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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
1 @Order(1) 2 public class WebMvcInit extends AbstractAnnotationConfigDispatcherServletInitializer { 3 protected Class<?>[] getRootConfigClasses() { 4 return new Class[]{RootConfig.class,WebSecurityConfig.class}; 5 } 6 7 protected Class<?>[] getServletConfigClasses() { 8 return new Class[]{WebConfig.class}; 9 } 10 11 protected String[] getServletMappings() { 12 return new String[]{"/"}; 13 } 14 15 @Override 16 protected Filter[] getServletFilters() { 17 return new Filter[]{new HiddenHttpMethodFilter()}; 18 } 19 20 }
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()
1 @Override 2 protected WebApplicationContext createRootApplicationContext() { 3 Class<?>[] configClasses = getRootConfigClasses(); 4 if (!ObjectUtils.isEmpty(configClasses)) { 5 AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext(); 6 rootAppContext.register(configClasses); 7 return rootAppContext; 8 } 9 else { 10 return null; 11 } 12 }
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
1 public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer { 2 3 /** Logger available to subclasses */ 4 protected final Log logger = LogFactory.getLog(getClass()); 5 6 7 @Override 8 public void onStartup(ServletContext servletContext) throws ServletException { 9 registerContextLoaderListener(servletContext); 10 } 11 12 /** 13 * Register a {@link ContextLoaderListener} against the given servlet context. The 14 * {@code ContextLoaderListener} is initialized with the application context returned 15 * from the {@link #createRootApplicationContext()} template method. 16 * @param servletContext the servlet context to register the listener against 17 */ 18 protected void registerContextLoaderListener(ServletContext servletContext) { 19 WebApplicationContext rootAppContext = createRootApplicationContext(); 20 if (rootAppContext != null) { 21 ContextLoaderListener listener = new ContextLoaderListener(rootAppContext); 22 listener.setContextInitializers(getRootApplicationContextInitializers()); 23 servletContext.addListener(listener); 24 } 25 else { 26 logger.debug("No ContextLoaderListener registered, as " + 27 "createRootApplicationContext() did not return an application context"); 28 } 29 } 30 31 /** 32 * Create the "<strong>root</strong>" application context to be provided to the 33 * {@code ContextLoaderListener}. 34 * <p>The returned context is delegated to 35 * {@link ContextLoaderListener#ContextLoaderListener(WebApplicationContext)} and will 36 * be established as the parent context for any {@code DispatcherServlet} application 37 * contexts. As such, it typically contains middle-tier services, data sources, etc. 38 * @return the root application context, or {@code null} if a root context is not 39 * desired 40 * @see org.springframework.web.servlet.support.AbstractDispatcherServletInitializer 41 */ 42 protected abstract WebApplicationContext createRootApplicationContext(); 43 44 /** 45 * Specify application context initializers to be applied to the root application 46 * context that the {@code ContextLoaderListener} is being created with. 47 * @since 4.2 48 * @see #createRootApplicationContext() 49 * @see ContextLoaderListener#setContextInitializers 50 */ 51 protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() { 52 return null; 53 } 54 55 }
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
1 @HandlesTypes(WebApplicationInitializer.class) 2 public class SpringServletContainerInitializer implements ServletContainerInitializer { 3 @Override 4 public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) 5 throws ServletException { 6 7 List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>(); 8 9 if (webAppInitializerClasses != null) { 10 for (Class<?> waiClass : webAppInitializerClasses) { 11 // Be defensive: Some servlet containers provide us with invalid classes, 12 // no matter what @HandlesTypes says... 13 if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && 14 WebApplicationInitializer.class.isAssignableFrom(waiClass)) { 15 try { 16 initializers.add((WebApplicationInitializer) waiClass.newInstance()); 17 } 18 catch (Throwable ex) { 19 throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex); 20 } 21 } 22 } 23 } 24 25 if (initializers.isEmpty()) { 26 servletContext.log("No Spring WebApplicationInitializer types detected on classpath"); 27 return; 28 } 29 30 servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath"); 31 AnnotationAwareOrderComparator.sort(initializers); 32 for (WebApplicationInitializer initializer : initializers) { 33 initializer.onStartup(servletContext); 34 } 35 } 36 37 }
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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原文地址：http://www.cnblogs.com/scau-chm/p/5929968.html
