SpringMVC启动流程源码分析

た入场券 2022-11-15 12:54 197阅读 0赞

Spring MVC向WEB容器中注入了两个对象：

*  ContextLoaderListener：由AbstractContextLoaderInitializer注入。
 *  DispatcherServlet：由AbstractDispatcherServletInitializer注入。

下面分别分析这两个对象在WEB容器启动时做了什么工作？

# ContextLoaderListener #

ContextLoaderListener实现了Servlet规范中的javax.servlet.ServletContextListener，WEB容器在启动过程中会回调ServletContextListener.contextInitialized()，所以ContextLoaderListener类需要重点关注contextInitialized()。

先来看看ContextLoaderListener是怎么被加入到ServletContext中的：

> org.springframework.web.context.AbstractContextLoaderInitializer\#registerContextLoaderListener

protected void registerContextLoaderListener(ServletContext servletContext) { 
    	// 创建父容器
    	WebApplicationContext rootAppContext = createRootApplicationContext();
    	if (rootAppContext != null) { 
    		// 创建ContextLoaderListener
    		ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
    		listener.setContextInitializers(getRootApplicationContextInitializers());
    		servletContext.addListener(listener);
    	}
    	else { 
    		logger.debug("No ContextLoaderListener registered, as " +
    				"createRootApplicationContext() did not return an application context");
    	}
    }

也就是在创建ContextLoaderListener时，父容器已经创建完成，注意只是创建完成，并没有调用refresh()方法完成初始化。

> org.springframework.web.context.ContextLoaderListener\#contextInitialized

public void contextInitialized(ServletContextEvent event) { 
    	initWebApplicationContext(event.getServletContext());
    }

> org.springframework.web.context.ContextLoader\#initWebApplicationContext

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) { 
    	/** * 首先通过WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE这个String类型的静态变量获取一个父容器 * 父容器作为全局变量存储在application对象中，如果存在则有且只能有一个 * 如果在初始化父容器时发现已经存在则直接抛出异常 * 这个值在下面的代码中会放入application中 */
    	if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) { 
    		throw new IllegalStateException(
    				"Cannot initialize context because there is already a root application context present - " +
    				"check whether you have multiple ContextLoader* definitions in your web.xml!");
    	}
    
    	servletContext.log("Initializing Spring root WebApplicationContext");
    	Log logger = LogFactory.getLog(ContextLoader.class);
    	if (logger.isInfoEnabled()) { 
    		logger.info("Root WebApplicationContext: initialization started");
    	}
    	long startTime = System.currentTimeMillis();
    
    	try { 
    		// Store context in local instance variable, to guarantee that
    		// it is available on ServletContext shutdown.
    		// context代表父容器，已经有值了
    		if (this.context == null) { 
    			this.context = createWebApplicationContext(servletContext);
    		}
    		if (this.context instanceof ConfigurableWebApplicationContext) { 
    			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
    			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 ->
    					// determine parent for root web application context, if any.
    					ApplicationContext parent = loadParentContext(servletContext); // null
    					cwac.setParent(parent);
    				}
    				// 调用refresh()方法完成父容器的初始化
    				configureAndRefreshWebApplicationContext(cwac, servletContext);
    			}
    		}
    		
    		// 将父容器放入到了servletContext中
    		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
    
    		ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    		if (ccl == ContextLoader.class.getClassLoader()) { 
    			currentContext = this.context;
    		}
    		else if (ccl != null) { 
    			currentContextPerThread.put(ccl, this.context);
    		}
    
    		if (logger.isInfoEnabled()) { 
    			long elapsedTime = System.currentTimeMillis() - startTime;
    			logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
    		}
    
    		return this.context;
    	}
    	catch (RuntimeException | Error ex) { 
    		logger.error("Context initialization failed", ex);
    		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
    		throw ex;
    	}
    }

总结：ContextLoaderListener主要负责父容器的初始化。

# DispatcherServlet #

先来看下DispatcherServlet的创建：

org.springframework.web.servlet.support.AbstractDispatcherServletInitializer\#onStartup

public void onStartup(ServletContext servletContext) throws ServletException { 
    	super.onStartup(servletContext); // 创建ContextLoaderListener
    	registerDispatcherServlet(servletContext); // 创建DispatcherServlet
    }
    
    protected void registerDispatcherServlet(ServletContext servletContext) { 
    	String servletName = getServletName();
    	Assert.hasLength(servletName, "getServletName() must not return null or empty");
    
    	WebApplicationContext servletAppContext = createServletApplicationContext(); // 子容器
    	Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");
    
    	// 创建DispatcherServlet
    	FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
    	Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
    	dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers()); // null
    
    	ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
    	if (registration == null) { 
    		throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
    				"Check if there is another servlet registered under the same name.");
    	}
    
    	registration.setLoadOnStartup(1);
    	registration.addMapping(getServletMappings()); // 处理哪些映射
    	registration.setAsyncSupported(isAsyncSupported()); // 默认为true，支持异步
    
    	Filter[] filters = getServletFilters(); // 过滤器
    	if (!ObjectUtils.isEmpty(filters)) { 
    		for (Filter filter : filters) { 
    			// 添加filter到servlet容器中
    			registerServletFilter(servletContext, filter);
    		}
    	}
    
    	customizeRegistration(registration);
    }
    
    protected FrameworkServlet createDispatcherServlet(WebApplicationContext servletAppContext) { 
    	return new DispatcherServlet(servletAppContext);
    }

