spring学习--DispatcherServlet工作原理
Posted GLLegolas
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在使用springmvc框架,会在web.xml文件配置一个DispatcherServlet,这正是web容器开始初始化,同时会在建立自己的上下文来持有SpringMVC的bean对象。
先从DispatcherServlet入手,从名字来看,它是一个Servlet。它的定义如下:
public class DispatcherServlet extends FrameworkServlet{
它是继承FrameworkServlet,来看一下整个的继承关系。
从继承关系来看,DispatcherServlet继承FrameworkServlet和HttpServletBean而继承HttpServlet,通过使用Servlet API 来对HTTP请求进行响应,成为SpringMVC的前端处理器。
先看一个时序图
注:作为Servlet,DispatcherServlet的启动和Servlet的启动相关联的。在Servlet初始化过程中,Servlet的init方法会被调用,以进行初始化,然而DispatcherServlet的基类,所以从HttpServletBean中的初始化过程开始。
DispatcherServlet的工作分为2部分,一部分是初始化(也就是图的上半部分),有initServletBean()启动,通过initWebApplicationContext()方法最终调用DispatcherServlet中的initStrategies()方法。另一部分(也就是图的下半部分),是对HTTP请求进行响应,作为Servlet,Web容器会调用Servlet的doGet()和doPost()方法,在经过FrameworkServlet的processRequest()简单处理后,会调用DispatcherServlet的doService方法,在这个方法调用中封装了doDispatch(),继续调用processDispatchResult方法返回调用信息。
接下来看看初始化的源码的流程:
HttpServletBean.init方法
/** * Map config parameters onto bean properties of this servlet, and * invoke subclass initialization. * @throws ServletException if bean properties are invalid (or required * properties are missing), or if subclass initialization fails. */ @Override public final void init() throws ServletException { if (logger.isDebugEnabled()) { logger.debug("Initializing servlet \'" + getServletName() + "\'"); } //获取Servlet初始化参数 // 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; } //调用子类的方法 // Let subclasses do whatever initialization they like. initServletBean(); if (logger.isDebugEnabled()) { logger.debug("Servlet \'" + getServletName() + "\' configured successfully"); } }
例如web.xml中配置参数:
<servlet> <servlet-name>TestServlet</servlet-name> <servlet-class>com.lzyer.TestServlet</servlet-class> <!--配置参数,可以通过ServletConfig获取参数--> <init-param> <param-name>servlet-name</param-name> <param-value>TestServlet</param-value> </init-param> </servlet> <servlet-mapping> <servlet-name>TestServlet</servlet-name> <url-pattern>/TestServlet</url-pattern> </servlet-mapping>
FrameworkServlet.initServletBean方法
/** * Overridden method of {@link HttpServletBean}, invoked after any bean properties * have been set. Creates this servlet\'s WebApplicationContext. */ @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 { //创建Web应用上下文 this.webApplicationContext = initWebApplicationContext(); initFrameworkServlet(); } catch (ServletException ex) { this.logger.error("Context initialization failed", ex); throw ex; } catch (RuntimeException ex) { this.logger.error("Context initialization failed", ex); throw ex; } if (this.logger.isInfoEnabled()) { long elapsedTime = System.currentTimeMillis() - startTime; this.logger.info("FrameworkServlet \'" + getServletName() + "\': initialization completed in " + elapsedTime + " ms"); } }
FrameworkServlet.initWebApplicationContext()
/** * Initialize and publish the WebApplicationContext for this servlet. * <p>Delegates to {@link #createWebApplicationContext} for actual creation * of the context. Can be overridden in subclasses. * @return the WebApplicationContext instance * @see #FrameworkServlet(WebApplicationContext) * @see #setContextClass * @see #setContextConfigLocation */ 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; }
先看WebApplicationContextUtils.getWebApplicationContext(getServletContext()),,主要是从ServletContext获取WebApplicationContext
public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) { Assert.notNull(sc, "ServletContext must not be null");<br> //从ServletContext中获取 Object attr = sc.getAttribute(attrName); if (attr == null) { return null; } if (attr instanceof RuntimeException) { throw (RuntimeException) attr; } if (attr instanceof Error) { throw (Error) attr; } if (attr instanceof Exception) { throw new IllegalStateException((Exception) attr); } if (!(attr instanceof WebApplicationContext)) { throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr); } return (WebApplicationContext) attr; }
findWebApplicationContext和上面一样从ContextServlet中查找,如果不存在就调用下面的createWebApplicationContext方法。
protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {<br> //获取contextClass Class<?> contextClass = getContextClass(); if (this.logger.isDebugEnabled()) { this.logger.debug("Servlet with name \'" + getServletName() + "\' will try to create custom WebApplicationContext context of class \'" + contextClass.getName() + "\'" + ", using parent context [" + parent + "]"); } 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"); }<br> //通过反射获取实例 ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); wac.setEnvironment(getEnvironment());<br> //设置双亲上下文 wac.setParent(parent); wac.setConfigLocation(getContextConfigLocation()); //配置和刷新应用 configureAndRefreshWebApplicationContext(wac); return wac; }
看一下getContextClass()到底是哪个类? XmlWebApplicationContext.class
最后configureAndRefreshWebApplicationContext调用refresh方法启动容器。
回到initWebApplicationContext方法中
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);
这个会触发SpringMVC初始化策略
/** * This implementation calls {@link #initStrategies}. */ @Override protected void onRefresh(ApplicationContext context) { initStrategies(context); } /** * Initialize the strategy objects that this servlet uses. * <p>May be overridden in subclasses in order to initialize further strategy objects. */ protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context);<br> //映射关系 initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); }
到此,SpringMVC的初始化的流程大概就是这样,下篇就是SpringMVC请求流程。
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