Spring MVC Web应用上下文

     Spring MVC是建立在Spring IoC容器的基础上，然而Spring IoC是一个独立的模块，因此并不能直接在Web环境中发挥作用。为了在Web环境中使用Ioc，就需要Spring提供一个适合于Web环境的相关容器（WebApplicationContext），以及基于此容器的IoC启动过程，将IoC导入，这个启动过程应该与Web容器启动过程相集成。下面着重介绍Web应用上下文。

WebApplicationContext 类继承体系   ：

      可以看到，Web应用上下文的实现是基于基本Ioc容器ApplicatoionContext相关接口，并在此基础上加入了适用于Web环境的相关功能扩展。

1）WebApplicationContext接口实现

public interface WebApplicationContext extends ApplicationContext {String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";String SCOPE_REQUEST = "request";String SCOPE_SESSION = "session";      ...      ...ServletContext getServletContext();}

在该接口中，定义了几个常量，比如ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE，这个常量用于索引在ServletContext中保存的根Web应用上下文。还有就是ServletContext getServletContext()方法，通过这个方法可以得到当前web容器的Servlet上下文。

2）ConfigurableWebApplicationContext实现

 public interface ConfigurableWebApplicationContext extends WebApplicationContext, ConfigurableApplicationContext {String APPLICATION_CONTEXT_ID_PREFIX = WebApplicationContext.class.getName() + ":";String SERVLET_CONFIG_BEAN_NAME = "servletConfig";//设置与该WebApplicationContext相关的ServletContext void setServletContext(ServletContext servletContext);//设置与该WebApplicationContext相关的Servlet的servletConfigvoid setServletConfig(ServletConfig servletConfig);ServletConfig getServletConfig();//设置该WebApplicationContext依赖的bean定义资源配置路径void setConfigLocation(String configLocation);void setConfigLocations(String... configLocations);String[] getConfigLocations();}

  ConfigurableWebApplicationContext继承自WebApplicationContext接口，并且提供了设置ServletContext 以及上下文配置资源路径的相关函数。

3）AbstractRefreshableWebApplicationContext 类

public abstract class AbstractRefreshableWebApplicationContext extends AbstractRefreshableConfigApplicationContextimplements ConfigurableWebApplicationContext, ThemeSource {/** Servlet context that this context runs in */private ServletContext servletContext;/** Servlet config that this context runs in, if any */private ServletConfig servletConfig;/** Namespace of this context, or {@code null} if root */private String namespace;/** the ThemeSource for this ApplicationContext */private ThemeSource themeSource;   public AbstractRefreshableWebApplicationContext() {      setDisplayName("Root WebApplicationContext");}@Overridepublic void setServletContext(ServletContext servletContext) {this.servletContext = servletContext;}@Overridepublic ServletContext getServletContext() {return this.servletContext;}@Overridepublic void setServletConfig(ServletConfig servletConfig) {this.servletConfig = servletConfig;      if (servletConfig != null && this.servletContext == null) {         setServletContext(servletConfig.getServletContext());}   }@Overridepublic ServletConfig getServletConfig() {return this.servletConfig;}@Overridepublic void setNamespace(String namespace) {this.namespace = namespace;      if (namespace != null) {         setDisplayName("WebApplicationContext for namespace '" + namespace + "'");}   }@Overridepublic String getNamespace() {return this.namespace;}@Overridepublic String[] getConfigLocations() {return super.getConfigLocations();}@Overridepublic String getApplicationName() {return (this.servletContext != null ? this.servletContext.getContextPath() : "");}/**    * Create and return a new {@link StandardServletEnvironment}. Subclasses may override    * in order to configure the environment or specialize the environment type returned.    */@Overrideprotected ConfigurableEnvironment createEnvironment() {return new StandardServletEnvironment();}/**    * Register request/session scopes, a {@link ServletContextAwareProcessor}, etc.    */@Overrideprotected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {      beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));beanFactory.ignoreDependencyInterface(ServletContextAware.class);beanFactory.ignoreDependencyInterface(ServletConfigAware.class);WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);}/**    * This implementation supports file paths beneath the root of the ServletContext.    * @see ServletContextResource    */@Overrideprotected Resource getResourceByPath(String path) {return new ServletContextResource(this.servletContext, path);}/**    * This implementation supports pattern matching in unexpanded WARs too.    * @see ServletContextResourcePatternResolver    */@Overrideprotected ResourcePatternResolver getResourcePatternResolver() {return new ServletContextResourcePatternResolver(this);}/**    * Initialize the theme capability.    */@Overrideprotected void onRefresh() {this.themeSource = UiApplicationContextUtils.initThemeSource(this);}/**    * {@inheritDoc}    * <p>Replace {@code Servlet}-related property sources.    */@Overrideprotected void initPropertySources() {      ConfigurableEnvironment env = getEnvironment();      if (env instanceof ConfigurableWebEnvironment) {         ((ConfigurableWebEnvironment) env).initPropertySources(this.servletContext, this.servletConfig);}   }@Overridepublic Theme getTheme(String themeName) {return this.themeSource.getTheme(themeName);}}

