Spring框架提供了构建Web应用程序的全功能MVC模块。通过策略接口，Spring框架是高度可配置的，而且支持多种视图技术。Spring MVC分离了控制器，模型对象，分派器以及处理程序对象的角色，这种分离让它们更容易进行定制。
Spring的MVC是基于Servlet功能实现的，通过实现Servlet接口的DispatcherServlet来封装其核心功能实现，通过将请求分派给处理程序，同时带有可配置的处理程序映射，视图解析，本地语言，主题解析以及上载文件支持。默认的处理程序是非常简单的Controller接口，只有一个方法ModelAndView handleRequest（request, response）。Spring提供了一个控制器层次结构，可以派生子类。如果应用程序需要处理用户输入表单，那么可以继承AbstractFormController。如果需要把多页输入处理到一个表单，那么可以继承AbstractWizardFormController。
一：SpringMVC 快速体验
（1）配置web.xml
一个Web中可以没有web.xml文件，也就是说，web.xml文件并不是Web工程必须的。web.xml文件用来初始化配置信息：比如Welcome页面，servlet,servlet-mapping,filter,listener,启动加载级别等。但是，SpringMVC的实现原理是通过Servlet拦截所有URL来达到控制的目的，所以web.xml的配置是必须的。
Spring的MVC之所以必须要配置web.xml，其实最关键的是要配置两个地方。
- contextConfigLocation：Spring的核心就是配置文件，可以说配置文件是Spring中必不可少的东西（java配置流行前），而这个参数就是使Web与Spring的配置文件相结合的一个关键配置。
- DispatcherServlet：包含了SpringMVC的请求逻辑，Spring使用此类拦截Web请求并进行相应的逻辑处理。
（2）创建Spring配置文件applicationContext.xml
（3）创建model
（4）创建controller
在请求的最后返回了ModelAndView类型的实例。ModelAndView类在SpringMVC中占用很重要的地位，控制器执行方法都必须返回一个ModelAndView，ModelAndView对象保存了视图以及视图显示的模型数据，例如其中的参数如下。
- 第一个参数userList：视图组件的逻辑名称。这里视图的逻辑名称就是userlist，视图解析器会使用该名称查找实际的View对象。
- 第二个参数users：传递给视图的，模型对象的名称。
- 第三个参数userList：传递给视图的，模型对象的值。
（5）创建视图文件userlist.jsp
（6）创建Servlet配置文件Spring-servlet.xml
因为SpringMVC是基于Servlet的实现，所以在Web启动的时候，服务器会首先尝试加载对应于Servlet的配置文件，而为了让项目更加模块化，通常我们将Web部分的配置都存于此配置文件中。
此时，启动服务器，输入网址：http://localhost:8080/Springmvc/userlist.htm.看到服务器返回界面：
二：ContextLoaderListener
在Web下，我们需要更多的是与Web环境相互结合，通常的办法是将路径以context--param的方式注册并使用ContextLoaderListener进行监听读取。
ContextLoaderListener的作用就是启动Web容器时，自动装配ApplicationContext的配置信息。因为它实现了ServletContextListener这个接口，在web.xml配置这个监听器，启动容器时，就会默认执行它实现的方法，使用ServletContextListener接口，开发者能够在为客户端请求提供服务之前向ServletContext中添加任意的对象。这个对象在ServletContext启动的时候被初始化，然后再ServletContextt整个运行期间都是可见的。
每一个Web应用都要一个ServletContext与之相关联。ServletContext对象在应用启动时被创建，在应用关闭的时候被销毁。ServletContext在全局范围内有效，类似于应用中的一个全局变量。
在ServletContextListener中的核心逻辑便是初始化WebApplicationContext实例并存放至ServletContext中。
（2.1）Spring中的ContextLoaderListener
ServletContext启动之后会调用ServletContextListener的contextInitialized方法，那么，我们就从这个函数开始进行分析。
/** * Interface for receiving notification events about ServletContext * lifecycle changes. * * <p>In order to receive these notification events, the implementation * class must be either declared in the deployment descriptor of the web * application, annotated with {@link javax.servlet.annotation.WebListener}, * or registered via one of the addListener methods defined on * {@link ServletContext}. * * <p>Implementations of this interface are invoked at their * {@link #contextInitialized} method in the order in which they have been * declared, and at their {@link #contextDestroyed} method in reverse * order. * * @see ServletContextEvent * * @since Servlet 2.3 */ public interface ServletContextListener extends EventListener { /** * Receives notification that the web application initialization * process is starting. * * <p>All ServletContextListeners are notified of context * initialization before any filters or servlets in the web * application are initialized. * * @param sce the ServletContextEvent containing the ServletContext * that is being initialized */ public void contextInitialized(ServletContextEvent sce); /** * Receives notification that the ServletContext is about to be * shut down. * * <p>All servlets and filters will have been destroyed before any * ServletContextListeners are notified of context * destruction. * * @param sce the ServletContextEvent containing the ServletContext * that is being destroyed */ public void contextDestroyed(ServletContextEvent sce); }
public class ContextLoaderListener extends ContextLoader implements ServletContextListener { /** * Create a new {@code ContextLoaderListener} that will create a web application * context based on the "contextClass" and "contextConfigLocation" servlet * context-params. See {@link ContextLoader} superclass documentation for details on * default values for each. * <p>This constructor is typically used when declaring {@code ContextLoaderListener} * as a {@code <listener>} within {@code web.xml}, where a no-arg constructor is * required. * <p>The created application context will be registered into the ServletContext under * the attribute name {@link WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} * and the Spring application context will be closed when the {@link #contextDestroyed} * lifecycle method is invoked on this listener. * @see ContextLoader * @see #ContextLoaderListener(WebApplicationContext) * @see #contextInitialized(ServletContextEvent) * @see #contextDestroyed(ServletContextEvent) */ public ContextLoaderListener() { } /** * Create a new {@code ContextLoaderListener} with the given application context. This * constructor is useful in Servlet 3.0+ environments where instance-based * registration of listeners is possible through the {@link javax.servlet.ServletContext#addListener} * API. * <p>The context may or may not yet be {@linkplain * org.springframework.context.ConfigurableApplicationContext#refresh() refreshed}. If it * (a) is an implementation of {@link ConfigurableWebApplicationContext} and * (b) has <strong>not</strong> already been refreshed (the recommended approach), * then the following will occur: * <ul> * <li>If the given context has not already been assigned an {@linkplain * org.springframework.context.ConfigurableApplicationContext#setId id}, one will be assigned to it</li> * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to * the application context</li> * <li>{@link #customizeContext} will be called</li> * <li>Any {@link org.springframework.context.ApplicationContextInitializer ApplicationContextInitializer}s * specified through the "contextInitializerClasses" init-param will be applied.</li> * <li>{@link org.springframework.context.ConfigurableApplicationContext#refresh refresh()} will be called</li> * </ul> * If the context has already been refreshed or does not implement * {@code ConfigurableWebApplicationContext}, none of the above will occur under the * assumption that the user has performed these actions (or not) per his or her * specific needs. * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples. * <p>In any case, the given application context will be registered into the * ServletContext under the attribute name {@link * WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and the Spring * application context will be closed when the {@link #contextDestroyed} lifecycle * method is invoked on this listener. * @param context the application context to manage * @see #contextInitialized(ServletContextEvent) * @see #contextDestroyed(ServletContextEvent) */ public ContextLoaderListener(WebApplicationContext context) { super(context); } /** * Initialize the root web application context. */ @Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); } /** * Close the root web application context. */ @Override public void contextDestroyed(ServletContextEvent event) { closeWebApplicationContext(event.getServletContext()); ContextCleanupListener.cleanupAttributes(event.getServletContext()); } }
