SpringMvc请求原理流程

springmvc是用户和服务沟通的桥梁，官网提供了springmvc的全面使用和解释：DispatcherServlet :: Spring Framework

流程

1.Tomcat启动

2.解析web.xml文件，根据servlet-class找到DispatcherServlet，根据init-param来获取spring的配置文件，spring的配置文件配置的主要内容就是参数的扫描路径(扫描Bean)和自定义Bean

   <servlet><servlet-name>mvc_shine_aa</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>classpath:mvc.xml</param-value></init-param><!-- Servlet默认懒加载，改成饿汉式加载（可选） --><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>mvc_shine_aa</servlet-name><!--<url-pattern>*.action</url-pattern>--><url-pattern>/</url-pattern></servlet-mapping>

mvc.xml文件

<!-- 注解扫描 --><context:component-scan base-package="com.qf.web"/><bean class="com.yuyu.AService"><property name="id" value="2"></property></bean>

3.创建DispatcherServlet实例，创建实例后会执行父类(FrameworkServlet)的父类(HttpServletBean)的init方法，HttpServletBean 在执行init()方法最后会执行initServletBean()方法，该方法在HttpServletBean是一个空方法，目的是让继承他的子类自己去完成接下来的初始化操作。

	@Overridepublic final void init() throws ServletException {// Set bean properties from init parameters.// 进行一些初始化操作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.// 核心代码，将剩余的初始化操作交给子类自己去实现initServletBean();}

接下来程序就走到了FrameworkServlet的initServletBean()方法，

	@Overrideprotected 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 {// 核心代码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");}}

该方法的核心就是

this.webApplicationContext = initWebApplicationContext();

这一步就是创建一个spring容器，该方法首先会尝试获取父容器，然后判断之前是否有创建过webApplicationContext容器。

	protected WebApplicationContext initWebApplicationContext() {WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());WebApplicationContext wac = null;if (this.webApplicationContext != null) {// A context instance was injected at construction time -> use itwac = 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, etcif (cwac.getParent() == null) {// The context instance was injected without an explicit parent -> set// the root application context (if any; may be null) as the parentcwac.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 idwac = findWebApplicationContext();}if (wac == null) {// 创建本地实例// No context instance is defined for this servlet -> create a local onewac = 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);}}if (this.publishContext) {// Publish the context as a servlet context attribute.String attrName = getServletContextAttributeName();getServletContext().setAttribute(attrName, wac);}return wac;}

如果之前没有创建容器，程序会走到

        if (wac == null) {
            // 核心代码，创建本地容器实例
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

通过createWebApplicationContext(rootContext)，之后就会成功创建并初始化一个spring容器了

	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");}// 创建容器实例，里面的属性都是空的 等待填充ConfigurableWebApplicationContext wac =(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);wac.setEnvironment(getEnvironment());// 设置容器的父容器wac.setParent(parent);String configLocation = getContextConfigLocation();if (configLocation != null) {wac.setConfigLocation(configLocation);}// 对容器进行初始化等操作 填充容器configureAndRefreshWebApplicationContext(wac);return wac;}

之后spring容器创建完成，就可以给外部提供访问了。

springmvc的父子容器

在前文中发现，DispatcherServlet在生成容器之前会去找一个rootContext父容器，那么这个父容器是什么呢？为什么要找父容器呢？

现在如果在配置文件中声明了两个servlet，并且对应的spring配置文件配置了不同的bean，但是扫描的bean路径都相同的话，就会出现两个DispatcherServlet容器里会有一部分重复的bean

<!-- servlet1 -->
<servlet><servlet-name>app1</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation1</param-name><param-value>spring1.xml</param-value></init-param><load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping><servlet-name>app1</servlet-name><url-pattern>/app1/*</url-pattern>
</servlet-mapping><!-- servlet2 -->
<servlet><servlet-name>app2</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation2</param-name><param-value>spring2.xml</param-value></init-param><load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping><servlet-name>app2</servlet-name><url-pattern>/app2/*</url-pattern>
</servlet-mapping>

springmvc为了解决这个问题就创造了一个父容器的概念，在springmvc官方提供的配置文件中就有一个<listener/>属性 该属性就是定义的父容器，tomcat在读取web.xml文件时，首先读取的就是<listener/>和<context-param>来创建父容器。之后再创建子容器，创建完子容器后就会将父容器放入子容器中，这样就可以避免Bean的重复创建。

<web-app><listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener><context-param><param-name>contextConfigLocation</param-name><param-value>/WEB-INF/app-context.xml</param-value></context-param><servlet><servlet-name>app</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value></param-value></init-param><load-on-startup>1</load-on-startup></servlet><servlet-mapping><servlet-name>app</servlet-name><url-pattern>/app/*</url-pattern></servlet-mapping></web-app>

springMvc的零配置

spring官方不止提供了通过配置文件来配置springMvc，还提供了配置类的形式来配置，这样就可以省略掉xml配置文件了。

public class MyWebApplicationInitializer implements WebApplicationInitializer {@Overridepublic void onStartup(ServletContext servletContext) {// Load Spring web application configurationAnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();// AppConfig对应之前的spring.xml配置文件配置包扫描路径context.register(AppConfig.class);// Create and register the DispatcherServletDispatcherServlet servlet = new DispatcherServlet(context);ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);registration.setLoadOnStartup(1);registration.addMapping("/app/*");}
}

@ComponentScan("com.yuyu")
public class AppConfig {
}

使用代码的方式来实现父子容器也非常简单

public class MyWebApplicationInitializer implements WebApplicationInitializer {@Overridepublic void onStartup(ServletContext servletContext) {// Load Spring web application configurationAnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();context.register(AppConfig.class);// 创建子容器，将context 父容器注入子容器AnnotationConfigWebApplicationContext context2 = new AnnotationConfigWebApplicationContext();context2.register(AppConfig.class);context2.setParent(context);// Create and register the DispatcherServletDispatcherServlet servlet = new DispatcherServlet(context);ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);registration.setLoadOnStartup(1);registration.addMapping("/app/*");}
}