Tomcat源码分析（四）：ServletContext应用启动之核心组件初始化

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概述

由我的上篇文章Tomcat源码分析（三）：ServletContext应用启动之配置解析可知，在Tomcat启动应用StandardContext时，是先通过listener事件机制的方式，交给ContextConfig，解析web.xml类来获取应用的配置信息，包含过滤器filter，监听器listener，如spring的ContextLoaderListener，servlet，如spring的DispatchServlet，以及session配置等。但是配置解析这边只是创建一个包装类，如过滤器的FilterDef，servlet的StandardWrapper，获取这些组件对应的类的全限定名以及init params等数据保存到包装类中，而不进行实际类对象的创建。
其次在Servlet3.0之后，提供了WebApplicationInitializer接口来支持以编程方式配置web.xml的内容，故在ContextConfig中也会查找应用中的WebApplicationInitializer的实例类，然后保存到StandardContext的initializers中。注意在ContextConfig并不会对WebApplicationInitializer进行解析，即调用其onStartup方法，而是留在StardardContext中执行。
```
/*** The ordered set of ServletContainerInitializers for this web application.*/// map的value为WebApplicationInitializer的集合
private Map<ServletContainerInitializer,Set<Class<?>>> initializers =new LinkedHashMap<>();
```
具体以上对象实例的创建，如servlet, filter，listener等，是之后在StandardContext中继续完成。即在StandardContext的startInternal方法中，以如下顺序继续完成启动和相关类对象实例的创建。

1. context初始化参数加载

context初始化参数主要包括两种，一种是在web.xml（或者WebApplicationInitializer，以下提到web.xml的类似）配置context-param设置，如下：
```
<context-param>  <param-name>contextConfigLocation</param-name>  <param-value>classpath:spring/applicationContext.xml</param-value>  
</context-param>
```

另外一种则是在Server.xml的Context节点下面的Parameter节点配置，或者在Context.xml中配置，这些是提供某个param默认值的方式，可以被web.xml中的覆盖。然后在Digester解析规则会添加Parameter解析器，具体为在ContextRuleSet中定义，如下：

digester.addObjectCreate(prefix + "Context/Parameter","org.apache.tomcat.util.descriptor.web.ApplicationParameter");
digester.addSetProperties(prefix + "Context/Parameter");
digester.addSetNext(prefix + "Context/Parameter","addApplicationParameter","org.apache.tomcat.util.descriptor.web.ApplicationParameter");

在ContextConfig中已经解析好了在web.xml中配置的param，然后接着在StandardContext中，将以上两种方式获取的param填充到servletContext中：

/*** Merge the context initialization parameters specified in the application* deployment descriptor with the application parameters described in the* server configuration, respecting the <code>override</code> property of* the application parameters appropriately.*/
private void mergeParameters() {Map<String,String> mergedParams = new HashMap<>();// web.xmlString names[] = findParameters();for (int i = 0; i < names.length; i++) {mergedParams.put(names[i], findParameter(names[i]));}// Parameter节点ApplicationParameter params[] = findApplicationParameters();for (int i = 0; i < params.length; i++) {if (params[i].getOverride()) {if (mergedParams.get(params[i].getName()) == null) {mergedParams.put(params[i].getName(),params[i].getValue());}} else {mergedParams.put(params[i].getName(), params[i].getValue());}}// 填充到ServletContext中ServletContext sc = getServletContext();for (Map.Entry<String,String> entry : mergedParams.entrySet()) {sc.setInitParameter(entry.getKey(), entry.getValue());}}

2. SCI（ServletContainerInitializers）启动onStartup

在ContextConfig查找到应用中的WebApplicationInitializer实现类中，填充保存在StandardContext的initializers集合中，然后在这步调用WebApplicationInitializer的onStartup方法，获取编程方式的应用配置信息。

// Call ServletContainerInitializers
for (Map.Entry<ServletContainerInitializer, Set<Class<?>>> entry :initializers.entrySet()) {try {// 在spring中，则是遍历WebApplicationInitializer集合，// 即entry.getValue()，// 调用WebApplicationInitializer的onStartup方法;// getServletContext()为传递当前应用的servletContextentry.getKey().onStartup(entry.getValue(),getServletContext());} catch (ServletException e) {log.error(sm.getString("standardContext.sciFail"), e);ok = false;break;}
}