这里涉及一个常用类WebApplicationContext：在Web应用中，我们会用到WebApplicationContext，WebApplicationContext继承自ApplicationContext，在ApplicationContext的基础上又追加了一些特定于Web的操作及属性，非常类似于我们通过编程方式使用Spring时使用的ClassPathXmlApplicationContext类提供的功能。
public class ContextLoader { /** * Config param for the root WebApplicationContext id, * to be used as serialization id for the underlying BeanFactory: {@value} */ public static final String CONTEXT_ID_PARAM = "contextId"; /** * Name of servlet context parameter (i.e., {@value}) that can specify the * config location for the root context, falling back to the implementation's * default otherwise. * @see org.springframework.web.context.support.XmlWebApplicationContext#DEFAULT_CONFIG_LOCATION */ public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation"; /** * Config param for the root WebApplicationContext implementation class to use: {@value} * @see #determineContextClass(ServletContext) */ public static final String CONTEXT_CLASS_PARAM = "contextClass"; /** * Config param for {@link ApplicationContextInitializer} classes to use * for initializing the root web application context: {@value} * @see #customizeContext(ServletContext, ConfigurableWebApplicationContext) */ public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses"; /** * Config param for global {@link ApplicationContextInitializer} classes to use * for initializing all web application contexts in the current application: {@value} * @see #customizeContext(ServletContext, ConfigurableWebApplicationContext) */ public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses"; /** * Optional servlet context parameter (i.e., "{@code locatorFactorySelector}") * used only when obtaining a parent context using the default implementation * of {@link #loadParentContext(ServletContext servletContext)}. * Specifies the 'selector' used in the * {@link ContextSingletonBeanFactoryLocator#getInstance(String selector)} * method call, which is used to obtain the BeanFactoryLocator instance from * which the parent context is obtained. * <p>The default is {@code classpath*:beanRefContext.xml}, * matching the default applied for the * {@link ContextSingletonBeanFactoryLocator#getInstance()} method. * Supplying the "parentContextKey" parameter is sufficient in this case. */ public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector"; /** * Optional servlet context parameter (i.e., "{@code parentContextKey}") * used only when obtaining a parent context using the default implementation * of {@link #loadParentContext(ServletContext servletContext)}. * Specifies the 'factoryKey' used in the * {@link BeanFactoryLocator#useBeanFactory(String factoryKey)} method call, * obtaining the parent application context from the BeanFactoryLocator instance. * <p>Supplying this "parentContextKey" parameter is sufficient when relying * on the default {@code classpath*:beanRefContext.xml} selector for * candidate factory references. */ public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey"; /** * Any number of these characters are considered delimiters between * multiple values in a single init-param String value. */ private static final String INIT_PARAM_DELIMITERS = ",; "; /** * Name of the class path resource (relative to the ContextLoader class) * that defines ContextLoader's default strategy names. */ private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties"; private static final Properties defaultStrategies; static { // Load default strategy implementations from properties file. // This is currently strictly internal and not meant to be customized // by application developers. try { ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class); defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); } catch (IOException ex) { throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage()); } } /** * Map from (thread context) ClassLoader to corresponding 'current' WebApplicationContext. */ private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread = new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1); /** * The 'current' WebApplicationContext, if the ContextLoader class is * deployed in the web app ClassLoader itself. */ private static volatile WebApplicationContext currentContext; /** * The root WebApplicationContext instance that this loader manages. */ private WebApplicationContext context; /** * Holds BeanFactoryReference when loading parent factory via * ContextSingletonBeanFactoryLocator. */ private BeanFactoryReference parentContextRef; /** Actual ApplicationContextInitializer instances to apply to the context */ private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers = new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>(); /** * Create a new {@code ContextLoader} that will create a web application context * based on the "contextClass" and "contextConfigLocation" servlet context-params. * See class-level documentation for details on default values for each. * <p>This constructor is typically used when declaring the {@code * ContextLoaderListener} subclass as a {@code <listener>} within {@code web.xml}, as * a no-arg constructor is required. * <p>The created application context will be registered into the ServletContext under * the attribute name {@link WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} * and subclasses are free to call the {@link #closeWebApplicationContext} method on * container shutdown to close the application context. * @see #ContextLoader(WebApplicationContext) * @see #initWebApplicationContext(ServletContext) * @see #closeWebApplicationContext(ServletContext) */ public ContextLoader() { } /** * Create a new {@code ContextLoader} with the given application context. This * constructor is useful in Servlet 3.0+ environments where instance-based * registration of listeners is possible through the {@link ServletContext#addListener} * API. * <p>The context may or may not yet be {@linkplain * ConfigurableApplicationContext#refresh() refreshed}. If it (a) is an implementation * of {@link ConfigurableWebApplicationContext} and (b) has <strong>not</strong> * already been refreshed (the recommended approach), then the following will occur: * <ul> * <li>If the given context has not already been assigned an {@linkplain * ConfigurableApplicationContext#setId id}, one will be assigned to it</li> * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to * the application context</li> * <li>{@link #customizeContext} will be called</li> * <li>Any {@link ApplicationContextInitializer}s specified through the * "contextInitializerClasses" init-param will be applied.