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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。