峰峰矿区公安局局长:我在写servlet时出现了HTTP Status 500 错误,这是什么原因?

HTTP Status 500 -

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type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Error instantiating servlet class HelloWorld
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118)
org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
java.lang.Thread.run(Thread.java:534)

root cause

java.lang.IllegalAccessException: Class org.apache.catalina.core.StandardWrapper can not access a member of class HelloWorld with modifiers ""
sun.reflect.Reflection.ensureMemberAccess(Reflection.java:57)
java.lang.Class.newInstance0(Class.java:302)
java.lang.Class.newInstance(Class.java:261)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118)
org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
java.lang.Thread.run(Thread.java:534)

note The full stack trace of the root cause is available in the Apache Tomcat/5.0.30 logs.

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Apache Tomcat/5.0.30
我先创建了一个文件夹在webapps下,然后写了个很简单的servlet,也进行了正确的部署,但是出现了这样的一个错误?郁闷????