</li> * <li>{@link ConfigurableApplicationContext#refresh refresh()} will be called</li> * </ul> * If the context has already been refreshed or does not implement * {@code ConfigurableWebApplicationContext}, none of the above will occur under the * assumption that the user has performed these actions (or not) per his or her * specific needs. * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples. * <p>In any case, the given application context will be registered into the * ServletContext under the attribute name {@link * WebApplicationContext#ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE} and subclasses are * free to call the {@link #closeWebApplicationContext} method on container shutdown * to close the application context. * @param context the application context to manage * @see #initWebApplicationContext(ServletContext) * @see #closeWebApplicationContext(ServletContext) */ public ContextLoader(WebApplicationContext context) { this.context = context; } /** * Specify which {@link ApplicationContextInitializer} instances should be used * to initialize the application context used by this {@code ContextLoader}. * @since 4.2 * @see #configureAndRefreshWebApplicationContext * @see #customizeContext */ @SuppressWarnings("unchecked") public void setContextInitializers(ApplicationContextInitializer<?>... initializers) { if (initializers != null) { for (ApplicationContextInitializer<?> initializer : initializers) { this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer); } } } /** * Initialize Spring's web application context for the given servlet context, * using the application context provided at construction time, or creating a new one * according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and * "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params. * @param servletContext current servlet context * @return the new WebApplicationContext * @see #ContextLoader(WebApplicationContext) * @see #CONTEXT_CLASS_PARAM * @see #CONFIG_LOCATION_PARAM */ public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { 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. if (this.context == null) { this.context = createWebApplicationContext(servletContext); } 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); } }
//ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT"; 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; } } /** * Instantiate the root WebApplicationContext for this loader, either the * default context class or a custom context class if specified. * <p>This implementation expects custom contexts to implement the * {@link ConfigurableWebApplicationContext} interface. * Can be overridden in subclasses. * <p>In addition, {@link #customizeContext} gets called prior to refreshing the * context, allowing subclasses to perform custom modifications to the context. * @param sc current servlet context * @return the root WebApplicationContext * @see ConfigurableWebApplicationContext */ protected WebApplicationContext createWebApplicationContext(ServletContext sc) { Class<?> contextClass = determineContextClass(sc); if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) { throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]"); } return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); } /** * Return the WebApplicationContext implementation class to use, either the * default XmlWebApplicationContext or a custom context class if specified. * @param servletContext current servlet context * @return the WebApplicationContext implementation class to use * @see #CONTEXT_CLASS_PARAM * @see org.springframework.web.context.support.XmlWebApplicationContext */ protected Class<?> determineContextClass(ServletContext servletContext) {
//CONTEXT_CLASS_PARAM = "contextClass" 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); } } 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); } } } protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) { 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())); } } 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); } customizeContext(sc, wac); wac.refresh(); } /** * Customize the {@link ConfigurableWebApplicationContext} created by this * ContextLoader after config locations have been supplied to the context * but before the context is <em>refreshed</em>. * <p>The default implementation {@linkplain #determineContextInitializerClasses(ServletContext) * determines} what (if any) context initializer classes have been specified through * {@linkplain #CONTEXT_INITIALIZER_CLASSES_PARAM context init parameters} and * {@linkplain ApplicationContextInitializer#initialize invokes each} with the * given web application context. * <p>Any {@code ApplicationContextInitializers} implementing * {@link org.springframework.core.Ordered Ordered} or marked with @{@link * org.springframework.core.annotation.Order Order} will be sorted appropriately. * @param sc the current servlet context * @param wac the newly created application context * @see #CONTEXT_INITIALIZER_CLASSES_PARAM * @see ApplicationContextInitializer#initialize(ConfigurableApplicationContext) */ protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) { List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses = determineContextInitializerClasses(sc); for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) { Class<?