WebApplicationInitializer

ServletContainInitalizer是Java EE 6中Servlet 3.0的新增接口；它的onStartup方法是一个web应用中我们的代码可以控制到的最早时间点。

它不需要通过web.xml部署描述符来定义，需要在/META-INF/services/javax.servlet.ServletContainerInitializer中列出具体的实现，Servlet容器在启动时会自动扫描加载它们并调用onStartUp方法。但是文件不能放在WAR文件的/META-INF/services中，而是需要放在JAR文件的/META-INF/services中，这样就很不方便。如果你使用spring的话，spring Framework提供了一个桥接口，在Spring中SpringServletContainerInitializer类实现了ServletContainerInitializer接口，Spring的JAR中列出了SpringServletContainerInitializer，如下。在SpringServletContainerInitializer中会扫描所有WebApplicationInitializer的实现，调用它们的onStartUp方法

如图，位于sping-web.jar/META-INF/services中

SpringServletContainerInitializer源码如下

/* * Copyright 2002-2014 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */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;/** * Servlet 3.0 {@link ServletContainerInitializer} designed to support code-based * configuration of the servlet container using Spring's {@link WebApplicationInitializer} * SPI as opposed to (or possibly in combination with) the traditional * {@code web.xml}-based approach. * * <h2>Mechanism of Operation</h2> * This class will be loaded and instantiated and have its {@link #onStartup} * method invoked by any Servlet 3.0-compliant container during container startup assuming * that the {@code spring-web} module JAR is present on the classpath. This occurs through * the JAR Services API {@link ServiceLoader#load(Class)} method detecting the * {@code spring-web} module's {@code META-INF/services/javax.servlet.ServletContainerInitializer} * service provider configuration file. See the * <a href="http://download.oracle.com/javase/6/docs/technotes/guides/jar/jar.html#Service%20Provider"> * JAR Services API documentation</a> as well as section <em>8.2.4</em> of the Servlet 3.0 * Final Draft specification for complete details. * * <h3>In combination with {@code web.xml}</h3> * A web application can choose to limit the amount of classpath scanning the Servlet * container does at startup either through the {@code metadata-complete} attribute in * {@code web.xml}, which controls scanning for Servlet annotations or through an * {@code <absolute-ordering>} element also in {@code web.xml}, which controls which * web fragments (i.e. jars) are allowed to perform a {@code ServletContainerInitializer} * scan. When using this feature, the {@link SpringServletContainerInitializer} * can be enabled by adding "spring_web" to the list of named web fragments in * {@code web.xml} as follows: * * <pre class="code"> * {@code * <absolute-ordering> *   <name>some_web_fragment</name> *   <name>spring_web</name> * </absolute-ordering> * }</pre> * * <h2>Relationship to Spring's {@code WebApplicationInitializer}</h2> * Spring's {@code WebApplicationInitializer} SPI consists of just one method: * {@link WebApplicationInitializer#onStartup(ServletContext)}. The signature is intentionally * quite similar to {@link ServletContainerInitializer#onStartup(Set, ServletContext)}: * simply put, {@code SpringServletContainerInitializer} is responsible for instantiating * and delegating the {@code ServletContext} to any user-defined * {@code WebApplicationInitializer} implementations. It is then the responsibility of * each {@code WebApplicationInitializer} to do the actual work of initializing the * {@code ServletContext}. The exact process of delegation is described in detail in the * {@link #onStartup onStartup} documentation below. * * <h2>General Notes</h2> * In general, this class should be viewed as <em>supporting infrastructure</em> for * the more important and user-facing {@code WebApplicationInitializer} SPI. Taking * advantage of this container initializer is also completely <em>optional</em>: while * it is true that this initializer will be loaded and invoked under all Servlet 3.0+ * runtimes, it remains the user's choice whether to make any * {@code WebApplicationInitializer} implementations available on the classpath. If no * {@code WebApplicationInitializer} types are detected, this container initializer will * have no effect. * * <p>Note that use of this container initializer and of {@code WebApplicationInitializer} * is not in any way "tied" to Spring MVC other than the fact that the types are shipped * in the {@code spring-web} module JAR. Rather, they can be considered general-purpose * in their ability to facilitate convenient code-based configuration of the * {@code ServletContext}. In other words, any servlet, listener, or filter may be * registered within a {@code WebApplicationInitializer}, not just Spring MVC-specific * components. * * <p>This class is neither designed for extension nor intended to be extended. * It should be considered an internal type, with {@code WebApplicationInitializer} * being the public-facing SPI. * * <h2>See Also</h2> * See {@link WebApplicationInitializer} Javadoc for examples and detailed usage * recommendations.<p> * * @author Chris Beams * @author Juergen Hoeller * @author Rossen Stoyanchev * @since 3.1 * @see #onStartup(Set, ServletContext) * @see WebApplicationInitializer */@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 */@Overridepublic 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);}}}