SpringMVC 源码之 DispatcherServlet 启动初始化解析

àì夳堔傛蜴生んèń 2023-02-23 03:46 122阅读 0赞

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

<servlet>
       <servlet-name>springmvc</servlet-name>
       <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
       <init-param>
          <param-name>contextConfigLocation</param-name>
          <param-value>classpath:config/springMVC/spring-*.xml</param-value>
       </init-param>
       <load-on-startup>1</load-on-startup>
    </servlet>
    
    
    <servlet-mapping>
       <servlet-name>springmvc</servlet-name>
       <url-pattern>/</url-pattern>
    </servlet-mapping>

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</load-on-startup>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：

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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！

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