> initializerContextClass = GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class); if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) { throw new ApplicationContextException(String.format( "Could not apply context initializer [%s] since its generic parameter [%s] " + "is not assignable from the type of application context used by this " + "context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(), wac.getClass().getName())); } this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass)); } AnnotationAwareOrderComparator.sort(this.contextInitializers); for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) { initializer.initialize(wac); } } /** * Return the {@link ApplicationContextInitializer} implementation classes to use * if any have been specified by {@link #CONTEXT_INITIALIZER_CLASSES_PARAM}. * @param servletContext current servlet context * @see #CONTEXT_INITIALIZER_CLASSES_PARAM */ protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> determineContextInitializerClasses(ServletContext servletContext) { List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes = new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>(); String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM); if (globalClassNames != null) { for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) { classes.add(loadInitializerClass(className)); } } String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM); if (localClassNames != null) { for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) { classes.add(loadInitializerClass(className)); } } return classes; } @SuppressWarnings("unchecked") private Class<ApplicationContextInitializer<ConfigurableApplicationContext>> loadInitializerClass(String className) { try { Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader()); if (!ApplicationContextInitializer.class.isAssignableFrom(clazz)) { throw new ApplicationContextException( "Initializer class does not implement ApplicationContextInitializer interface: " + clazz); } return (Class<ApplicationContextInitializer<ConfigurableApplicationContext>>) clazz; } catch (ClassNotFoundException ex) { throw new ApplicationContextException("Failed to load context initializer class [" + className + "]", ex); } } /** * Template method with default implementation (which may be overridden by a * subclass), to load or obtain an ApplicationContext instance which will be * used as the parent context of the root WebApplicationContext. If the * return value from the method is null, no parent context is set. * <p>The main reason to load a parent context here is to allow multiple root * web application contexts to all be children of a shared EAR context, or * alternately to also share the same parent context that is visible to * EJBs. For pure web applications, there is usually no need to worry about * having a parent context to the root web application context. * <p>The default implementation uses * {@link org.springframework.context.access.ContextSingletonBeanFactoryLocator}, * configured via {@link #LOCATOR_FACTORY_SELECTOR_PARAM} and * {@link #LOCATOR_FACTORY_KEY_PARAM}, to load a parent context * which will be shared by all other users of ContextsingletonBeanFactoryLocator * which also use the same configuration parameters. * @param servletContext current servlet context * @return the parent application context, or {@code null} if none * @see org.springframework.context.access.ContextSingletonBeanFactoryLocator */ protected ApplicationContext loadParentContext(ServletContext servletContext) { ApplicationContext parentContext = null; String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM); String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM); if (parentContextKey != null) { // locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml" BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector); Log logger = LogFactory.getLog(ContextLoader.class); if (logger.isDebugEnabled()) { logger.debug("Getting parent context definition: using parent context key of '" + parentContextKey + "' with BeanFactoryLocator"); } this.parentContextRef = locator.useBeanFactory(parentContextKey); parentContext = (ApplicationContext) this.parentContextRef.getFactory(); } return parentContext; } /** * Close Spring's web application context for the given servlet context. If * the default {@link #loadParentContext(ServletContext)} implementation, * which uses ContextSingletonBeanFactoryLocator, has loaded any shared * parent context, release one reference to that shared parent context. * <p>If overriding {@link #loadParentContext(ServletContext)}, you may have * to override this method as well. * @param servletContext the ServletContext that the WebApplicationContext runs in */ public void closeWebApplicationContext(ServletContext servletContext) { servletContext.log("Closing Spring root WebApplicationContext"); try { if (this.context instanceof ConfigurableWebApplicationContext) { ((ConfigurableWebApplicationContext) this.context).close(); } } finally { ClassLoader ccl = Thread.currentThread().getContextClassLoader(); if (ccl == ContextLoader.class.getClassLoader()) { currentContext = null; } else if (ccl != null) { currentContextPerThread.remove(ccl); } servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE); if (this.parentContextRef != null) { this.parentContextRef.release(); } } } /** * Obtain the Spring root web application context for the current thread * (i.e. for the current thread's context ClassLoader, which needs to be * the web application's ClassLoader). * @return the current root web application context, or {@code null} * if none found * @see org.springframework.web.context.support.SpringBeanAutowiringSupport */ public static WebApplicationContext getCurrentWebApplicationContext() { ClassLoader ccl = Thread.currentThread().getContextClassLoader(); if (ccl != null) { WebApplicationContext ccpt = currentContextPerThread.get(ccl); if (ccpt != null) { return ccpt; } } return currentContext; } }
initWebApplicationContext函数主要是体现了创建WebApplicationContext实例的一个功能架构，从函数中我们看到了初始化的大致步骤。
（1）WebApplicationContext存在性的验证。
在配置中只允许声明一次ServletContextListener，多次声明会扰乱Spring的执行逻辑，所以这里首先做的就是对此验证，在Spring中如果创建WebApplicationContext实例会记录在ServletContext中以便全局调用，而使用的key就是WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
所以验证的方式就是查看ServletContext实例中是否有对应key的属性。
（2）创建WebApplicationContext实例。
如果通过验证，则Spring将创建WebApplicationContext实例的工作委托给了createWebApplicationContext函数。
其中，在ContextLoader类中有这样的静态代码块：
static { // Load default strategy implementations from properties file. // This is currently strictly internal and not meant to be customized // by application developers. try { ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class); defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); } catch (IOException ex) { throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage()); } }
根据以上静态代码块的内容，我们推断在当前类ContextLoader通用目录下必定会存在属性文件ContextLoader.properties，查看后果然存在，内容如下：
