传统的使用SpringMVC的方法是在web.xml中配置DispatchServlet，在SpringMVC 3.1版本后提供了一种新的启动方式。在介绍这种新的方式之前，有必要先介绍一下Servlet 3.0的一个新特性 ServletContainerInitializer 。先看一下javadoc对该接口的介绍
实现该接口的类会成为web应用的入口，并且在启动期间可以注入servlets，filters，和listeners。
该接口的实现类可能会包含 **javax.servlet.annotation.HandlesTypes** 注解，该注解的value对应的Class<?> []， 用于表示该接口的实现类的onStartup方法会接收Class<?> []对应的实现或者子类作为参数。
如果该接口的实现类不包含 **javax.servlet.annotation.HandlesTypes** 注解，或者应用中没有该注解中对应的Class，则需要传递null给onStartup方法。
如果实现了该接口则需要在jar的META-INF/services目录下放置一个文件，改文件的名称必须是该接口的全名称。
从上面javadoc的注释可以看出，当应用启动的时候，如果有 ServletContainerInitializer 的实现类并且有相应的配置，则会使用该类作为web的入口。
SpringMVC新的启动方式正是基于上面提到的 ServletContainerInitializer 接口，其实现类是 SpringServletContainerInitializer ，下面看看该类的实现，
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();
if (webAppInitializerClasses != null) {
for (Class<?> waiClass : webAppInitializerClasses) {
// Be defensive: Some servlet containers provide us with invalid classes,
// no matter what @HandlesTypes says...
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
initializers.add((WebApplicationInitializer) waiClass.newInstance());
}
catch (Throwable ex) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
}
}
}
}
if (initializers.isEmpty()) {
servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
return;
}
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
for (WebApplicationInitializer initializer : initializers) {
initializer.onStartup(servleContext);
}
}
}
上面的代码功能比较明确，针对每一个传入的 WebApplicationInitializer 的实现类进行实例化，然后将这些实例化的对象进行排序操作。排序的规则是
如果类带有 org.springframework.core.annotation.Order 标签，则获取对应的Order值，如果类实现 org.springframework.core.Ordered 接口，则调用getOrder方法获取Order值，如果都没有则为最低Order值。获取所有实例的Order值后，根据Order值进行排序。
排序好后，依次调用实例的onStartup方法进行初始化。
SpringServletContainerInitialize 作为SpringMVC初始化的入口，最终的实现依赖于 WebApplicationInitializer 的实现类 AbstractAnnotationConfigDispatcherServletInitializer ，
public abstract class AbstractAnnotationConfigDispatcherServletInitializer
extends AbstractDispatcherServletInitializer {
/**
* {@inheritDoc}
* <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},
* providing it the annotated classes returned by {@link #getRootConfigClasses()}.
* Returns {@code null} if {@link #getRootConfigClasses()} returns {@code null}.
*/
@Override
protected WebApplicationContext createRootApplicationContext() {
Class<?>[] configClasses = getRootConfigClasses();
if (!ObjectUtils.isEmpty(configClasses)) {
AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
rootAppContext.register(configClasses);
return rootAppContext;
}
else {
return null;
}
}
/**
* {@inheritDoc}
* <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},
* providing it the annotated classes returned by {@link #getServletConfigClasses()}.
*/
@Override
protected WebApplicationContext createServletApplicationContext() {
AnnotationConfigWebApplicationContext servletAppContext = new AnnotationConfigWebApplicationContext();
Class<?>[] configClasses = getServletConfigClasses();
if (!ObjectUtils.isEmpty(configClasses)) {
servletAppContext.register(configClasses);
}
return servletAppContext;
}
/**
* Specify {@link org.springframework.context.annotation.Configuration @Configuration}
* and/or {@link org.springframework.stereotype.Component @Component} classes to be
* provided to the {@linkplain #createRootApplicationContext() root application context}.
* @return the configuration classes for the root application context, or {@code null}
* if creation and registration of a root context is not desired
*/
protected abstract Class<?>[] getRootConfigClasses();
/**
* Specify {@link org.springframework.context.annotation.Configuration @Configuration}
* and/or {@link org.springframework.stereotype.Component @Component} classes to be
* provided to the {@linkplain #createServletApplicationContext() dispatcher servlet
* application context}.
* @return the configuration classes for the dispatcher servlet application context or
* {@code null} if all configuration is specified through root config classes.
*/
protected abstract Class<?>[] getServletConfigClasses();
}
该类提供了两个抽象方法 getRootConfigClasses 和 getServletConfigClasses ，前者用于获取创建RootApplicationContext的配置，后者用于获取创建ServletApplicationContext的配置。另外两个方法 createRootApplicationContext ，createServletApplicationContext 用于创建RootApplicationContext和ServletApplicationContext。这个类的入口onStartup在其父类 AbstractDispatcherServletInitializer 中。
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
super.onStartup(servletContext);
registerDispatcherServlet(servletContext);
}
onStartup会首先调用其父类 AbstractContextLoaderInitializer 的onStartup方法进行初始化，
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
registerContextLoaderListener(servletContext);
}
protected void registerContextLoaderListener(ServletContext servletContext) {
WebApplicationContext rootAppContext = createRootApplicationContext();
if (rootAppContext != null) {
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");
}
}
AbstractContextLoaderInitializer 的onStartup主要用于初始化RootApplicationContext。在RootApplicationContext初始化好后，AbstractDispatcherServletInitializer 会继续初始化DispatcherServlet。
protected void registerDispatcherServlet(ServletContext servletContext) {
String servletName = getServletName();
Assert.hasLength(servletName, "getServletName() must not return empty or null");
WebApplicationContext servletAppContext = createServletApplicationContext();
Assert.notNull(servletAppContext,
"createServletApplicationContext() did not return an application " +
"context for servlet [" + servletName + "]");
FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());
ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
Assert.notNull(registration,
"Failed to register servlet with name '" + servletName + "'." +
"Check if there is another servlet registered under the same name.");
registration.setLoadOnStartup(1);
registration.addMapping(getServletMappings());
registration.setAsyncSupported(isAsyncSupported());
Filter[] filters = getServletFilters();
if (!ObjectUtils.isEmpty(filters)) {
for (Filter filter : filters) {
registerServletFilter(servletContext, filter);
}
}
customizeRegistration(registration);
}
registerDispatcherServlet 调用 createServletApplicationContext 创建ServletApplicationContext，然后创建DispatcherServlet，设置ServletMapping，然后注册Filter。
经过上面的流程，SpringMVC的加载过程就完成了后续的请求处理就和通过web.xml启动的SpringMVC一样了。