Error: 500 Internal Servlet Error

[David Klemitz]

Hi Folks,

I have successfully run the Jakarta-Tomcat Server (3.2.3) and successfully
completed some tutorial examples from the Marty Hall book
'Servlets and JSP's. However, I attempted to start the http server
'EchoServer' ( after having successfully compiling it into byte code)
and received Error 500. Below is the trace given at the Tomcat Server window

Error: 500
Location: /servlet/coreservlets.EchoServer

Internal Servlet Error:
java.lang.NullPointerException

at java.lang.ClassLoader.resolveClass0(Native Method)
at java.lang.ClassLoader.resolveClass(ClassLoader.java:588)
at
org.apache.tomcat.loader.AdaptiveClassLoader.loadClass(AdaptiveClassLoader.j
ava:518)
at
org.apache.tomcat.loader.AdaptiveServletLoader.loadClass(AdaptiveServletLoad
er.java:174)
at
org.apache.tomcat.core.ServletWrapper.loadServlet(ServletWrapper.java:265)
at
org.apache.tomcat.core.ServletWrapper.init(ServletWrapper.java:289)
at org.apache.tomcat.core.Handler.service(Handler.java:254)
at
org.apache.tomcat.core.ServletWrapper.service(ServletWrapper.java:372)
at
org.apache.tomcat.core.ContextManager.internalService(ContextManager.java:81
2)
at
org.apache.tomcat.core.ContextManager.service(ContextManager.java:758)
at
org.apache.tomcat.service.http.HttpConnectionHandler.processConnection(HttpC
onnectionHandler.java:213)
at
org.apache.tomcat.service.TcpWorkerThread.runIt(PoolTcpEndpoint.java:416)
at
org.apache.tomcat.util.ThreadPool$ControlRunnable.run(ThreadPool.java:501)
at java.lang.Thread.run(Thread.java:484)

Apart from asking for suggestions on how to clear this error and get the
echoserver running, I would like to ask what tools are available
to investigate this sort of error ? As far as my knowledge goes at the
moment , once a *.java file is compiled into byte code (*.class) it
really is a black box.

Mind you, I suspect the issues is more with the server as the trace
suggests. As with normal debugging procedures do you deal with the first
error
first ie

java.lang.NullPointerException

On a slightly separate note, is there a consistent mapping of servlet
(*.class) files to requested URLs? I have read the Tomcat FAQ
and it seems short on this detail.

Any help appreciated

regards

David Klemitz