部份功能。
ServletContextEvent event = new ServletContextEvent(getServletContext());
ServletContextEvent tldEvent = new ServletContextEvent(noPluggabilityServletContext);
最后就是遍历所有ServletContextListener，调用监听方法。如果是动态添加的监听器，是禁止一些操作，所以需要传入无插入性的ServletContext。
Object instances[] = getApplicationLifecycleListeners();
for (int i = 0; i < instances.length; i++) {
if (!(instances[i] instanceof ServletContextListener))
continue;
ServletContextListener listener = (ServletContextListener) instances[i];
try {
if (noPluggabilityListeners.contains(listener)) {
listener.contextInitialized(tldEvent);
} else {
listener.contextInitialized(event);
}
} catch (Throwable t) {
}
}
完整源码：
public boolean listenerStart() {
if (log.isDebugEnabled())
log.debug("Configuring application event listeners");
String listeners[] = findApplicationListeners();
Object results[] = new Object[listeners.length];
boolean ok = true;
for (int i = 0; i < results.length; i++) {
if (getLogger().isDebugEnabled())
getLogger().debug(" Configuring event listener class '" + listeners[i] + "'");
try {
String listener = listeners[i];
results[i] = getInstanceManager().newInstance(listener);
} catch (Throwable t) {
t = ExceptionUtils.unwrapInvocationTargetException(t);
ExceptionUtils.handleThrowable(t);
getLogger().error(sm.getString("standardContext.applicationListener", listeners[i]), t);
ok = false;
}
}
if (!ok) {
getLogger().error(sm.getString("standardContext.applicationSkipped"));
return (false);
}
ArrayList<Object> eventListeners = new ArrayList<>();
ArrayList<Object> lifecycleListeners = new ArrayList<>();
for (int i = 0; i < results.length; i++) {
if ((results[i] instanceof ServletContextAttributeListener)
|| (results[i] instanceof ServletRequestAttributeListener)
|| (results[i] instanceof ServletRequestListener)
|| (results[i] instanceof HttpSessionIdListener)
|| (results[i] instanceof HttpSessionAttributeListener)) {
eventListeners.add(results[i]);
}
if ((results[i] instanceof ServletContextListener)
|| (results[i] instanceof HttpSessionListener)) {
lifecycleListeners.add(results[i]);
}
}
for (Object eventListener : getApplicationEventListeners()) {
eventListeners.add(eventListener);
}
setApplicationEventListeners(eventListeners.toArray());
for (Object lifecycleListener : getApplicationLifecycleListeners()) {
lifecycleListeners.add(lifecycleListener);
if (lifecycleListener instanceof ServletContextListener) {
noPluggabilityListeners.add(lifecycleListener);
}
}
setApplicationLifecycleListeners(lifecycleListeners.toArray());
if (getLogger().isDebugEnabled())
getLogger().debug("Sending application start events");
getServletContext();
context.setNewServletContextListenerAllowed(false);
Object instances[] = getApplicationLifecycleListeners();
if (instances == null || instances.length == 0) {
return ok;
}
ServletContextEvent event = new ServletContextEvent(getServletContext());
ServletContextEvent tldEvent = null;
if (noPluggabilityListeners.size() > 0) {
noPluggabilityServletContext = new NoPluggabilityServletContext(getServletContext());
tldEvent = new ServletContextEvent(noPluggabilityServletContext);
}
for (int i = 0; i < instances.length; i++) {
if (!(instances[i] instanceof ServletContextListener))
continue;
ServletContextListener listener = (ServletContextListener) instances[i];
try {
fireContainerEvent("beforeContextInitialized", listener);
if (noPluggabilityListeners.contains(listener)) {
listener.contextInitialized(tldEvent);
} else {
listener.contextInitialized(event);
}
fireContainerEvent("afterContextInitialized", listener);
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
fireContainerEvent("afterContextInitialized", listener);
getLogger().error(sm.getString("standardContext.listenerStart", instances[i].getClass().getName()), t);
ok = false;
}
}
return (ok);
}