SpringMVC 源码之 DispatcherServlet 启动初始化解析

上篇我们详解了 ContextLoaderListener启动过程，这次我们详解DispatcherServlet,先看web.xml 中的相关配置

   springmvc
   org.springframework.web.servlet.DispatcherServlet
   
      contextConfigLocation
      classpath:config/springMVC/spring-*.xml
   
   1




   springmvc
   /

先解释参数含义：DispatcherServlet 核心servlet 类；contextConfigLocation 是springmvc容器加载配置默认的地方，对应的param-value 值是配置文件的地址；load-on-startup，该标签的作用是当 Tomcat 服务器发布 Web 项目时创建 DispatcherServlet 对象并执行起父类的 init 方法，load-on-startup>x中x的取值1，2，3，4，5代表的是优先级，1时优先级最高。url-pattern：过滤所有地址。

1、web服务启动时，同时初始化 DispatcherServlet 父类的init ，因为容器启动首先是启动servlet，DispatcherServlet继承和实现关系如下：

根据以上关系找到init方法，

package javax.servlet;

import java.io.IOException;

public interface Servlet {
    // Ctrl+t 找到实现这个方法的实现类
    void init(ServletConfig var1) throws ServletException;

    ServletConfig getServletConfig();

    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    String getServletInfo();

    void destroy();
}

2、来到GenericServlet 类中 init方法

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {

public void init(ServletConfig config) throws ServletException {
    this.config = config;
    this.init();//模板设计模式，子类继续实现，ctrl+t 进入
}

3、来到HttpServletBean 中的 init 方法，注意：HttpServlet继承GenericServlet 类 

public abstract class HttpServletBean extends HttpServlet implements EnvironmentCapable, EnvironmentAware {

@Override
public final void init() throws ServletException {

   // Set bean properties from init parameters.
   PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
   if (!pvs.isEmpty()) {
      try {
         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) {
         if (logger.isErrorEnabled()) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
         }
         throw ex;
      }
   }

   // Let subclasses do whatever initialization they like.
   //核心方法，点击进入子类实现的方法
   initServletBean();
}

4、来到FrameworkServlet  的initServletBean方法

public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {

@Override
protected final void initServletBean() throws ServletException {
   getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
   if (logger.isInfoEnabled()) {
      logger.info("Initializing Servlet '" + getServletName() + "'");
   }
   long startTime = System.currentTimeMillis();

   try {
      //核心方法初始化容器，重点看
      this.webApplicationContext = initWebApplicationContext();
      initFrameworkServlet();
   }
   catch (ServletException | RuntimeException ex) {
      logger.error("Context initialization failed", ex);
      throw ex;
   }

   if (logger.isDebugEnabled()) {
      String value = this.enableLoggingRequestDetails ?
            "shown which may lead to unsafe logging of potentially sensitive data" :
            "masked to prevent unsafe logging of potentially sensitive data";
      logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
            "': request parameters and headers will be " + value);
   }

   if (logger.isInfoEnabled()) {
      logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
   }
}

点击来到

protected WebApplicationContext initWebApplicationContext() {
   //这里会从servletContext中获取到父容器，就是通过监听器加载的容器
   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);
            }
            //容器加载配置文件和初始化，重点看，点击进入
            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.
      synchronized (this.onRefreshMonitor) {
         onRefresh(wac);//重要初始化，ctrl+t 点击进入子类
      }
   }

   if (this.publishContext) {
      // Publish the context as a servlet context attribute.
      String attrName = getServletContextAttributeName();
      getServletContext().setAttribute(attrName, wac);
   }

   return wac;
}

点击进入：

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);
   wac.refresh();//初始化入口，ctrl+t 进入，后就一目了然。
}

5、onRefresh(wac);ctrl+t 点击进入核心子类DispatcherServlet ，

public class DispatcherServlet extends FrameworkServlet {

@Override
protected void onRefresh(ApplicationContext context) {
   initStrategies(context);//点击进入
}

来到initStrategies方法

protected void initStrategies(ApplicationContext context) {
   initMultipartResolver(context);//初始化文件上传处理类
   initLocaleResolver(context);//初始化本地化Resolver
   initThemeResolver(context);//初始化主题Resolver
   initHandlerMappings(context);//初始化一些个与处理的HandlerMappings
   initHandlerAdapters(context);
   initHandlerExceptionResolvers(context);//初始化异常处理的handler
   initRequestToViewNameTranslator(context);//初始化请求路径转换为ViewName 的Translator
//初始化ViewResolvers 这个就是针对视图处理的Resolvers 比如jsp处理Resolvers 或者freemarker处理Resolvers
   initViewResolvers(context);
   initFlashMapManager(context);
}

6，这是DispatcherServlet 启动的整个流程，下篇我们详解 http请求 到 DispatcherServlet 的过程，会更精彩！