最近看起spring源碼，突然想知道沒有web.xml的配置，spring是怎么通過一個繼承于AbstractAnnotationConfigDispatcherServletInitializer的類來啟動自己的。鑒于能力有限以及第一次看源碼和發(fā)博客，不到之處請望諒~
我用的IDE是IntelliJ IDEA，這個比myEclipse看源碼方便一點(diǎn)，而且黑色背景挺喜歡。然后項(xiàng)目是在maven下的tomcat7插件運(yùn)行。spring版本是4.3.2.RELEASE。
如果寫過純注解配置的spring web，應(yīng)該知道需要繼承一個初始化類來裝載bean，然后從這個類開始就會加載我們自定義的功能和bean了，下面是我的一個WebInitializer
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@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類的結(jié)構(gòu)，這個也是IDEA的一個uml功能，在類那里右鍵Diagrams->show Diagrams就有啦
然后我們直接點(diǎn)進(jìn)AbstractAnnotationConfigDispatcherServletInitializer，可以看到這個類很簡單，只有四個方法，然后我們關(guān)注下createRootApplicationContext()
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@Override protected WebApplicationContext createRootApplicationContext() { Class<?>[] configClasses = getRootConfigClasses(); if (!ObjectUtils.isEmpty(configClasses)) { AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext(); rootAppContext.register(configClasses); return rootAppContext; } else { return null; } } |
這個方法大概意思是獲取用戶（程序員）傳過來的RootClasses，然后注冊里面的bean，這些都不是我們關(guān)注的，不過這個方法應(yīng)該是要在啟動后執(zhí)行的，所以我們可以從這個方法往上找
IDEA下Ctrl+G可以找調(diào)用某個方法或類，然后設(shè)置尋找范圍為project and library
我們找到，AbstractContextLoaderInitializer下registerContextLoaderListener(ServletContext servletContext)方法調(diào)用子類的createRootApplicationContext()獲取WebApplicationContext，繼續(xù)找registerContextLoaderListener(ServletContext servletContext)方法的調(diào)用者，結(jié)果發(fā)現(xiàn)就是該類下的onStartup(ServletContext servletContext)，下面貼下AbstractContextLoaderInitializer類
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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; }} |
注意的是這里我們跳過了AbstractDispatcherServletInitializer抽象類（看uml圖），這個類主要配置DispatcherServlet，這里就是spring mvc等功能的實(shí)現(xiàn)了。
那誰來加載AbstractContextLoaderInitializer？WebApplicationInitializer已經(jīng)是接口，不會再有一個抽象類來調(diào)用了，于是我嘗試性地搜WebApplicationInitializer接口，因?yàn)閟pring這種大項(xiàng)目肯定是面向接口的，所以調(diào)用的地方一般是寫接口，然后我們找到了SpringServletContainerInitializer類，它實(shí)現(xiàn)了ServletContainerInitializer接口，這個類大概是說把所有WebApplicationInitializer都startUp一遍，可以說這個類很接近我們的目標(biāo)了。下面貼下SpringServletContainerInitializer
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@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都啟動一遍。那么問題來了，誰來啟動SpringServletContainerInitializer，spring肯定不能自己就能啟動的，在
web環(huán)境下，就只有web容器了。我們可以在上面某一個地方打個斷點(diǎn)，然后Debug一下（事實(shí)上，完全可以全程Debug = =，這樣準(zhǔn)確又快捷，不過這樣少了點(diǎn)尋找的意味，沿路風(fēng)景還是挺不錯的）
可以看到包org.apache.catalina.core下的StandardContext類的startInternal方法，這個已經(jīng)是tomcat的范圍了，所以我們的目標(biāo)算是達(dá)到了。注意的是ServletContainerInitializer接口并不是spring包下的，而是javax.servlet
我猜測，tomcat通過javax.servlet的ServletContainerInitializer接口來找容器下實(shí)現(xiàn)這個接口的類，然后調(diào)用它們的OnStartUp，然后spring的SpringServletContainerInitializer就可以把所有WebApplicationInitializer都啟動一遍，其中就有我們自己寫的WebInitializer，另外spring security用注解配置也是實(shí)現(xiàn)WebApplicationInitializer啟動的，所以這樣spring的擴(kuò)展性很強(qiáng)。這幾天再看下tomcat源碼，了解下tomcat的機(jī)制。
以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持服務(wù)器之家。