3. Listener监听器配置与调用

这些一般是应用在web.xml和WebApplicationInitializer中配置的监听器，而不是Tomcat自身添加的监听器，如ContextConfig是Tomcat自身使用Digester解析server.xml时添加的监听器，用于解析配置文件。而这里说的监听器是应用在web.xml或者WebApplicationInitializer中，即配置文件中添加的，类型包括ServletContextListener，HttpSessionListener，ServletRequestListener等，如spring的ContextLoaderListener就是一个ServletContextListener接口的实现类。

实现在StandardContext的listenerStart方法，核心实现如下：即包括：

EventListener和LifeCycleListener的创建
生命周期监听器LifeCycleListener的调用

/*** Configure the set of instantiated application event listeners* for this Context.* @return <code>true</code> if all listeners wre* initialized successfully, or <code>false</code> otherwise.*/
public boolean listenerStart() {...// 1. 创建listener对象实例// Instantiate the required listenersString listeners[] = findApplicationListeners();Object results[] = new Object[listeners.length];boolean ok = true;for (int i = 0; i < results.length; i++) {...String listener = listeners[i];results[i] = getInstanceManager().newInstance(listener);...}...// 2. 分类：分为EventListener和LifeCycleListener，// 并保存到StandardContext中// EventListener是在应用运行过程中调用的// LifetCycleListener是应用启动，关闭等中调用的// Sort listeners in two arraysArrayList<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]);}}// Listener instances may have been added directly to this Context by// ServletContextInitializers and other code via the pluggability APIs.// Put them these listeners after the ones defined in web.xml and/or// annotations then overwrite the list of instances with the new, full// list.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());...// 3. 调用LifetCycleListener，处理应用启动初始化事件// 这里只调用LifetCycleListener，// 不调用EventListener// Send application start events// Ensure context is not nullgetServletContext();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++) {// 只调用ServletContextListener// spring的ContextLoaderListener就是// ServletContextListener的实现类if (!(instances[i] instanceof ServletContextListener))continue;ServletContextListener listener =(ServletContextListener) instances[i];try {fireContainerEvent("beforeContextInitialized", listener);if (noPluggabilityListeners.contains(listener)) {listener.contextInitialized(tldEvent);} else {// 调用contextInitialized方法// 可以通过event获取ServletContext// spring的ContextLoaderListener// 是在这里加载spring的root WebApplicationContet// 并作为一个attribute填充到ServletContext中listener.contextInitialized(event);}fireContainerEvent("afterContextInitialized", listener);}...}return (ok);
}

4. 过滤器Filter的实例化

遍历StandardContext的filterDefs集合，创建ApplicationFilterConfig，然后填充到filterConfigs中。其中在创建ApplicationFilterConfig时，会创建filter实例并进行初始化。

