浅析Spring MVC:DispatcherServlet

使用Spring Web MVC，开发者可以直接访问官方文档Spring Web MVC文档Version 5.2.1.RELEASE，本文及Spring MVC系列文章都参考于此文档及源码。

先看文档是如何介绍Spring MVC的：

Spring Web MVC is the original web framework built on the Servlet API and has been included in the Spring Framework from the very beginning. The formal name, “Spring Web MVC,” comes from the name of its source module (spring-webmvc), but it is more commonly known as “Spring MVC”

同时也提到到了Spring WebFlux。

1. Spring MVC引入

这里使用maven：

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>${spring.version}</version>
</dependency>

2. DispatcherServlet

1. 声明

The DispatcherServlet, as any Servlet, needs to be declared and mapped according to the Servlet specification by using Java configuration or in web.xml

可以看到需要通过Java configuration或者web.xml来申明配置DispatcherServlet，也就是Web容器寻找的入口。

1. Java configuration

   public class MyWebApplicationInitializer implements WebApplicationInitializer {
   
   @Override
   public void onStartup(ServletContext servletCxt) {
   
   	// Load Spring web application configuration
   	AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
   	ac.register(AppConfig.class);
   	ac.refresh();
   
   	// Create and register the DispatcherServlet
   	DispatcherServlet servlet = new DispatcherServlet(ac);
   	ServletRegistration.Dynamic registration = servletCxt.addServlet("app", servlet);
   	registration.setLoadOnStartup(1);
   	registration.addMapping("/app/*");
   }
   }

2. web.xml

   <web-app>
   
       <listener>
           <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
       </listener>
   
       <context-param>
           <param-name>contextConfigLocation</param-name>
           <param-value>/WEB-INF/app-context.xml</param-value>
       </context-param>
   
       <servlet>
           <servlet-name>app</servlet-name>
           <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
           <init-param>
               <param-name>contextConfigLocation</param-name>
               <param-value></param-value>
           </init-param>
           <load-on-startup>1</load-on-startup>
       </servlet>
   
       <servlet-mapping>
           <servlet-name>app</servlet-name>
           <url-pattern>/app/*</url-pattern>
       </servlet-mapping>
   
   </web-app>

2. 上下文

DispatcherServlet expects a WebApplicationContext (an extension of a plain ApplicationContext) for its own configuration。

WebApplicationContext和ServletContext通过监听器实现共存亡的关系。可以从源码中看到：

ContextLoaderListener,ContextCleanupListener等实现了Servlet中的ServletContextListener。

例如：ContextLoaderListener中：

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

	public ContextLoaderListener() {
	}

	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}

	/**
	 * 项目启动，创建ServletContext时初始化WebApplicationContext
	 */
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

	/**
	 * 项目关闭，销毁ServletContext时调用ContextCleanupListener销毁清理WebApplicationContext
	 */
	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}

}

2. 实例

可以在源码中看到初始化DispatcherServlet时使用onRefresh方法，onRefresh又调用了initStrategies，在initStrategies中初始化了这9个实例:

initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);

可以自定义扩张或者替换这些组件。当没有配置时，会调用默认的配置。

例如在initHandlerAdapters中，没有找到自定义配置的handlerAdapters则会调用getDefaultStrategies获取默认配置：

if (this.handlerAdapters == null) {
	this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
	if (logger.isTraceEnabled()) {
		logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
				"': using default strategies from DispatcherServlet.properties");
	}
}

再看一下如何获取的默认配置：

//获取默认配置属性
//Create a List of default strategy objects for the given strategy interface.
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<>(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(
						"Unresolvable class definition for DispatcherServlet's default strategy class [" +
						className + "] for interface [" + key + "]", err);
			}
		}
		return strategies;
	}
	else {
		return new LinkedList<>();
	}
}

可以看到defaultStrategies作为配置：

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, DispatcherServlet.class);
		defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
	}
	catch (IOException ex) {
		throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
	}
}

而这里加载的DEFAULT_STRATEGIES_PATH可以在申明中找到：

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

也就是说会通过默认DispatcherServlet.properties配置文件来初始化默认实例。

3. 配置

上面说到需要通过WebApplicationInitializer或者web.xml配置容器。

An abstract base class implementation of WebApplicationInitializer named AbstractDispatcherServletInitializer makes it even easier to register the DispatcherServlet by overriding methods to specify the servlet mapping and the location of the DispatcherServlet configuration.

