WebApplicationInitializer (spring 3.x.x以上版本)
实现WebApplicationinitializer接口的类都可以在web应用程序启动时被加载。
那么来想一个问题：为什么实现了WebApplicationInitializer这个接口后，onStartup方法就会自动执行？
我们来简单分析一下它的实现原理，下面先贴上WebApplicationInitializer接口相关代码：
servlet的ServletContainerInitializer接口：
package javax.servlet; import java.util.Set; public interface ServletContainerInitializer { /** * Receives notification during startup of a web application of the classes * within the web application that matched the criteria defined via the * {@link javax.servlet.annotation.HandlesTypes} annotation. * * @param c The (possibly null) set of classes that met the specified * criteria * @param ctx The ServletContext of the web application in which the * classes were discovered * * @throws ServletException If an error occurs */ void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException; }
SpringServletContainerInitializer 类（从本类开始以下代码都是spring的代码）：
package org.springframework.web; import java.lang.reflect.Modifier; import java.util.LinkedList; import java.util.List; import java.util.ServiceLoader; import java.util.Set; import javax.servlet.ServletContainerInitializer; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.annotation.HandlesTypes; import org.springframework.core.annotation.AnnotationAwareOrderComparator; @HandlesTypes(WebApplicationInitializer.class) public class SpringServletContainerInitializer implements ServletContainerInitializer { /** * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer} * implementations present on the application classpath. * * <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)}, * Servlet 3.0+ containers will automatically scan the classpath for implementations * of Spring's {@code WebApplicationInitializer} interface and provide the set of all * such types to the {@code webAppInitializerClasses} parameter of this method. * * <p>If no {@code WebApplicationInitializer} implementations are found on the * classpath, this method is effectively a no-op. An INFO-level log message will be * issued notifying the user that the {@code ServletContainerInitializer} has indeed * been invoked but that no {@code WebApplicationInitializer} implementations were * found. * * <p>Assuming that one or more {@code WebApplicationInitializer} types are detected, * they will be instantiated (and <em>sorted</em> if the @{@link * org.springframework.core.annotation.Order @Order} annotation is present or * the {@link org.springframework.core.Ordered Ordered} interface has been * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)} * method will be invoked on each instance, delegating the {@code ServletContext} such * that each instance may register and configure servlets such as Spring's * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener}, * or any other Servlet API componentry such as filters. * * @param webAppInitializerClasses all implementations of * {@link WebApplicationInitializer} found on the application classpath * @param servletContext the servlet context to be initialized * @see WebApplicationInitializer#onStartup(ServletContext) * @see AnnotationAwareOrderComparator */ @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; } AnnotationAwareOrderComparator.sort(initializers); servletContext.log("Spring WebApplicationInitializers detected on classpath: " + initializers); for (WebApplicationInitializer initializer : initializers) { initializer.onStartup(servletContext); } } }
WebApplicationInitializer接口(本文讨论的接口)：
package org.springframework.web; import javax.servlet.ServletContext; import javax.servlet.ServletException public interface WebApplicationInitializer { /** * Configure the given {@link ServletContext} with any servlets, filters, listeners * context-params and attributes necessary for initializing this web application. See * examples {@linkplain WebApplicationInitializer above}. * @param servletContext the {@code ServletContext} to initialize * @throws ServletException if any call against the given {@code ServletContext} * throws a {@code ServletException} */ void onStartup(ServletContext servletContext) throws ServletException; }
javax.servlet.ServletContainerInitializer文件（在路径：spring-framework/spring-web/src/main/resources/META-INF/services/下）
该文件文件名就是“javax.servlet.ServletContainerInitializer”，其内容只有下面这行
并且必须在 spring-web-X.X.X.RELEASE-sources.jar 文件下，的META_INF/services路径下有个名为“ javax.servlet.ServletContainerInitializer”的文件，文件中的内容与 ServletContainerIntializer的实现类全限定名相同(请参考上面贴出的代码)。这样，web应用启动时，程序就会找到WebApplicationInitializer这个接口的实现类，并调用onStartup方法。
如果说得不对的地方，请指正，谢谢
参考：
http://blog.csdn.net/sunhuwh/article/details/25441217
http://blog.csdn.net/sunhuwh/article/details/25504061
spring源码：https://github.com/spring-projects/spring-framework
