Spring MVC系列（6）-DispatcherServlet初始化源码分析

怼烎@ 2023-10-10 13:22 37阅读 0赞

### 前言 ###

我们知道servlet程序需要使用servlet容器运行，那么DispatcherServlet是如何在spring boot内嵌tomcat容器进行初始化的呢？

### 1. 启动tomcat ###

首先点击启动类，启动spring boot 程序，然后进入了ServletWebServerApplicationContext类的createWebServer方法，开始创建一个web服务器。

private void createWebServer() {
        
            WebServer webServer = this.webServer;
            ServletContext servletContext = this.getServletContext();
            if (webServer == null && servletContext == null) {
        
                StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
                // 1. 获取容器工厂
                ServletWebServerFactory factory = this.getWebServerFactory();
                createWebServer.tag("factory", factory.getClass().toString());
                // 2. 使用工厂获取容器
                this.webServer = factory.getWebServer(new ServletContextInitializer[]{
        this.getSelfInitializer()});
                createWebServer.end();
                this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
                this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
            } else if (servletContext != null) {
        
                try {
        
                    this.getSelfInitializer().onStartup(servletContext);
                } catch (ServletException var5) {
        
                    throw new ApplicationContextException("Cannot initialize servlet context", var5);
                }
            }
    		// 3. 初始化PropertySources配置
            this.initPropertySources();
        }

可以看到这段代码走完后，打印了tomcat启动相关日志。  
![在这里插入图片描述][a1f80ba4b6674c8aa07f43a23e5eca9a.png]  
之后进入StandardWrapper的initServlet方法，开始初始化我们的Servlet。该方法的参数Servlet，就是DispatcherServlet，是由springboot自动配置功能事先注入到了IOC容器中的。

initServlet最终调用DispatcherServlet的init方法，并将当ServletConfig传递了过去。

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16]

### 2. 初始化WebApplicationContext ###

在上面最后的init方法中，DispatcherServlet调用的是父类HttpServletBean的init()方法，该方法最后又调用了initServletBean() 方法。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 1]  
initServletBean() 方法调用的是父类FrameworkServlet。

protected final void initServletBean() throws ServletException {
        
        	// 1. 打印日志 Initializing Spring DispatcherServlet 'dispatcherServlet'
            this.getServletContext().log("Initializing Spring " + this.getClass().getSimpleName() + " '" + this.getServletName() + "'");
            if (this.logger.isInfoEnabled()) {
        
            // 2. 打印日志 Initializing Servlet 'dispatcherServlet'
                this.logger.info("Initializing Servlet '" + this.getServletName() + "'");
            }
    
            long startTime = System.currentTimeMillis();
    
            try {
        
            	// 3. 初始化WebApplicationContext
                this.webApplicationContext = this.initWebApplicationContext();
                // 4. 初始化FrameworkServlet
                this.initFrameworkServlet();
            } catch (RuntimeException | ServletException var4) {
        
                this.logger.error("Context initialization failed", var4);
                throw var4;
            }
    
            if (this.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";
                this.logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails + "': request parameters and headers will be " + value);
            }
    
            if (this.logger.isInfoEnabled()) {
        
                this.logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
            }
    
        }

在initServletBean() 方法中，首先会调用FrameworkServlet类的initWebApplicationContext()方法，初始化WebApplicationContext。

protected WebApplicationContext initWebApplicationContext() {
        
       		// 1. 查询WebApplicationContext 
            WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(this.getServletContext());
            WebApplicationContext wac = null;
            if (this.webApplicationContext != null) {
        
            	// 2. 已创建WebApplicationContext 
                wac = this.webApplicationContext;
                if (wac instanceof ConfigurableWebApplicationContext) {
        
                    ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)wac;
                    if (!cwac.isActive()) {
        
                        if (cwac.getParent() == null) {
        
                            cwac.setParent(rootContext);
                        }
    
                        this.configureAndRefreshWebApplicationContext(cwac);
                    }
                }
            }
    		// 3.再查询一遍，看其父类的是否已经注册了对应的context.
            if (wac == null) {
        
                wac = this.findWebApplicationContext();
            }
    		// 4. 实在没找到，创建一个
            if (wac == null) {
        
                wac = this.createWebApplicationContext(rootContext);
            }
    		// 5. 刷新容器
            if (!this.refreshEventReceived) {
        
                synchronized(this.onRefreshMonitor) {
        
                    this.onRefresh(wac);
                }
            }
    
            if (this.publishContext) {
        
                String attrName = this.getServletContextAttributeName();
                this.getServletContext().setAttribute(attrName, wac);
            }
    
            return wac;
        }

如果不存在，就会创建一个WebApplicationContext 。

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
        
        Class<?> contextClass = getContextClass();
        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");
        }
        // 1. 通过反射创建 IOC 容器对象
        ConfigurableWebApplicationContext wac =
            (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    
        wac.setEnvironment(getEnvironment());
        // 2. 设置父容器
        wac.setParent(parent);
        String configLocation = getContextConfigLocation();
        if (configLocation != null) {
        
            wac.setConfigLocation(configLocation);
        }
        configureAndRefreshWebApplicationContext(wac);
    
        return wac;
    }

最后看下我们创建的WebApplicationContext。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 2]

#### 3. 刷新容器 ####

FrameworkServlet中的initWebApplicationContext创建WebApplicationContext后，刷新容器，调用onRefresh(wac)，此方法在DispatcherServlet中进行了重写，调用了initStrategies(context)方法，初始化策略，即初始化DispatcherServlet的各个组件。

protected void initStrategies(ApplicationContext context) {
        
       initMultipartResolver(context);
       initLocaleResolver(context);
       initThemeResolver(context);
       initHandlerMappings(context);
       initHandlerAdapters(context);
       initHandlerExceptionResolvers(context);
       initRequestToViewNameTranslator(context);
       initViewResolvers(context);
       initFlashMapManager(context);
    }

最终回到FrameworkServlet的initServletBean方法，打印日志。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 3]  
控制台日志如下，Completed initialization表示我们DispatcherServlet初始化完成了。  
![在这里插入图片描述][4c89b7447bf34307bddc79d87e37413a.png]

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[4c89b7447bf34307bddc79d87e37413a.png]: https://img-blog.csdnimg.cn/4c89b7447bf34307bddc79d87e37413a.png