# Default WebApplicationContext implementation class for ContextLoader. # Used as fallback when no explicit context implementation has been specified as context-param. # Not meant to be customized by application developers. org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext
综合以上代码分析，在初始化的过程中，在没有配置contextClass情况下，程序首先会读取ContextLoader类的同目录下的属性文件ContextLoader.properties，并根据其中的配置提取将要实现WebApplicationContext接口的实现类，并根据这个实现类通过反射的方式进行实例的创建。
（3）将实例记录在servletContext中。
（4）映射当前的类加载器与创建的实例到全局变量currentContextPerThread中。
三：DispatcherServlet
（3.1）介绍
在Spring中，ContextLoaderListener只是辅助功能，用于创建WebApplicationContext类型实例，而真正的逻辑实现其实是在DispatcherServlet中进行的，DispatcherServlet是实现servlet接口的实现类。
servlet是一个java编写的程序，此程序是基于HTTP协议的，在服务器端运行的（如Tomcat），是按照servlet规范编写的一个Java类。主要是处理客户端的请求并将其结果发送到客户端。servlet的生命周期是由servlet的容器来控制的，它可以分为3个阶段：初始化，运行和销毁。
（1）初始化阶段
- servlet容器加载servlet类，把servlet类的.class文件中的数据读到内存中。
- servlet容器创建一个ServletConfig对象。ServletConfig对象包含了servlet的初始化配置信息。
- servlet容器创建一个servlet对象。
- servlet容器调用servlet对象的init方法进行初始化。
（2）运行阶段。
当servlet容器接收到一个请求时，servlet容器会针对这个请求创建servletRequest和servletResponse对象，然后调用service方法。并把这两个参数传递给service方法。service方法通过servletRequest对象获得请求的信息。并处理该请求。再通过servletResponse对象生成这个请求的响应结果。然后销毁servletRequest和servletResponse对象。我们不管这个请求是post提交的还是get提交的，最终这个请求都会由service方法来处理。
（3）销毁阶段。
当Web应用被终止时，servlet容器会先调用servlet对象的destory方法，然后再销毁servlet对象，同时也会销毁与servlet对象相关联的servletConfig对象。我们可以在destory方法的实现张红，释放servlet所占用的资源，如关闭数据库连接，关闭文件输入输出流等。
servlet的框架是由两个Java包组成：javax.servlet和javax.servlet.http。在javax.servlet包中定义了所有的servlet类都必须实现或扩展的通用接口和类，在javax.servlet.http包中定义了采用HTTP通信协议的HttpServlet类。
servlet被设计成请求驱动，servlet的请求可能包含多个数据项，当Web容器接收到某个servlet请求时，servlet把请求封装成一个HttpServletRequest对象，然后把对象传给servlet的对应的服务方法。
（3.2）DispatcherServlet的初始化
在servlet初始化阶段会调用其init方法，所以我们首先要查看在DispatcherServlet中是否重写了init方法。我们在其父类HttpServletBean中找到了该方法。
/** * 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 {
//解析init-param并封装至pvs中 PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties); //将当前的Servlet类转化为一个BeanWrapper，从而能够以Spring的方式来对init-param的值进行注入
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this); ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext()); bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment())); //空实现，留给子类覆盖
initBeanWrapper(bw); bw.setPropertyValues(pvs, true); } catch (BeansException ex) { if (logger.isErrorEnabled()) { 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"); } }
DispatcherServlet的初始化过程主要是通过将当前的servlet类型实例转换为BeanWrapper类型实例，以便使用Spring中提供的注入功能进行对应属性的注入。这些属性如contextAttribute，contextClass，nameSpace，contextConfigLocation等，都可以在web.xml文件中以初始化参数的方式配置在servlet的声明中。DispatcherServlet继承自FrameworkServlet，FrameworkServlet类上包含对应的同名属性，Spring会保证这些参数被注入到对应的值中。属性注入主要包含以下几个步骤：
（1）封装及验证初始化参数
ServletConfigPropertyValues除了封装属性外还有对属性验证的功能。
HttpServletBean中静态类：
/** * PropertyValues implementation created from ServletConfig init parameters. */ private static class ServletConfigPropertyValues extends MutablePropertyValues { /** * Create new ServletConfigPropertyValues. * @param config ServletConfig we'll use to take PropertyValues from * @param requiredProperties set of property names we need, where * we can't accept default values * @throws ServletException if any required properties are missing */ public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties) throws ServletException { Set<String> missingProps = (requiredProperties != null && !requiredProperties.isEmpty() ? new HashSet<String>(requiredProperties) : null); Enumeration<String> paramNames = config.getInitParameterNames(); while (paramNames.hasMoreElements()) { String property = paramNames.nextElement(); Object value = config.getInitParameter(property); addPropertyValue(new PropertyValue(property, value)); if (missingProps != null) { missingProps.remove(property); } } // Fail if we are still missing properties. if (!CollectionUtils.isEmpty(missingProps)) { throw new ServletException( "Initialization from ServletConfig for servlet '" + config.getServletName() + "' failed; the following required properties were missing: " + StringUtils.collectionToDelimitedString(missingProps, ", ")); } } }
从代码中得知，封装属性主要是对初始化的参数进行封装，也就是servlet中配置的<init-param>中配置的封装。
（2）将当前servlet实例转化成BeanWrapper实例
PropertyAccessorFactory.forBeanPropertyAccess是Spring中提供的工具方法，主要用于将指定实例转化为Spring中可以处理的BeanWrapper类型的实例。
（3）注册相对于Resource的属性编辑器
这里使用属性编辑器的目的是在对当前实例（DispatcherServlet）属性注入过程中一旦遇到Resource类型的属性就会使用ResourceEditor去解析。
（4）属性注入
BeanWrapper为Spring中的方法，支持Spring的自动注入。其实我们最常用的属性组人无非是contextAttribute，contextClass，nameSpace，contextConfigLocation等属性。
（5）servletBean的初始化
在ContextLoaderListener加载的时候已经创建了WebApplicationContext实例，而在这个函数中最重要的就是对这个实例进行进一步的补充初始化。
继续查看initServletBean()。父类FrameworkServlet覆盖了HttpServletBean中的initServletBean函数，如下：
/** * 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();
//设计为子类覆盖 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"); } }
上面的函数设计了计时器来统计初始化的执行时间，而且提供了一个扩展方法initFrameworkServlet()用于子类的覆盖操作，而作为关键的初始化逻辑实现委托给了initWebApplicationContext()。
（3.3）WebApplicationContext的初始化
initWebApplicationContext函数的主要工作就是创建或刷新WebApplicationContext实例并对servlet功能所使用的变量进行初始化。
FrameworkServlet类中 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() {
//通过属性WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE属性在servletContext中获取
//一般是ContextLoaderListener的初始化函数initWebApplicationContext中设置的 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, etc if (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; }
本函数中的初始化主要包含几个部分。
1. 寻找或创建对应的WebApplicationContext实例
WebApplicationContext的寻找及创建包括以下几个步骤。
（1）通过构造函数的注入进行初始化。
当进入initWebApplicationContext函数后通过判断this.webApplicationContext != null 后，便可以确定this.webApplicationContext是否是通过构造函数来初始化的。在Web中包含SpringWeb的核心逻辑的DispatcherServlet只可能被声明为一次，在Spring中已经存在验证，所以这就确保了如果this.webApplicationContext != null，则可以直接判定this.webApplicationContext已经通过构造函数初始化。
（2）通过contextAttribute进行初始化
如上findWebApplicationContext()函数，通过在web.xml文件中配置的servlet参数contextAttribute来查找ServletContext中对应的属性，默认为WebApplicationContext.class.getName() + ".ROOT"，也就是在ContextLoaderListener加载时会创建WebApplicationContext实例，并将实例以WebApplicationContext.class.getName() + ".ROOT"为key放入ServletContext中，当然也可以重写初始化逻辑使用自己创建的WebApplicationContext，并在servlet的配置中通过初始化参数contextAttribute指定key。
（3）重新创建WebApplicationContext实例
如果通过以上两种方式并没有找到任何突破，那就没有办法了，只能在这里重新创建新的实例了。
FrameworkServlet类中
