传统的SpringMVC项目中，需要在web.xml中配置Contextlistener。ContextLoaderListener是负责引导启动和关闭Spring的Root上下文的监听器。主要将处理委托给ContextLoader和ContextCleanupListener。
类的继承关系。
ContextLoaderListener实现了ServletContextListener接口。该接口主要定义了两个行为：监听上下文创建（contextInitialized）和监听上下文销毁（contextDestroyed）。ContextLoaderListener只是将方法的处理委托给ContextLoader。
package org.springframework.web.context;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
public ContextLoaderListener() {
}
public ContextLoaderListener(WebApplicationContext context) {
super(context);
}
//初始化Root WebApplication
@Override
public void contextInitialized(ServletContextEvent event) {
//委托给ContextLoader
initWebApplicationContext(event.getServletContext());
}
//销毁Root WebApplication
@Override
public void contextDestroyed(ServletContextEvent event) {
//委托给ContextLoader
closeWebApplicationContext(event.getServletContext());
//委托给ContextCleanupListener
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
}
ContextLoader的initWebApplicationContext方法负责创建root web application context。
//初始化root web application context
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//校验是否已经存在root application，
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException(
"Cannot initialize context because there is already a root application context present - "
"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
//创建WebApplicationContext，并保存到变量中，等关闭时使用
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
//如果是ConfigurableWebApplicationContext，则配置上下文并刷新
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//在ServletContext中设置属性，表明已经配置了root web application context
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name ["
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " elapsedTime " ms");
}
return this.context;
}
catch (RuntimeException ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
catch (Error err) {
logger.error("Context initialization failed", err);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
throw err;
}
}该方法主要分为四步：
- 校验是否已经存在root WebApplicationContext
- 创建root WebApplicationContext
- 配置上下文并刷新
- 绑定root WebApplicationContext到servlet context上
这四步具体分析如下：
1.校验
校验的过程比较简单，主要是判断Servlet Context是否已经绑定过WebApplicationContext
2.创建对象
createWebApplicationContext()方法负责创建WebApplicationContext对象，主要过程是确定WebApplicationContext类，并实例化：
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//查找WebApplicationContext的类
Class<?> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" contextClass.getName()
"] is not of type [" ConfigurableWebApplicationContext.class.getName() "]");
}
//实例化对象
return
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}决定WebApplicationContext类的主要策略是先判断ServletContext是否配置了contextClass属性，如果是，则用该属性指定的类作为WebApplicationContext类，否则则是否默认策略。默认策略是根绝classpath下的ContextLoader.properties中配置的类作为WebApplicatioNCOntext类：
protected Class<?> determineContextClass(ServletContext servletContext) {
//优先根据servletContext配置的contextClass属性
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" contextClassName "]", ex);
}
}
//在通过classpath下的`ContextLoader.properties`文件制定的类作为WebApplicationContext类
//默认是XmlWebApplicationContext
else {
contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
try {
return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load default context class [" contextClassName "]", ex);
}
}
}配置与刷新
创建WebApplicationContext对象后，ContextLoader会判断是否是ConfigurableWebApplicationContext。ConfigurableWebApplicationContext拓展了WebApplicationContext的能力。其不仅扩展了自身方法，还继承了ConfigurableApplicationContext接口。
配置与刷新的步骤也正是利用了这些接口提供的能力。
//配置并刷新上下文
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
//设置ID
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
//WebApplicationContext绑定Servlet Context
wac.setServletContext(sc);
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}
// The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
//自定义上下文初始过程，留给用户的拓展机制
customizeContext(sc, wac);
//刷新上下文，加载bean
wac.refresh();
}创建流程图如下：