做项目时碰到Controller不能使用aop进行拦截，从网上搜索得知：使用spring mvc 启动了两个context：applicationContext 和WebapplicationContext。
首先我们来了解applicationContext 和WebapplicationContext区别和联系吧
1. ApplicationContext和WebApplicationContext是继承关系
/**
* Interface to provide configuration for a web application. This is read-only while
* the application is running, but may be reloaded if the implementation supports this.
*
* <p>This interface adds a {@code getServletContext()} method to the generic
* ApplicationContext interface, and defines a well-known application attribute name
* that the root context must be bound to in the bootstrap process.
*
* <p>Like generic application contexts, web application contexts are hierarchical.
* There is a single root context per application, while each servlet in the application
* (including a dispatcher servlet in the MVC framework) has its own child context.
*
* <p>In addition to standard application context lifecycle capabilities,
* WebApplicationContext implementations need to detect {@link ServletContextAware}
* beans and invoke the {@code setServletContext} method accordingly.*/
public interface WebApplicationContext extends ApplicationContext {}
2. ContextLoaderListener 创建基于web的应用根applicationContext， 并将applicationContext放入到ServletContext， applicationContext加载或者卸载spring管理的beans。在structs和spring mvc的控制层都是这样使用的。
3. DispatcherServlet创建自己的WebApplicationContext并管理这个WebApplicationContext里面的 handlers/controllers/view-resolvers.
4. 当ContextLoaderListener和DispatcherServlet一起使用时, ContextLoaderListener 先创建一个根applicationContext，然后DispatcherSerlvet创建一个子applicationContext并且绑定到根applicationContext。
首先来看看web.xml的定义：
一般的web应用，通过ContextLoaderListener监听，ContextLoaderListener中加载的context成功后，spring 将 applicationContext存放在ServletContext的key值为"org.springframework.web.context.WebApplicationContext.ROOT"的attribute中。
<listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath*:conf/applicationContext*.xml</param-value> </context-param>
DispatcherServlet加载的context成功后，会将applicationContext存放在org.springframework.web.servlet.FrameworkServlet.CONTEXT. + (servletName)的attribute中。
<servlet> <servlet-name>mvcServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:conf/spring-dispatcher-servlet.xml</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet>
当然，如果没有指定*servlet.xml 配置，则默认使用DispatcherServlet的默认配置DispatcherServlet.properties
# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.
org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter
org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
简单的来说：spring bean的管理在applicationContext中，ContextLoaderListener的作用：
1. 将applicationContext的生命周期和servletContext的生命周期联系到一起。
2. 自动管理applicationContext的创建，ContextLoaderListener 给我们提供了一个便利，不用显式的去创建applicationContext。
DispatcherServlet 相关bean的管理在WebApplicationContext，ServletContextListener创建WebApplicationContext，WebApplicationContext可以访问ServletContext/ServletContextAware这些bean，还可以访问getServletContext方法。
正式的官方文档：
在一个web应用中可以使用多个DispatcherServlet，每个servlet通过自己的命名空间来获取自己的webapplicationContext，然后加载此applicationContext里面的hangdlerMapping，hangdlerAdapter等等。ContextLoaderListener加载根application，所有子applicationContext共享根applicationContext。
从版本3.1 后，spring 支持将DispatcherServlet注入到根applicationContext，而不用创建自己的webapplicationContext，这主要为支持servlet 3.0 以上版本环境的要求，因为servlet 3.0 以上版本支持使用编程的方式来注册servlet实例。
spring支持使用多层层次applicationContext，通常我们使用两层结构就够了。
接下来，通过深入源代码层来探究WebApplicationContext是如何创建的？
1. DispatcherServlet的初始化过程使用到applicationContext。
我们知道DispatcherServlet直接继承自FrameworkServlet，而FrameworkServlet又继承了HttpServletBean和 ApplicationContextAware。
2. FrameworkServlet实现了ApplicationContextAware接口的setApplicationContext()方法，可知DispatcherServlet的applicationContext来自FrameworkServlet。
/**
* Called by Spring via {@link ApplicationContextAware} to inject the current
* application context. This method allows FrameworkServlets to be registered as
* Spring beans inside an existing {@link WebApplicationContext} rather than
* {@link #findWebApplicationContext() finding} a
* {@link org.springframework.web.context.ContextLoaderListener bootstrapped} context.
* <p>Primarily added to support use in embedded servlet containers.
* @since 4.0
*/
@Override
public void setApplicationContext(ApplicationContext applicationContext) {
if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
this.webApplicationContext = (WebApplicationContext) applicationContext;
this.webApplicationContextInjected = true;
}
}
3. FrameworkServlet的setApplicationContext()方法中WebApplicationContext是如何实例化的呢？
FrameworkServlet继承自HttpServletBean，HttpServletBean的初始化方法：
/**
* Map config parameters onto bean properties of this servlet, and
* invoke subclass initialization.
* @throws ServletException if bean properties are invalid (or required
* properties are missing), or if subclass initialization fails.
*/
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// Set bean properties from init parameters.try {
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}
// Let subclasses do whatever initialization they like.
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
FrameworkServlet 实现了initServletBean();
/**
* Overridden method of {@link HttpServletBean}, invoked after any bean properties
* have been set. Creates this servlet's WebApplicationContext.
*/
@Override
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}
最终追踪到FrameworkServlet 的initWebApplicationContext()方法
/**
* Initialize and publish the WebApplicationContext for this servlet.
* <p>Delegates to {@link #createWebApplicationContext} for actual creation
* of the context. Can be overridden in subclasses.
