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

 




org.springframework.web.SpringServletContainerInitializer


 

至此，相关代码/文件，都在上面给出了。

       那么请看SpringServletContainerInitializer类，它实现了 Servlet 的ServletContainerInitializer接口，并且有"@HandlesTypes(WebApplicationInitializer.class) "注解，这个注解指明了ServletContainerInitializer接口可以使用的类。


     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