DispatcherServlet初始化和请求处理过程

1.DispatcherServlet的继承关系

在idea中，右键选择DispatcherServlet的Diagrams可以查看类的继承关系，如图所示

可以看到，DispatchServlet继承子FrameworkServlet，而FrameworkServlet继承子HttpServletBean;

2.HttpSerlvetBean初始化过程分析

public final void init() throws ServletException {
   if (logger.isDebugEnabled()) {
      logger.debug("Initializing servlet '" + getServletName() + "'");
   }

   // Set bean properties from init parameters.
   try {
      PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
      BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
      ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
      bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
      initBeanWrapper(bw);
      bw.setPropertyValues(pvs, true);
   }
   catch (BeansException ex) {
      logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
      throw ex;
   }

   // Let subclasses do whatever initialization they like.
   initServletBean();

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

分析：

PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);

获取web.xml的参数，构造PropertyValues对象；这里的psv值通过debug可以观察到实际是servlet的名称和具体的配置路径；

之后设置DispatcherServlet的属性；其实这段代码的目的就是获取servlet的context参数，设置为DispatcherServlet的contextConfigLocation参数，构造spring mvc的容器上下文；最后

initServletBean();

protected void initServletBean() throws ServletException {
}

该方法是占位方法，子类可以重写该方法，来做更多的事情；

3.FrameworkServlet

源代码如下所示，省略部分日志代码

@Override
protected final void initServletBean() throws ServletException {
   getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
   if (this.logger.isInfoEnabled()) {
      this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
   }
   long startTime = System.currentTimeMillis();

   try {
      this.webApplicationContext = initWebApplicationContext();
      initFrameworkServlet();
   }

重要的方法有initWebApplicationContext，代码

protected WebApplicationContext initWebApplicationContext() {
   WebApplicationContext rootContext =
         WebApplicationContextUtils.getWebApplicationContext(getServletContext());
   WebApplicationContext wac = null;

   if (this.webApplicationContext != null) {
      // A context instance was injected at construction time -> use it
      wac = this.webApplicationContext;
      if (wac instanceof ConfigurableWebApplicationContext) {
         ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
         if (!cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, 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;
}

首先判断WebApplicationContext是否为空，因为DispatcherServlet有一个构造函数webApplicationContext

public DispatcherServlet(WebApplicationContext webApplicationContext) {
   super(webApplicationContext);
}

所以要先进行判断，当使用该构造函数来生成DispatcherServlet的时候执行这段逻辑；

之后调用

wac = findWebApplicationContext();

以contextAttribute为key从ServletContext中找WebApplication。一般不会设置contextAttribute属性，所以这里返回值为null；然后调用

wac = createWebApplicationContext(rootContext);

来创建上下文，具体的代码如下

wac.setEnvironment(getEnvironment());
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());

configureAndRefreshWebApplicationContext(wac);

设置跟上下文为父上下文，然后配置ServletConfig,ServletContext等实例到这个上下文中；

最后onRefresh(wac)；模板方法，子类DispatcherServlet会覆盖这个方法；

之后将创建的容器上下文设置到ServletContext中；

String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);

4.DispatcherServlet

DispatcherServlet覆盖了父类FrameworkServlet中的方法，代码如下

@Override
protected void onRefresh(ApplicationContext context) {
   initStrategies(context);
}

/**
 * Initialize the strategy objects that this servlet uses.
 * 
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);
}

这里主要初始化各种策略接口的实现类，比如常用的异常处理初始化initHandlerExceptionResolvers，视图处理初始化initViewResolvers，请求映射初始化方法initHandlerMappings；

总结：1.HttpServletBean 主要做一些初始化的事情，将web.xml中的配置参数设置到servlet中。比如servlet标签的子标签init-param标签中的参数；

2.FrameworkServlet 将Servlet与Spring容器上下文关联；也就是初始化FarmeworkServlet属性webApplicationContext,这个属性代表springmvc上下文，它有个父类上下文；

3.DispatcherServlet 初始化各个功能的实现，比如异常处理，视图处理，请求映射等功能；

5.DispatcherServlet对http请求的处理过程

首先了解下HttpServlet对于http请求的处理过程；

1.所有的请求都会调用service方法；

protected void service(HttpServletRequest req, HttpServletResponse resp)

2.service中，根据请求类型再分别调用以下方法

else if (method.equals(METHOD_POST)) {
    doPost(req, resp);
    
} else if (method.equals(METHOD_PUT)) {
    doPut(req, resp);
    
} else if (method.equals(METHOD_DELETE)) {
    doDelete(req, resp);
    
} else if (method.equals(METHOD_OPTIONS)) {
    doOptions(req,resp);
    
} else if (method.equals(METHOD_TRACE)) {
    doTrace(req,resp);

3.FrameworkServlet子类继承子HttpServlet，并重写了上述方法，代码如下：

@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

   processRequest(request, response);
}

4.FrameworkServlet的processRequest方法代码如下

其中doService的具体代码如下，可以看到是抽象方法，具体的实现是在DispatchServlet中，

5.doService具体实现逻辑如下

这里判断请求如果是include请求，那么就保存请求的快照，doDispatch方法处理之后，快照中的数据会覆盖新的request中的数据；

6.doDispatch方法的处理逻辑

这里首先通过getHandler得到当前请求对应的handler；getHandler方法的具体代码如下

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   for (HandlerMapping hm : this.handlerMappings) {
      if (logger.isTraceEnabled()) {
         logger.trace(
               "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
      }
      HandlerExecutionChain handler = hm.getHandler(request);
      if (handler != null) {
         return handler;
      }
   }
   return null;
}

这里的handlerMapping的数据结构其实是个List,里面保存了当前servlet处理请求的映射；

/** List of HandlerMappings used by this servlet */
private List handlerMappings;

根据handler再得到HandlerAdapter，HandlerAdapter是一个接口，其中的handle方法如下所示

/**
 * Use the given handler to handle this request.
 * The workflow that is required may vary widely.
 * @param request current HTTP request
 * @param response current HTTP response
 * @param handler handler to use. This object must have previously been passed
 * to the {@code supports} method of this interface, which must have
 * returned {@code true}.
 * @throws Exception in case of errors
 * @return ModelAndView object with the name of the view and the required
 * model data, or {@code null} if the request has been handled directly
 */
ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

处理对应的request请求，返回一个视图；这里就是http请求的处理逻辑；

参考文章:

https://www.cnblogs.com/fangjian0423/p/springMVC-directory-summary.html