多层目录下的Servlet如何访问？

我做了个简单的Servlet,放在应用程序根目录下，访问正常，可是一移到文件夹里面就不行了，映射应该是对的，错误报类名错误，放在根目录下就没这个错误，这是为何？

exception

javax.servlet.ServletException: Error allocating a servlet instance
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:793)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:702)
org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:571)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:644)
java.lang.Thread.run(Thread.java:534)


root cause

java.lang.NoClassDefFoundError: jsp_instance/HelloWorld/HelloWorld (wrong name: HelloWorld)
java.lang.ClassLoader.defineClass0(Native Method)
java.lang.ClassLoader.defineClass(ClassLoader.java:537)
java.security.SecureClassLoader.defineClass(SecureClassLoader.java:123)
org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:1634)
org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:860)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1307)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1189)
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:793)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:702)
org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:571)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:644)
java.lang.Thread.run(Thread.java:534)