/*** Configure and initialize the set of filters for this Context.* @return <code>true</code> if all filter initialization completed* successfully, or <code>false</code> otherwise.*/
public boolean filterStart() {if (getLogger().isDebugEnabled()) {getLogger().debug("Starting filters");}// Instantiate and record a FilterConfig for each defined filterboolean ok = true;synchronized (filterConfigs) {filterConfigs.clear();for (Entry<String,FilterDef> entry : filterDefs.entrySet()) {String name = entry.getKey();if (getLogger().isDebugEnabled()) {getLogger().debug(" Starting filter '" + name + "'");}try {// 创建ApplicationFilterConfig实例，// 并放到StandardContext的filterConfigs中ApplicationFilterConfig filterConfig =new ApplicationFilterConfig(this, entry.getValue());filterConfigs.put(name, filterConfig);} catch (Throwable t) {t = ExceptionUtils.unwrapInvocationTargetException(t);ExceptionUtils.handleThrowable(t);getLogger().error(sm.getString("standardContext.filterStart", name), t);ok = false;}}}return ok;
}/*** Construct a new ApplicationFilterConfig for the specified filter* definition.** @param context The context with which we are associated* @param filterDef Filter definition for which a FilterConfig is to be*  constructed** @exception ClassCastException if the specified class does not implement*  the <code>javax.servlet.Filter</code> interface* @exception ClassNotFoundException if the filter class cannot be found* @exception IllegalAccessException if the filter class cannot be*  publicly instantiated* @exception InstantiationException if an exception occurs while*  instantiating the filter object* @exception ServletException if thrown by the filter's init() method* @throws NamingException* @throws InvocationTargetException* @throws SecurityException* @throws NoSuchMethodException* @throws IllegalArgumentException*/
ApplicationFilterConfig(Context context, FilterDef filterDef)throws ClassCastException, ClassNotFoundException, IllegalAccessException,InstantiationException, ServletException, InvocationTargetException, NamingException,IllegalArgumentException, NoSuchMethodException, SecurityException {super();this.context = context;this.filterDef = filterDef;// Allocate a new filter instance if necessaryif (filterDef.getFilter() == null) {getFilter();} else {this.filter = filterDef.getFilter();// 创建filter实例getInstanceManager().newInstance(filter);// 初始化filter，如添加filter配置的param等initFilter();}
}

5. Servlet的实例化

主要为查找配置的Servlet中onStartup > 0的serlvet，然后加载Servlet对应的类，创建servlet实例并初始化。具体为通过StandardWrapper的load方法来完成。

/*** Load and initialize all servlets marked "load on startup" in the* web application deployment descriptor.** @param children Array of wrappers for all currently defined*  servlets (including those not declared load on startup)* @return <code>true</code> if load on startup was considered successful*/
public boolean loadOnStartup(Container children[]) {// 筛选loadOnStartup > 0的servlet// Collect "load on startup" servlets that need to be initializedTreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();for (int i = 0; i < children.length; i++) {Wrapper wrapper = (Wrapper) children[i];int loadOnStartup = wrapper.getLoadOnStartup();if (loadOnStartup < 0)continue;Integer key = Integer.valueOf(loadOnStartup);ArrayList<Wrapper> list = map.get(key);if (list == null) {list = new ArrayList<>();map.put(key, list);}list.add(wrapper);}// Load the collected "load on startup" servletsfor (ArrayList<Wrapper> list : map.values()) {for (Wrapper wrapper : list) {try {// 通过StandardWrapper的load方法来完成// 类加载，实例创建，实例初始化wrapper.load();} catch (ServletException e) {getLogger().error(sm.getString("standardContext.loadOnStartup.loadException",getName(), wrapper.getName()), StandardWrapper.getRootCause(e));// NOTE: load errors (including a servlet that throws// UnavailableException from the init() method) are NOT// fatal to application startup// unless failCtxIfServletStartFails="true" is specifiedif(getComputedFailCtxIfServletStartFails()) {return false;}}}}return true;}/*** Load and initialize an instance of this servlet, if there is not already* at least one initialized instance.  This can be used, for example, to* load servlets that are marked in the deployment descriptor to be loaded* at server startup time.* <p>* <b>IMPLEMENTATION NOTE</b>:  Servlets whose classnames begin with* <code>org.apache.catalina.</code> (so-called "container" servlets)* are loaded by the same classloader that loaded this class, rather than* the classloader for the current web application.* This gives such classes access to Catalina internals, which are* prevented for classes loaded for web applications.** @exception ServletException if the servlet init() method threw*  an exception* @exception ServletException if some other loading problem occurs*/
@Override
public synchronized void load() throws ServletException {instance = loadServlet();if (!instanceInitialized) {initServlet(instance);}if (isJspServlet) {StringBuilder oname = new StringBuilder(getDomain());oname.append(":type=JspMonitor");oname.append(getWebModuleKeyProperties());oname.append(",name=");oname.append(getName());oname.append(getJ2EEKeyProperties());try {jspMonitorON = new ObjectName(oname.toString());Registry.getRegistry(null, null).registerComponent(instance, jspMonitorON, null);} catch( Exception ex ) {log.info("Error registering JSP monitoring with jmx " +instance);}}
}