SpringMVC之细说DispatcherServlet

DispatcherServlet的启动以及初始化

Spring MVC是一个MVC模式的实现,在使用Spring MVC 时,主要需要在web.xml配置文件中设置DispatcherServlet,这个Servlet是实现Spring mvc 的前端控制器,所有的Web请求都需要通过它来处理,进行匹配、转发、数据处理。DispatcherServlet是实现Spring MVC最核心的部分。

在使用SpringMVC 时我们通常需要如下配置:

Web容器启动,并开始加载各个Servlet,DispatcherServlet将建立自己的上下文来持有Spring MVC的bean对象,在建立这个自己持有的IoC容器时,会从ServletContext中得到根上下文作为DispatcherServlet持有的上下午的双亲上下文。有了这个根上下文,在对自己持有的上下文进行初始化,从而完成SpringMVC运行环境。在配置DispatcherServlet时可以设置初始化参数指定Ioc容器bean定义配置文件,名:<param-name>contextConfigLocation</param-nam>,值:<param-value>classpath:springmvc.xml</param-value>,如果不指定,默认使用/WEB-INF/DispatcherServlet名-servlet.xml。上述配置如不指定 <init-param>,将使用/WEB-INF/mvc-dispatcher-servlet.xml。

DispatcherServlet类继承体系

DispatcherServlet通过继承FrameworkServlet和HttpServletBean而继承了HttpServlet,通过使用ServletAPI来对HTTP请求进行处理成为了Spring MVC的前端处理器,同时也实现了MVC模块与WEB容器集成

1、DispatcherServlet的启动和初始化

DispatcherServlet启动

首先作为一个Servlet,在其生命周期中,将调用init()方法,完成初始化相关工作init()方法在HttpServletBean中进行了重写,代码如下:

@Override
public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
        logger.debug("Initializing servlet '" + getServletName() + "'");
        }
              //获取Servlet的初始化参数,并进行设置
              // 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;
        }
             //这里调用子类FrameWorkServlet中重写的initServletBean()方法继续进行初始化
              // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
        logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
        }

在FrameWorkServlet类中,initServletBean()方法如下:

@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 {
              //这里通过调用initWebApplicationContext(),初始化web应用上下文
        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");
        }
        }

继续看initWebApplicationContext()函数:

protected WebApplicationContext initWebApplicationContext() {
        //这里通过WebApplicationContextUtils工具类来获取根web应用上下文,这个上下文就是之前提到的全局应用根上下文,保存在ServletContext中,这个根上下文将作为当前mvc servlet上下文的双亲上下文。
        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) {
        //这里真正创建需要与该Servlet相关联的WebApplicationContext
              // 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.
               //这里,ioc容器已经创建初始化完成,执行onRefresh(wac)进一步初始化MVC其他模块,后面说明
        onRefresh(wac);
        }

        if (this.publishContext) {
        //这里将当前建立的上下文保存到ServletContext中,attrName 是与当前servlet名相关联,保证唯一性
               // 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;
        }

下面来看这个当前上下文是如何创建的,createWebApplicationContext()函数如下:

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");
        }
        //通过BeanUtils反射实例化具体的上下文对象,contextClass通过getContextClass()获取,取得的class是Class<?> DEFAULT_CONTEXT_CLASS = XmlWebApplicationContext.class;
        ConfigurableWebApplicationContext wac =
        (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
        //对上下文对象进行设置
        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        wac.setConfigLocation(getContextConfigLocation());
              //这里对刚刚创建的WebApplicationContext,进行配置和初始化操作,完成容器的最终初始化
        configureAndRefreshWebApplicationContext(wac);

        return wac;
        }

继续configureAndRefreshWebApplicationContext()函数:

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());
        }
        }
        //设置ServletContext引用,以及其他对象的设置
        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);
              //嗲用容器的refresh()方法,完成日期初始化
        wac.refresh();
        }

可以看到对上下文进行了相关配置,设置ServletContext,设置初始化参数等,最后通过调用容器的refresh()方法启动整个容器,就像一般的IoC容器初始化过程一样。

至此,web应用上下文的启动、初始化已经完成,DispatcherServlet运行MVC需要的IoC容器已经建立起来,但是DispatcherServlet作为Spring MVC前端控制器,还需要初始化MVC需要的其他模块,回到initWebApplicationContext()函数,看到在wac =createWebApplicationContext(rootContext)后面,调用了onRefresh(was)函数,onRefresh函数在DispatcherServlet类中重写,代码如下:

@Override
protected void onRefresh(ApplicationContext context) {
//调用initStrategies完成MVC模块初始化
        initStrategies(context);
        }

可以看到MVC初始化是在DispatcherServlet的initStrategies()方法中完成,在该方法中初始化各种MVC框架实现元素,如至此request映射的HandlerMappings,以及handler适配处理器HandlerAdapters,以及视图生成器ViewResolver等。

protected void initStrategies(ApplicationContextcontext) { initMultipartResolver(context);//初始化文件上传解析器 initLocaleResolver(context);//初始化本地解析器 initThemeResolver(context);//初始化主题解析器 initHandlerMappings(context);//初始化处理器映射器 initHandlerAdapters(context);//初始化处理器适配器 initHandlerExceptionResolvers(context);//初始化异常解析器 initRequestToViewNameTranslator(context);//初始化请求到视图名称解析器 initViewResolvers(context);//初始化视图解析器 initFlashMapManager(context); }

DispatcherServlet.properties的默认配置:

# 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

原文发布于微信公众号 - 国产程序员(Monday_lida)

原文发表时间:2018-06-06

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