可以继承AbstractDispatcherServletInitializer注册一个Servlet。

public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

	@Override
	protected Class<?>[] getRootConfigClasses() {
		return null;
	}

	@Override
	protected Class<?>[] getServletConfigClasses() {
		return new Class<?>[] { MyWebConfig.class };
	}

	@Override
	protected String[] getServletMappings() {
		return new String[] { "/" };
	}

	//添加Filter 实例并将其自动映射快捷方法
	@Override
	protected Filter[] getServletFilters() {
		return new Filter[] {
			new HiddenHttpMethodFilter(), new CharacterEncodingFilter() };
	}
}

如果使用xml配置，也可以使用AbstractDispatcherServletInitializer。

如果需要进一步的自定义DispatcherServlet，可以重写createDispatcherServlet方法。

4. 处理一个请求

可以在源码中看到如何处理一个请求的。在DispatcherServlet中doService方法：

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(request);
        
	//把request中数据备份到attributesSnapshot中，以便还原
	Map<String, Object> attributesSnapshot = null;
	if (WebUtils.isIncludeRequest(request)) {
		attributesSnapshot = new HashMap<>();
		Enumeration<?> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// request设置WebApplicationContext属性
	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
	// 绑定解析器
	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

	if (this.flashMapManager != null) {
		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
		if (inputFlashMap != null) {
			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
		}
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
	}

	try {
		//交由doDispatch处理请求
		doDispatch(request, response);
	}
	finally {
		//还原request
		if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}
}

5. 拦截

通过实现HandlerMapping，并且使用setter注册可以实现处理程序的拦截器。

1. preHandle(..)：执行实际的处理程序之前

2. postHandle(..)：处理程序执行后

3. afterCompletion(..)：完整请求完成后

6. 异常

if an exception occurs during request mapping or is thrown from a request handler (such as a @Controller), the DispatcherServlet delegates to a chain of HandlerExceptionResolver beans to resolve the exception and provide alternative handling, which is typically an error response.

如果异常在请求映射或者期间发生，DispatcherServlet会委托HandlerExceptionResolver来处理。

HandlerExceptionResolver的实现有：

1. SimpleMappingExceptionResolver: 异常类名称和错误视图名称之间的映射。

2. DefaultHandlerExceptionResolver: 解决了Spring MVC引发的异常，并将它们映射到HTTP状态代码。

3. ResponseStatusExceptionResolver: 使用@ResponseStatus注释解决异常，并根据注释中的值将它们映射到HTTP状态代码。

4. ExceptionHandlerExceptionResolver: 通过调用@ExceptionHandlera @Controller或 @ControllerAdviceclass中的方法来解决异常。

可以在Spring configuration设置HandlerExceptionResolver beans,并且能够设置order优先级来处理异常。也可以使用@ResponseStatus,@ExceptionHandler

可以在定义它的错误页面<location>/error</location>

我们可以看到Spring MVC提供了一个抽象方法AbstractHandlerExceptionResolver，他实现了HandlerExceptionResolver,当然就可以通过AbstractHandlerExceptionResolver来自定义异常的解析。从来做到项目异常的统一管理。

7. 视图

在处理请求doService中，可以看到request绑定了一个视图解析器ViewResolver。

在源码包view中，可以看到freemarker,groovy,json,script,xml。

可以申明多个解析器，并通过设置order链接。

可以redirect,forward

5. 国际化

Spirng MVC通过LocaleResolver对象完成国际化。我们也在doService看到request绑定了一个解析器LocaleResolver。可以使用RequestContext.getLocale()

可以从i18n包中如下解析器：

1. Time Zone: 获取请求语言时区环境
2. Header Resolver: 检查请求头信息，如accept-language
3. Cookie Resolver: 请求Cookie解析，可以通过CookieLocaleResolver设置属性

<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">

    <property name="cookieName" value="clientlanguage"/>

    <!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->
    <property name="cookieMaxAge" value="100000"/>
    <property name="cookiePath" value="">

</bean>

4. Session Resolver: Session解析
5. Locale Interceptor: 拦截器，可以自定义添加LocaleChangeInterceptor来改变语言区域环境

<bean id="localeChangeInterceptor"
        class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
    <property name="paramName" value="siteLanguage"/>
</bean>

<bean id="localeResolver"
        class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/>

<bean id="urlMapping"
        class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
    <property name="interceptors">
        <list>
            <ref bean="localeChangeInterceptor"/>
        </list>
    </property>
    <property name="mappings">
        <value>/**/*.view=someController</value>
    </property>
</bean>

6. 主题

需要实现ThemeSource接口，当然他提供了一个抽象类AbstractThemeResolver便于我们实现。

Spring还提供了ThemeChangeInterceptor进行更改。

7. Multipart Resolver

multipart/form-data

8. 日志

enableLoggingRequestDetails