/** * 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. * Delegates to #createWebApplicationContext(ApplicationContext). * @param parent the parent WebApplicationContext to use, or {@code null} if none * @return the WebApplicationContext for this servlet * @see org.springframework.web.context.support.XmlWebApplicationContext * @see #createWebApplicationContext(ApplicationContext) */ protected WebApplicationContext createWebApplicationContext(WebApplicationContext parent) { return createWebApplicationContext((ApplicationContext) parent); } /** * 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) { //获取servlet的初始化参数contextClass，如果没有配置默认为XmlWebApplicationContext.class
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"); }
//通过反射方式实例化contextClass ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); wac.setEnvironment(getEnvironment());
//parent为在ContextLoaderListener中创建的实例
//在ContextLoaderListener加载的时候初始化的WebApplicationContext类型实例 wac.setParent(parent); wac.setConfigLocation(getContextConfigLocation());
//初始化Spring环境包括加载配置文件等 configureAndRefreshWebApplicationContext(wac); return wac; } 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 information if (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);
//加载配置文件及整合parent到wac wac.refresh(); }
/** * ApplicationListener endpoint that receives events from this servlet's WebApplicationContext * only, delegating to {@code onApplicationEvent} on the FrameworkServlet instance. */ private class ContextRefreshListener implements ApplicationListener<ContextRefreshedEvent> { @Override public void onApplicationEvent(ContextRefreshedEvent event) { FrameworkServlet.this.onApplicationEvent(event); } }
/** * Callback that receives refresh events from this servlet's WebApplicationContext. * <p>The default implementation calls {@link #onRefresh}, * triggering a refresh of this servlet's context-dependent state. * @param event the incoming ApplicationContext event */ public void onApplicationEvent(ContextRefreshedEvent event) { this.refreshEventReceived = true; onRefresh(event.getApplicationContext()); }
DispatcherServlet类中重写onRefresh（ApplicationContext context）方法
/** * This implementation calls {@link #initStrategies}. */ @Override protected void onRefresh(ApplicationContext context) { initStrategies(context); } /** * Initialize the strategy objects that this servlet uses. * <p>May be overridden in subclasses in order to initialize further strategy objects. */ protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); }
2. configureAndRefreshWebApplicationContext
无论是通过构造函数注入还是单独创建，都免不了会调用configureAndRefreshWebApplicationContext方法来对已经创建的WebApplicationContext实例进行配置及刷新。
无论调用方式如何变化，只有是使用ApplicationContext所提供的功能到最后都免不了使用公共父类AbstractApplicationContext提供的refresh()进行配置文件加载。
3.刷新
onRefresh是FrameworkServlet类中提供的模板方法，在其子类DispatcherServlet中进行了重写，主要用于刷新Spring在Web功能实现中所必须使用的全局变量。这里从DispatcherServlet类源码中选择几个进行讲解。
（1）初始化MultipartResolver
在Spring中，MultipartResolver主要用来处理文件上传。默认情况下，Spring是没有multipart处理的。常用配置如下：
/** * Initialize the MultipartResolver used by this class. * <p>If no bean is defined with the given name in the BeanFactory for this namespace, * no multipart handling is provided. */ private void initMultipartResolver(ApplicationContext context) { try {
//MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver" this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class); if (logger.isDebugEnabled()) { logger.debug("Using MultipartResolver [" + this.multipartResolver + "]"); } } catch (NoSuchBeanDefinitionException ex) { // Default is no multipart resolver. this.multipartResolver = null; if (logger.isDebugEnabled()) { logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME + "': no multipart request handling provided"); } } }
（2）初始化HandlerMappings
当客户端发出Request时DispatcherServlet会将Request提交给HandlerMapping，然后HandlerMapping根据WebApplicationContext的配置来回传给DispatcherServlet相应的Controller.
在基于SpringMVC的Web应用程序中，我们可以为DispatcherServlet提供多个HandlerMapping供其使用。DispatcherServlet在选用HandlerMapping的过程中，将根据我们所指定的一系列HandlerMapping的优先级进行排序，然后优先使用优先级在前的HandlerMapping。如果当期的HandlerMapping能够返回可用的Handler，DispatcherServlet则使用返回的Handler进行Web请求的处理，而不再继续询问其他的HandlerMapping。否则，DispatcherServlet将继续按照各个HandlerMapping的优先级进行询问，直到获取一个可用的Handler为止。初始化配置如下：
/** * Initialize the HandlerMappings used by this class. * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace, * we default to BeanNameUrlHandlerMapping. */ private void initHandlerMappings(ApplicationContext context) { this.handlerMappings = null; if (this.detectAllHandlerMappings) { //默认为true // Find all HandlerMappings in the ApplicationContext, including ancestor contexts. Map<String, HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values()); // We keep HandlerMappings in sorted order. AnnotationAwareOrderComparator.sort(this.handlerMappings); } } else { try {
//HANDLER_MAPPING_BEAN_NAME = "handlerMapping" HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class); this.handlerMappings = Collections.singletonList(hm); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default HandlerMapping later. } } // Ensure we have at least one HandlerMapping, by registering // a default HandlerMapping if no other mappings are found. if (this.handlerMappings == null) { this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class); if (logger.isDebugEnabled()) { logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default"); } } }
在最后一步中，如果还是没有定义handlermapping的化，则SpringMVC将按照org.Springframework.web.servlet.DispatcherServlet所在目录下的DispatcherServlet.properties中所定义的org.Springframework.web.servlet.HandlerMapping的内容来加载默认的handlerMapping（用户没有自定义Strategies的情况下）。
（3）初始化HandlerAdapters。
从名字也能联想到这是一个典型的适配器模式的使用，适配器模式将一个类的接口适配成用户所期待的。
/** * Initialize the HandlerAdapters used by this class. * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace, * we default to SimpleControllerHandlerAdapter. */ private void initHandlerAdapters(ApplicationContext context) { this.handlerAdapters = null; if (this.detectAllHandlerAdapters) { // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts. Map<String, HandlerAdapter> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerAdapters = new ArrayList<HandlerAdapter>(matchingBeans.values()); // We keep HandlerAdapters in sorted order. AnnotationAwareOrderComparator.sort(this.handlerAdapters); } } else { try { HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class); this.handlerAdapters = Collections.singletonList(ha); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default HandlerAdapter later. } } // Ensure we have at least some HandlerAdapters, by registering // default HandlerAdapters if no other adapters are found. if (this.handlerAdapters == null) { this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class); if (logger.isDebugEnabled()) { logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default"); } } }
