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Eclipse Java EE IDE for Web Developers 创建的 Dynamic Web project 编译后的class文件默认存放路径问题，默认存放路径是build/classes导致应用部署后，Tomcat 抛出异常：
<!-- lang: java -->
The value for the useBean class attribute XXX is invalid
Java Servlet 3.0 Specification（java Servlet 3.0 规范是这样描述的）
10.5 Directory Structure A Web application exists as a structured hierarchy of directories. The root of this hierarchy serves as the document root for files that are part of the application. For example, for a Web application with the context path /catalog in a Web container, the index.html file at the base of the Web application hierarchy or in a JAR file inside WEB-INF/lib that includes the index.html under META-INF/resources directory can be served to satisfy a request from /catalog/index.html. If an index.html is present both in the root context and in the META-INF/resources directory of a JAR file in the WEB-INF/lib directory of the application, then the file that is available in the root context MUST be used. The rules for matching URLs to context path are laid out in Chapter 12, “Mapping Requests to Servlets”. Since the context path of an application determines the URL namespace of the contents of the Web application, Web containers must reject Web applications defining a context path that could cause potential conflicts in this URL namespace. This may occur, for example, by attempting to deploy a second Web application with the same context path. Since requests are matched to resources in a case-sensitive manner, this determination of potential conflict must be performed in a case-sensitive manner as well. A special directory exists within the application hierarchy named “WEB-INF”. This directory contains all things related to the application that aren’t in the document root of the application. Most of the WEB-INF node is not part of the public document tree of the application. Except for static resources and JSPs packaged in the META- INF/resources of a JAR file that resides in the WEB-INF/lib directory, no other files contained in the WEB-INF directory may be served directly to a client by the container. However, the contents of the WEB-INF directory are visible to servlet code using the getResource and getResourceAsStream method calls on the ServletContext, and may be exposed using the RequestDispatcher calls. Hence, if the Application Developer needs access, from servlet code, to application specific configuration information that he does not wish to be exposed directly to the Web client, he may place it under this directory. Since requests are matched to resource mappings in a case-sensitive manner, client requests for ‘/WEB-INF/foo’, ‘/WEb- iNf/foo’, for example, should not result in contents of the Web application located under /WEB-INF being returned, nor any form of directory listing thereof. The contents of the WEB-INF directory are: ■ The /WEB-INF/web.xml deployment descriptor. ■ The /WEB-INF/classes/ directory for servlet and utility classes. The classes in this directory must be available to the application class loader. ■ The /WEB-INF/lib/*.jar area for Java ARchive files. These files contain servlets, beans, static resources and JSPs packaged in a JAR file and other utility classes useful to the Web application. The Web application class loader must be able to load classes from any of these archive files. The Web application class loader must load classes from the WEB-INF/classes directory first, and then from library JARs in the WEB-INF/lib directory. Also, except for the case where static resources are packaged in JAR files, any requests from the client to access the resources in WEB-INF/ directory must be returned with a SC_NOT_FOUND(404) response.
** 规范的定义：服务器的class loader类加载器默认加载 /WEB-INF/classes目录下的 *.class文件 **
**1** 在创建 Dynamic Web project 时，就可以设置*.class文件的输出存放目录：
File>new > Dynamic Web project > next > 在default output folder地方的"build/classes"改成"WEB-INF\classes" > finish就可以了 2 如果在创建的时候忘记更改的话： 可以右击项目>build path > configure build path > 选择 source选项，将default output folder地方的"build/classes"改成"WebContent/WEB-INF/classes" > finish就可以了