为了实现不通过web.xml 启动JavaEE项目,容器提供了javax.servlet.ServletContainerInitializer。第三方的应用需要基于SPI机制,来实现javax.servlet.ServletContainerInitializer 接口。也就是需要在对应的jar包的META-INF/services 目录创建一个名为javax.servlet.ServletContainerInitializer的文件,文件内容指定具体的ServletContainerInitializer实现类,那么,当web容器启动时就会运行这个初始化器做一些组件内的初始化工作。
一般伴随着ServletContainerInitializer一起使用的还有HandlesTypes注解,通过HandlesTypes可以将感兴趣的一些类注入到ServletContainerInitializerde的onStartup方法作为参数传入。
自定义实现
1. 自定义SpringServletContainerInitializer 类
@HandlesTypes(WebApplicationInitializer.class) public class SpringServletContainerInitializer implements ServletContainerInitializer { @Override public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext ctx) throws ServletException { List<WebApplicationInitializer> initializers = new LinkedList<>(); if (webAppInitializerClasses != null) { for (Class<?> waiClass : webAppInitializerClasses) { // TODO } } } }
2. 在MEAT-INF中添加实现类信息。
Spring 中的实现
1. 定义实现ServletContainerInitializer的类
@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); } } }
2. META-INF 中添加
总结
Spring 提供的SpringServletContainerInitializer 是ServletContainerInitializer 接口真正的实现类,但是Spring另外定义了WebApplicationInitializer 接口作为真正的启动接口,Spring 对该接口的实现都是抽象类,因此在上面方法中过滤时,都被丢弃。
引用
- http://blog.csdn.net/wangyangzhizhou/article/details/52013779