详解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。

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