如果无法找到对应的bean，那么系统会尝试加载默认的适配器。
/** * Create a List of default strategy objects for the given strategy interface. * <p>The default implementation uses the "DispatcherServlet.properties" file (in the same * package as the DispatcherServlet class) to determine the class names. It instantiates * the strategy objects through the context's BeanFactory. * @param context the current WebApplicationContext * @param strategyInterface the strategy interface * @return the List of corresponding strategy objects */ @SuppressWarnings("unchecked") protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) { String key = strategyInterface.getName(); String value = defaultStrategies.getProperty(key); if (value != null) { String[] classNames = StringUtils.commaDelimitedListToStringArray(value); List<T> strategies = new ArrayList<T>(classNames.length); for (String className : classNames) { try { Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader()); Object strategy = createDefaultStrategy(context, clazz); strategies.add((T) strategy); } catch (ClassNotFoundException ex) { throw new BeanInitializationException( "Could not find DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]", ex); } catch (LinkageError err) { throw new BeanInitializationException( "Error loading DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]: problem with class file or dependent class", err); } } return strategies; } else { return new LinkedList<T>(); } }
在该函数中，Spring会尝试从defaultStrategies中加载对应的HandlerAdapter的属性，那么defaultStrategies是如何初始化的呢？
在当前类DispatcherServlet中存在这样一段初始化代码块：
static { // Load default strategy implementations from properties file. // This is currently strictly internal and not meant to be customized // by application developers. try {
//DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties" ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class); defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); } catch (IOException ex) { throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage()); } }
在系统加载的时候，defaultStrategies根据当前路径DispatcherServlet.properties来初始化本身。DispatcherServlet.properties内容如下：
# 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
（4）初始化HandlerExceptionResolvers
基于HandlerExceptionResolver接口的异常处理，使用这种方式只需要实现resolveException方法，该方法返回一个ModelAndView对象，在方法内部对异常的类型进行判断，然后尝试生成对应的ModelAndView对象，如果该方法返回了null，则Spring会继续寻找其它的实现了HandlerExceptionResolver接口的bean。
/** * Initialize the HandlerExceptionResolver used by this class. * <p>If no bean is defined with the given name in the BeanFactory for this namespace, * we default to no exception resolver. */ private void initHandlerExceptionResolvers(ApplicationContext context) { this.handlerExceptionResolvers = null; if (this.detectAllHandlerExceptionResolvers) { // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts. Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerExceptionResolvers = new ArrayList<HandlerExceptionResolver>(matchingBeans.values()); // We keep HandlerExceptionResolvers in sorted order. AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers); } } else { try { HandlerExceptionResolver her = context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class); this.handlerExceptionResolvers = Collections.singletonList(her); } catch (NoSuchBeanDefinitionException ex) { // Ignore, no HandlerExceptionResolver is fine too. } } // Ensure we have at least some HandlerExceptionResolvers, by registering // default HandlerExceptionResolvers if no other resolvers are found. if (this.handlerExceptionResolvers == null) { this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class); if (logger.isDebugEnabled()) { logger.debug("No HandlerExceptionResolvers found in servlet '" + getServletName() + "': using default"); } } }
（5）初始化ViewResolvers
在SpringMVC中，当Controller将请求处理结果放入到ModelAndView中以后，DispatcherServlet会根据ModelAndView选择合适的视图进行渲染。那么在SpringMVC中是如何选择合适的View呢？View对象是如何创建的呢？答案就在ViewResolver中。ViewResolver接口定义了resolverViewName方法，根据viewName创建合适类型的View实现。
/** * Initialize the ViewResolvers used by this class. * <p>If no ViewResolver beans are defined in the BeanFactory for this * namespace, we default to InternalResourceViewResolver. */ private void initViewResolvers(ApplicationContext context) { this.viewResolvers = null; if (this.detectAllViewResolvers) { // Find all ViewResolvers in the ApplicationContext, including ancestor contexts. Map<String, ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false); if (!matchingBeans.isEmpty()) { this.viewResolvers = new ArrayList<ViewResolver>(matchingBeans.values()); // We keep ViewResolvers in sorted order. AnnotationAwareOrderComparator.sort(this.viewResolvers); } } else { try {
//VIEW_RESOLVER_BEAN_NAME = "viewResolver" ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class); this.viewResolvers = Collections.singletonList(vr); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default ViewResolver later. } } // Ensure we have at least one ViewResolver, by registering // a default ViewResolver if no other resolvers are found. if (this.viewResolvers == null) { this.viewResolvers = getDefaultStrategies(context, ViewResolver.class); if (logger.isDebugEnabled()) { logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default"); } } }
ViewResolver接口
public interface ViewResolver { /** * Resolve the given view by name. * <p>Note: To allow for ViewResolver chaining, a ViewResolver should * return {@code null} if a view with the given name is not defined in it. * However, this is not required: Some ViewResolvers will always attempt * to build View objects with the given name, unable to return {@code null} * (rather throwing an exception when View creation failed). * @param viewName name of the view to resolve * @param locale Locale in which to resolve the view. * ViewResolvers that support internationalization should respect this. * @return the View object, or {@code null} if not found * (optional, to allow for ViewResolver chaining) * @throws Exception if the view cannot be resolved * (typically in case of problems creating an actual View object) */ View resolveViewName(String viewName, Locale locale) throws Exception; }
AbstractCachingViewResolver类
