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 "<span>root</span>" 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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