先创建子容器，然后将子容器作为DispatcherServlet的构造方法的参数传入，注意这个子容器只是刚刚被创建，并没有调用refresh()进行初始化，那么什么时候被初始化呢？

看一下DispatcherServlet的继承关系：

![在这里插入图片描述][20210401152359165.png]

WEB容器启动后会调用Servlet的init()方法进行初始化，此方法的实现实在父类HttpServletBean中，源码如下：

org.springframework.web.servlet.HttpServletBean\#init

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.
    	// 留给子类实现
    	initServletBean();
    }

org.springframework.web.servlet.FrameworkServlet\#initServletBean

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 { 
    		// 对子容器进行初始化
    		this.webApplicationContext = initWebApplicationContext();
    		initFrameworkServlet(); // NOP
    	}
    	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");
    	}
    }

org.springframework.web.servlet.FrameworkServlet\#initWebApplicationContext

protected WebApplicationContext initWebApplicationContext() { 
    	// 从ServletContext中获取父容器
    	WebApplicationContext rootContext =
    			WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    	WebApplicationContext wac = null;
    
    	if (this.webApplicationContext != null) { 
    		wac = this.webApplicationContext; // 子容器
    		if (wac instanceof ConfigurableWebApplicationContext) { 
    			ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
    			if (!cwac.isActive()) { 
    				if (cwac.getParent() == null) { 
    					cwac.setParent(rootContext); // 父子间建立关系
    				}
    				// 调用refresh()方法对子容器进行初始化
    				configureAndRefreshWebApplicationContext(cwac);
    			}
    		}
    	}
    	if (wac == null) {  // 不会进入
    		wac = findWebApplicationContext();
    	}
    	if (wac == null) {  // 不会进入
    		wac = createWebApplicationContext(rootContext);
    	}
    
    	if (!this.refreshEventReceived) {  // refreshEventReceived=true不会进入
    		synchronized (this.onRefreshMonitor) { 
    			onRefresh(wac);
    		}
    	}
    
    	if (this.publishContext) { 
    		String attrName = getServletContextAttributeName();
    		// 将子容器也放入到ServletContext中
    		getServletContext().setAttribute(attrName, wac);
    	}
    
    	return wac;
    }

org.springframework.web.servlet.FrameworkServlet\#configureAndRefreshWebApplicationContext

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());
    	// 添加了一个监听子容器刷新的事件
    	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); // NOP
    	applyInitializers(wac);
    	// invoke refresh method
    	wac.refresh();
    }

在子容器初始化之前添加了一个监听容器刷新的事件ContextRefreshListener，当容器刷新完成后将会调用ContextRefreshListener.onApplicationEvent()方法。

从泛型来看监听的是容器刷新事件ContextRefreshedEvent，然后又会调用FrameworkServlet.onApplicationEvent()。

private class ContextRefreshListener implements ApplicationListener<ContextRefreshedEvent> { 
    
    	@Override
    	public void onApplicationEvent(ContextRefreshedEvent event) { 
    		FrameworkServlet.this.onApplicationEvent(event);
    	}
    }

org.springframework.web.servlet.FrameworkServlet\#onApplicationEvent

public void onApplicationEvent(ContextRefreshedEvent event) { 
    	this.refreshEventReceived = true;
    	synchronized (this.onRefreshMonitor) { 
    		onRefresh(event.getApplicationContext());
    	}
    }

org.springframework.web.servlet.DispatcherServlet\#onRefresh

protected void onRefresh(ApplicationContext context) { 
    	initStrategies(context);
    }
    
    protected void initStrategies(ApplicationContext context) { 
    	// 这里的各种类都会使用默认的
    	// 默认的配置位于spring-webmvc org/springframework/web/servlet/DispatcherServlet.properties
    	initMultipartResolver(context); // null
    	initLocaleResolver(context); // AcceptHeaderLocaleResolver
    	initThemeResolver(context); // FixedThemeResolver
    	initHandlerMappings(context); // BeanNameUrlHandlerMapping、RequestMappingHandlerMapping、RouterFunctionMapping
    	initHandlerAdapters(context); // HttpRequestHandlerAdapter、SimpleControllerHandlerAdapter、RequestMappingHandlerAdapter、HandlerFunctionAdapter
    	initHandlerExceptionResolvers(context); // ExceptionHandlerExceptionResolver、ResponseStatusExceptionResolver、DefaultHandlerExceptionResolver
    	initRequestToViewNameTranslator(context); // DefaultRequestToViewNameTranslator
    	initViewResolvers(context); // InternalResourceViewResolver
    	initFlashMapManager(context); // SessionFlashMapManager
    }

DispatcherServlet.properties  
的内容如下：

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.method.annotation.RequestMappingHandlerMapping,\
    	org.springframework.web.servlet.function.support.RouterFunctionMapping
    
    org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
    	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
    	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,\
    	org.springframework.web.servlet.function.support.HandlerFunctionAdapter
    
    org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\
    	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

onRefresh()方法用于初始化SpringMVC开发中的九大组件：

1.  文件上传解析器
2.  本地化解析器
3.  主题解析器
4.  HandlerMapping处理器映射器
5.  HandlerAdapter处理器适配器
6.  异常解析器
7.  请求到视图名称的转换器
8.  视图解析器
9.  flashMap管理器

[20210401152359165.png]: /images/20221022/1a3e9a7a306a4326878fbf612403eb4a.png