spring注解配置启动过程

　　最近看起spring源码，突然想知道没有web.xml的配置，spring是怎么通过一个继承于AbstractAnnotationConfigDispatcherServletInitializer的类来启动自己的。鉴于能力有限以及第一次看源码和发博客，不到之处请望谅~

　　我用的IDE是IntelliJ IDEA，这个比myEclipse看源码方便一点，而且黑色背景挺喜欢。然后项目是在maven下的tomcat7插件运行。spring版本是4.3.2.RELEASE。

　　如果写过纯注解配置的spring web，应该知道需要继承一个初始化类来装载bean，然后从这个类开始就会加载我们自定义的功能和bean了，下面是我的一个WebInitializer

 @Order(1)
 public class WebMvcInit extends AbstractAnnotationConfigDispatcherServletInitializer {
     protected Class<?>[] getRootConfigClasses() {
         return new Class[]{RootConfig.class,WebSecurityConfig.class};
     }

     protected Class<?>[] getServletConfigClasses() {
         return new Class[]{WebConfig.class};
     }

     protected String[] getServletMappings() {
         return new String[]{"/"};
     }

     @Override
     protected Filter[] getServletFilters() {
         return new Filter[]{new HiddenHttpMethodFilter()};
     }

 }

　　首先看下AbstractAnnotationConfigDispatcherServletInitializer类的结构，这个也是IDEA的一个uml功能，在类那里右键Diagrams->show Diagrams就有啦

　　然后我们直接点进AbstractAnnotationConfigDispatcherServletInitializer，可以看到这个类很简单，只有四个方法，然后我们关注下createRootApplicationContext()

 @Override
     protected WebApplicationContext createRootApplicationContext() {
         Class<?>[] configClasses = getRootConfigClasses();
         if (!ObjectUtils.isEmpty(configClasses)) {
             AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
             rootAppContext.register(configClasses);
             return rootAppContext;
         }
         else {
             return null;
         }
     }

　　这个方法大概意思是获取用户（程序员）传过来的RootClasses，然后注册里面的bean，这些都不是我们关注的，不过这个方法应该是要在启动后执行的，所以我们可以从这个方法往上找

　　IDEA下Ctrl+G可以找调用某个方法或类，然后设置寻找范围为project and library

　　我们找到，AbstractContextLoaderInitializer下registerContextLoaderListener(ServletContext servletContext)方法调用子类的createRootApplicationContext()获取WebApplicationContext，继续找registerContextLoaderListener(ServletContext servletContext)方法的调用者，结果发现就是该类下的onStartup(ServletContext servletContext)，下面贴下AbstractContextLoaderInitializer类

 public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {

     /** Logger available to subclasses */
     protected final Log logger = LogFactory.getLog(getClass());

     @Override
     public void onStartup(ServletContext servletContext) throws ServletException {
         registerContextLoaderListener(servletContext);
     }

     /**
      * Register a {@link ContextLoaderListener} against the given servlet context. The
      * {@code ContextLoaderListener} is initialized with the application context returned
      * from the {@link #createRootApplicationContext()} template method.
      * @param servletContext the servlet context to register the listener against
      */
     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");
         }
     }

     /**
      * Create the "<strong>root</strong>" application context to be provided to the
      * {@code ContextLoaderListener}.
      * <p>The returned context is delegated to
      * {@link ContextLoaderListener#ContextLoaderListener(WebApplicationContext)} and will
      * be established as the parent context for any {@code DispatcherServlet} application
      * contexts. As such, it typically contains middle-tier services, data sources, etc.
      * @return the root application context, or {@code null} if a root context is not
      * desired
      * @see org.springframework.web.servlet.support.AbstractDispatcherServletInitializer
      */
     protected abstract WebApplicationContext createRootApplicationContext();

     /**
      * Specify application context initializers to be applied to the root application
      * context that the {@code ContextLoaderListener} is being created with.
      * @since 4.2
      * @see #createRootApplicationContext()
      * @see ContextLoaderListener#setContextInitializers
      */
     protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
         return null;
     }

 }

　　注意的是这里我们跳过了AbstractDispatcherServletInitializer抽象类（看uml图），这个类主要配置DispatcherServlet，这里就是spring mvc等功能的实现了。

　　那谁来加载AbstractContextLoaderInitializer？WebApplicationInitializer已经是接口，不会再有一个抽象类来调用了，于是我尝试性地搜WebApplicationInitializer接口，因为spring这种大项目肯定是面向接口的，所以调用的地方一般是写接口，然后我们找到了SpringServletContainerInitializer类，它实现了ServletContainerInitializer接口，这个类大概是说把所有WebApplicationInitializer都startUp一遍，可以说这个类很接近我们的目标了。下面贴下SpringServletContainerInitializer

 @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(servletContext);
         }
     }

 }

　　在最后的foreach把所有的WebApplicationInitializer都启动一遍。那么问题来了，谁来启动SpringServletContainerInitializer，spring肯定不能自己就能启动的，在

web环境下，就只有web容器了。我们可以在上面某一个地方打个断点，然后Debug一下（事实上，完全可以全程Debug = =，这样准确又快捷，不过这样少了点寻找的意味，沿路风景还是挺不错的）

　　可以看到包org.apache.catalina.core下的StandardContext类的startInternal方法，这个已经是tomcat的范围了，所以我们的目标算是达到了。注意的是ServletContainerInitializer接口并不是spring包下的，而是javax.servlet

　　我猜测，tomcat通过javax.servlet的ServletContainerInitializer接口来找容器下实现这个接口的类，然后调用它们的OnStartUp，然后spring的SpringServletContainerInitializer就可以把所有WebApplicationInitializer都启动一遍，其中就有我们自己写的WebInitializer，另外spring security用注解配置也是实现WebApplicationInitializer启动的，所以这样spring的扩展性很强。这几天再看下tomcat源码，了解下tomcat的机制。