public abstract class AbstractCachingViewResolver extends WebApplicationObjectSupport implements ViewResolver { /** Default maximum number of entries for the view cache: 1024 */ public static final int DEFAULT_CACHE_LIMIT = 1024; /** Dummy marker object for unresolved views in the cache Maps */ private static final View UNRESOLVED_VIEW = new View() { @Override public String getContentType() { return null; } @Override public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) { } }; /** The maximum number of entries in the cache */ private volatile int cacheLimit = DEFAULT_CACHE_LIMIT; /** Whether we should refrain from resolving views again if unresolved once */ private boolean cacheUnresolved = true; /** Fast access cache for Views, returning already cached instances without a global lock */ private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<Object, View>(DEFAULT_CACHE_LIMIT); /** Map from view key to View instance, synchronized for View creation */ @SuppressWarnings("serial") private final Map<Object, View> viewCreationCache = new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) { @Override protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) { if (size() > getCacheLimit()) { viewAccessCache.remove(eldest.getKey()); return true; } else { return false; } } }; /** * Specify the maximum number of entries for the view cache. * Default is 1024. */ public void setCacheLimit(int cacheLimit) { this.cacheLimit = cacheLimit; } /** * Return the maximum number of entries for the view cache. */ public int getCacheLimit() { return this.cacheLimit; } /** * Enable or disable caching. * <p>This is equivalent to setting the {@link #setCacheLimit "cacheLimit"} * property to the default limit (1024) or to 0, respectively. * <p>Default is "true": caching is enabled. * Disable this only for debugging and development. */ public void setCache(boolean cache) { this.cacheLimit = (cache ? DEFAULT_CACHE_LIMIT : 0); } /** * Return if caching is enabled. */ public boolean isCache() { return (this.cacheLimit > 0); } /** * Whether a view name once resolved to {@code null} should be cached and * automatically resolved to {@code null} subsequently. * <p>Default is "true": unresolved view names are being cached, as of Spring 3.1. * Note that this flag only applies if the general {@link #setCache "cache"} * flag is kept at its default of "true" as well. * <p>Of specific interest is the ability for some AbstractUrlBasedView * implementations (FreeMarker, Velocity, Tiles) to check if an underlying * resource exists via {@link AbstractUrlBasedView#checkResource(Locale)}. * With this flag set to "false", an underlying resource that re-appears * is noticed and used. With the flag set to "true", one check is made only. */ public void setCacheUnresolved(boolean cacheUnresolved) { this.cacheUnresolved = cacheUnresolved; } /** * Return if caching of unresolved views is enabled. */ public boolean isCacheUnresolved() { return this.cacheUnresolved; } @Override public View resolveViewName(String viewName, Locale locale) throws Exception { if (!isCache()) { return createView(viewName, locale); } else { Object cacheKey = getCacheKey(viewName, locale); View view = this.viewAccessCache.get(cacheKey); if (view == null) { synchronized (this.viewCreationCache) { view = this.viewCreationCache.get(cacheKey); if (view == null) { // Ask the subclass to create the View object. view = createView(viewName, locale); if (view == null && this.cacheUnresolved) { view = UNRESOLVED_VIEW; } if (view != null) { this.viewAccessCache.put(cacheKey, view); this.viewCreationCache.put(cacheKey, view); if (logger.isTraceEnabled()) { logger.trace("Cached view [" + cacheKey + "]"); } } } } } return (view != UNRESOLVED_VIEW ? view : null); } } /** * Return the cache key for the given view name and the given locale. * <p>Default is a String consisting of view name and locale suffix. * Can be overridden in subclasses. * <p>Needs to respect the locale in general, as a different locale can * lead to a different view resource. */ protected Object getCacheKey(String viewName, Locale locale) { return viewName + '_' + locale; } /** * Provides functionality to clear the cache for a certain view. * <p>This can be handy in case developer are able to modify views * (e.g. Velocity templates) at runtime after which you'd need to * clear the cache for the specified view. * @param viewName the view name for which the cached view object * (if any) needs to be removed * @param locale the locale for which the view object should be removed */ public void removeFromCache(String viewName, Locale locale) { if (!isCache()) { logger.warn("View caching is SWITCHED OFF -- removal not necessary"); } else { Object cacheKey = getCacheKey(viewName, locale); Object cachedView; synchronized (this.viewCreationCache) { this.viewAccessCache.remove(cacheKey); cachedView = this.viewCreationCache.remove(cacheKey); } if (logger.isDebugEnabled()) { // Some debug output might be useful... if (cachedView == null) { logger.debug("No cached instance for view '" + cacheKey + "' was found"); } else { logger.debug("Cache for view " + cacheKey + " has been cleared"); } } } } /** * Clear the entire view cache, removing all cached view objects. * Subsequent resolve calls will lead to recreation of demanded view objects. */ public void clearCache() { logger.debug("Clearing entire view cache"); synchronized (this.viewCreationCache) { this.viewAccessCache.clear(); this.viewCreationCache.clear(); } } /** * Create the actual View object. * <p>The default implementation delegates to {@link #loadView}. * This can be overridden to resolve certain view names in a special fashion, * before delegating to the actual {@code loadView} implementation * provided by the subclass. * @param viewName the name of the view to retrieve * @param locale the Locale to retrieve the view for * @return the View instance, or {@code null} if not found * (optional, to allow for ViewResolver chaining) * @throws Exception if the view couldn't be resolved * @see #loadView */ protected View createView(String viewName, Locale locale) throws Exception { return loadView(viewName, locale); } /** * Subclasses must implement this method, building a View object * for the specified view. The returned View objects will be * cached by this ViewResolver base class. * <p>Subclasses are not forced to support internationalization: * A subclass that does not may simply ignore the locale parameter. * @param viewName the name of the view to retrieve * @param locale the Locale to retrieve the view for * @return the View instance, or {@code null} if not found * (optional, to allow for ViewResolver chaining) * @throws Exception if the view couldn't be resolved * @see #resolveViewName */ protected abstract View loadView(String viewName, Locale locale) throws Exception; }