案例: 最近在上线某活动时，之前访问正常的url更新后访问返回404。review提交日志，web.xml schema 换作3.0时无意中添加了metadata-complete=“true”，遂删除之，重启解决。

事毕，重新翻阅了一下servlet-3_0-final-spec。看到这么一段：

8.1 Annotations and pluggability

In a web application, classes using annotations will have their annotations processed only if they are located in the WEB-INF/classes directory, or if they are packaged in a jar file located in WEB-INF/lib within the application. The web application deployment descriptor contains a new “metadata-complete” attribute on the web-app element. The “metadata-complete” attribute defines whether the web descriptor is complete, or whether the class files of the jar file should be examined for annotations and web fragments at deployment time. If “metadata-complete” is set to “true”, the deployment tool MUST ignore any servlet annotations present in the class files of the application and web fragments. If the metadata-complete attribute is not specified or is set to “false”, the deployment tool must examine the class files of the application for annotations, and scan for web fragments. Following are the annotations that MUST be supported by a Servlet 3.0 compliant web container.

汗颜了，基于Anotation的注解式Servlet出来这么久，也就到今天才真正在生产环境中投入使用。