最近看起spring源碼，突然想知道沒有web.xml的配置，spring是怎麼通過一個繼承於AbstractAnnotationConfigDispatcherServletInitializer的類來啟動自己的。鑒於能力有限以及第一次看源碼和發博客，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，而且黑色背景挺喜歡。然後項目是在maven下的tomcat7插件運行。spring版本是4.3.2.RELEASE。
如果寫過純注解配置的spring web，應該知道需要繼承一個初始化類來裝載bean，然後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
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類的結構，這個也是IDEA的一個uml功能，在類那裡右鍵Diagrams->show Diagrams就有啦
然後我們直接點進AbstractAnnotationConfigDispatcherServletInitializer，可以看到這個類很簡單，只有四個方法，然後我們關注下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 }
這個方法大概意思是獲取用戶（程序員）傳過來的RootClasses，然後注冊裡面的bean，這些都不是我們關注的，不過這個方法應該是要在啟動後執行的，所以我們可以從這個方法往上找
IDEA下Ctrl+G可以找調用某個方法或類，然後設置尋找范圍為project and library
我們找到，AbstractContextLoaderInitializer下registerContextLoaderListener(ServletContext servletContext)方法調用子類的createRootApplicationContext()獲取WebApplicationContext，繼續找registerContextLoaderListener(ServletContext servletContext)方法的調用者，結果發現就是該類下的onStartup(ServletContext servletContext)，下面貼下AbstractContextLoaderInitializer類
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 }
注意的是這裡我們跳過了AbstractDispatcherServletInitializer抽象類（看uml圖），這個類主要配置DispatcherServlet，這裡就是spring mvc等功能的實現了。
那誰來加載AbstractContextLoaderInitializer？WebApplicationInitializer已經是接口，不會再有一個抽象類來調用了，於是我嘗試性地搜WebApplicationInitializer接口，因為spring這種大項目肯定是面向接口的，所以調用的地方一般是寫接口，然後我們找到了SpringServletContainerInitializer類，它實現了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了。下面貼下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 }
在最後的foreach把所有的WebApplicationInitializer都啟動一遍。那麼問題來了，誰來啟動SpringServletContainerInitializer，spring肯定不能自己就能啟動的，在
web環境下，就只有web容器了。我們可以在上面某一個地方打個斷點，然後Debug一下（事實上，完全可以全程Debug = =，這樣准確又快捷，不過這樣少了點尋找的意味，沿路風景還是挺不錯的）
可以看到包org.apache.catalina.core下的StandardContext類的startInternal方法，這個已經是tomcat的范圍了，所以我們的目標算是達到了。注意的是ServletContainerInitializer接口並不是spring包下的，而是javax.servlet
我猜測，tomcat通過javax.servlet的ServletContainerInitializer接口來找容器下實現這個接口的類，然後調用它們的OnStartUp，然後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用注解配置也是實現WebApplicationInitializer啟動的，所以這樣spring的擴展性很強。這幾天再看下tomcat源碼，了解下tomcat的機制。