4）Spring MVC使用的WebApplicationContext实现

   Spring MVC使用一个默认的WebApplicationContext来作为Ioc容器，这个Ioc容器就是XmlWebApplicationContext。通过类继承体系可以看到其实现继承于AbstractRefreshableWebApplicationContext抽象类，而AbstractRefreshableWebApplicationContext又继承自AbstractRefreshableConfigApplicationContext。

public class XmlWebApplicationContext extends AbstractRefreshableWebApplicationContext {  //设置的默认的BeanDefinition的地方，在/WEB-INF/applicationContext.xml文件中public static final String DEFAULT_CONFIG_LOCATION = "/WEB-INF/applicationContext.xml";  //默认的bean配置文件目录前缀和文件后缀public static final String DEFAULT_CONFIG_LOCATION_PREFIX = "/WEB-INF/";public static final String DEFAULT_CONFIG_LOCATION_SUFFIX = ".xml";  //loadBeanDefinitions函数，在Ioc容器启动时，通过该函数加载、注册BeanDefinition@Overrideprotected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {     //用XmlBeanDefinitionReader 类来对BeanDefinition信息进行解析XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);beanDefinitionReader.setEnvironment(getEnvironment());    //设置ResourceLoader资源导入器，因ApplicationContext是DefaultResource子类，因此这里传入this，使用该DefaultResourceLoader来定位BeanDefinition资源文件beanDefinitionReader.setResourceLoader(this);beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));initBeanDefinitionReader(beanDefinitionReader);    //通过定义好的XmlBeanDefinitionReader 来载入BeanDefinition。loadBeanDefinitions(beanDefinitionReader);}protected void initBeanDefinitionReader(XmlBeanDefinitionReader beanDefinitionReader) {   }//对多个BeanDefinition定义文件，通过XmlBeanDefinitionReader 逐个载入protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {      String[] configLocations = getConfigLocations();      if (configLocations != null) {for (String configLocation : configLocations) {            reader.loadBeanDefinitions(configLocation);}      }   } //获取默认资源位置，默认的位置是/WEB-INF/applicationContext.xml@Overrideprotected String[] getDefaultConfigLocations() {if (getNamespace() != null) {return new String[] {DEFAULT_CONFIG_LOCATION_PREFIX + getNamespace() + DEFAULT_CONFIG_LOCATION_SUFFIX};}else {return new String[] {DEFAULT_CONFIG_LOCATION};}   }}

  XmlWebApplicationContext中基本的上下文功能都通过类继承来获得，需要处理的就是获取Bean定义信息。这里是获取web容器环境指定的资源如/WEB-INF/applicationContext.xml。在获取到bean定义后，就如同Spring基本Ioc上下文容器ApplicationContext一样通过XmlBeanDefinitionReader来载入bean定义信息，最终完成上下文初始化。