* @return the WebApplicationContext instance
* @see #FrameworkServlet(WebApplicationContext)
* @see #setContextClass
* @see #setContextConfigLocation
*/protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etcif (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here. onRefresh(wac);
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}
return wac;
}
我们来分析整个流程吧：
1. 获取根applicationContext。
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
/**
* Find the root {@link WebApplicationContext} for this web app, typically
* loaded via {@link org.springframework.web.context.ContextLoaderListener}.
* <p>Will rethrow an exception that happened on root context startup,
* to differentiate between a failed context startup and no context at all.
* @param sc ServletContext to find the web application context for
* @return the root WebApplicationContext for this web app, or {@code null} if none
* @see org.springframework.web.context.WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
*/public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
/**
* Find a custom {@link WebApplicationContext} for this web app.
* @param sc ServletContext to find the web application context for
* @param attrName the name of the ServletContext attribute to look for
* @return the desired WebApplicationContext for this web app, or {@code null} if none
*/public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
Assert.notNull(sc, "ServletContext must not be null");
Object attr = sc.getAttribute(attrName);
if (attr == null) {
return null;
}
if (attr instanceof RuntimeException) {
throw (RuntimeException) attr;
}
if (attr instanceof Error) {
throw (Error) attr;
}
if (attr instanceof Exception) {
throw new IllegalStateException((Exception) attr);
}
if (!(attr instanceof WebApplicationContext)) {
throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
}
return (WebApplicationContext) attr;
}
2. 判断webapplicationContext是否存在？存在的话就重利用该webapplicationContext
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
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 informationif (this.contextId != null) {
wac.setId(this.contextId);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(getServletContext().getContextPath()) + "/" + getServletName());
}
}
wac.setServletContext(getServletContext());
wac.setServletConfig(getServletConfig());
wac.setNamespace(getNamespace());
wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
// 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(getServletContext(), getServletConfig());
}
postProcessWebApplicationContext(wac);
applyInitializers(wac);
wac.refresh();
}
不存在的话，根据配置的属性名去查询：
/**
* Retrieve a {@code WebApplicationContext} from the {@code ServletContext}
* attribute with the {@link #setContextAttribute configured name}. The
* {@code WebApplicationContext} must have already been loaded and stored in the
* {@code ServletContext} before this servlet gets initialized (or invoked).
* <p>Subclasses may override this method to provide a different
* {@code WebApplicationContext} retrieval strategy.
* @return the WebApplicationContext for this servlet, or {@code null} if not found
* @see #getContextAttribute()
*/protected WebApplicationContext findWebApplicationContext() {
String attrName = getContextAttribute();
if (attrName == null) {
return null;
}
WebApplicationContext wac =
WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
if (wac == null) {
throw new IllegalStateException("No WebApplicationContext found: initializer not registered?");
}
return wac;
}
3. 如果还查询不到WebapplicationContext，那么就创建一个新的WebapplicationContext，并绑定到root WebapplicationContext上：
/**
* Instantiate the WebApplicationContext for this servlet, either a default
* {@link org.springframework.web.context.support.XmlWebApplicationContext}
* or a {@link #setContextClass custom context class}, if set.
* <p>This implementation expects custom contexts to implement the
* {@link org.springframework.web.context.ConfigurableWebApplicationContext}
* interface. Can be overridden in subclasses.
* <p>Do not forget to register this servlet instance as application listener on the
* created context (for triggering its {@link #onRefresh callback}, and to call
* {@link org.springframework.context.ConfigurableApplicationContext#refresh()}
* before returning the context instance.
* @param parent the parent ApplicationContext to use, or {@code null} if none
* @return the WebApplicationContext for this servlet
* @see org.springframework.web.context.support.XmlWebApplicationContext
*/protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
Class<?> contextClass = getContextClass();
if (this.logger.isDebugEnabled()) {
this.logger.debug("Servlet with name '" + getServletName() +
"' will try to create custom WebApplicationContext context of class '" +
contextClass.getName() + "'" + ", using parent context [" + parent + "]");
}
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
configureAndRefreshWebApplicationContext(wac);
return wac;
}
4. 将子applicationContext发布到servlet context上。
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}
/**
* Return the ServletContext attribute name for this servlet's WebApplicationContext.
* <p>The default implementation returns
* {@code SERVLET_CONTEXT_PREFIX + servlet name}.
* @see #SERVLET_CONTEXT_PREFIX
* @see #getServletName
*/
public String getServletContextAttributeName() {
return SERVLET_CONTEXT_PREFIX + getServletName();
}
/**
* Prefix for the ServletContext attribute for the WebApplicationContext.
* The completion is the servlet name.
*/
public static final String SERVLET_CONTEXT_PREFIX = FrameworkServlet.class.getName() + ".CONTEXT.";
最后，ContextLoaderListener启动时如何产生applicationContext呢？
见参考我的这篇文章：http://www.cnblogs.com/davidwang456/archive/2013/03/12/2956125.html
小结：
我们就以FrameworkServlet的官方说明来结束吧。没有比它更合适的！希望你喜欢。
公共抽象类FrameworkServlet继承HttpServletBean；Spring的web框架实现ApplicationContextAwareBase servlet。在一个以java为基础的整体解决方案中提供与Spring应用程序上下文的集成。
这个类提供以下功能:
管理每个servlet的WebApplicationContext实例。servlet的配置由servlet的名称空间中的bean决定。
在请求处理上发布事件，不管请求是否成功处理。子类必须实现doService(javax.servlet.http。HttpServletRequest,javax.servlet。
参考资料：
http://starscream.iteye.com/blog/1107036
http://www.softwarevol.com/en/tutorial/Spring-ContextLoaderListener-And-DispatcherServlet-Concepts
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