在使用SpringMVC时会在Web.xml配置如下代码：
<!-- 加入SpringMVC配置 begin-->
<servlet>
<servlet-name>dispatcher</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:/dispatcher-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>dispatcher</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
<!-- end -->
先看看DispatcherServlet类的继承关系
DispatcherServlet类的继承关系
作为Servlet，DispatcherServlet的启动与Servlet的启动过程是想联系的，在Servlet启动的过程中,Servlet的init()方法会被调用，以进行初始化，首先会调用HttpServletBean的init（）方法，因为HttpServletBean继承自HttpServlet
- 运用了依赖注入思想完成了<init-param>配置元素的读取
public void setContextConfigLocation(String contextConfigLocation) {
this.contextConfigLocation = contextConfigLocation;
}
读取在web.xml配置的 classpath:/dispatcher-servlet.xml
- HttpServletBean类init（）方法
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}
// 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;
}
// 让子类去初始化
initServletBean();
if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}
- initServletBean的初始化过程在FrameworkServlet中完成
@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 {
//初始化WebApplicationContext上下文
this.webApplicationContext = initWebApplicationContext();
//空实现
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");
}
}
protected WebApplicationContext initWebApplicationContext() {
//调用WebApplicationContextUtils静态类得到跟上下文，这个跟上下文是保存在ServletContext中的
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
//如果webApplicationContext已经不为空，表示这个Servlet类是通过编程式注册到容器中的（Servlet 3.0+中的ServletContext.addServlet() ），上下文也由编程式传入。
//若这个传入的上下文还没被初始化，将rootContext上下文设置为它的父上下文，然后将其初始化，否则直接使用
if (this.webApplicationContext != null) {
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
if (cwac.getParent() == null) {
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// 此时以contextAttribute属性的值为键，在ServletContext中查找上下文，查找得到，说明上下文已经以别的方式初始化并注册在contextAttribute下，直接使用
wac = findWebApplicationContext();
}
if (wac == null) {
//没有上下文实例定义servlet ->创建一个
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
//刷新上下文
onRefresh(wac);
}
//把当前创建的上下文保存到ServletContext中去，使用的属性名是和当前Servlet名相关的
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 + "]");
}
}
//返回当前创建的上下文
return wac;
}
那么这个MVC的上下文就建立起来了，具体取得跟上下文的过程在WebApplicationContextUtils中实现的这个根上下文是在ContextLoader设置到ServletContext中去的,使用的属性是ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE，这个根上下文是DispatherServlet建立的上下文的父上下文，所以根上下管理的Bean也是可以被DispatherServlet的上下文使用的。通过getBean向Ioc容器获取Bean时，容器会先到他的父Ioc容器中获得getBean
- WebApplicationContextUtils.getWebApplicationContext
public static WebApplicationContext getWebApplicationContext(ServletContext sc) {
return getWebApplicationContext(sc, WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
}
public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
Assert.notNull(sc, "ServletContext must not be null");
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;
}
- FrameworkServlet的createWebApplicationContext()方法
protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
// getContextClass()--> XmlWebApplicationContext.class
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");
}
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
wac.setEnvironment(getEnvironment());
//设置父上下文
wac.setParent(parent);
wac.setConfigLocation(getContextConfigLocation());
//刷新上下文
configureAndRefreshWebApplicationContext(wac);
return wac;
}
