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From bugzi...@apache.org
Subject DO NOT REPLY [Bug 3896] New: - ClassCastException when invoking certain subclasses of HTTPServlet
Date Mon, 01 Oct 2001 10:58:43 GMT
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http://nagoya.apache.org/bugzilla/show_bug.cgi?id=3896

ClassCastException when invoking certain subclasses of HTTPServlet

           Summary: ClassCastException when invoking certain subclasses of
                    HTTPServlet
           Product: Tomcat 4
           Version: 4.0 Final
          Platform: Sun
        OS/Version: Solaris
            Status: NEW
          Severity: Normal
          Priority: Other
         Component: Catalina
        AssignedTo: tomcat-dev@jakarta.apache.org
        ReportedBy: faensen@inf.fu-berlin.de


When invoking a servlet that is a subclass of org.webmacro.http.WMServlet (which 
in turn is a direct subclass of HTTPServlet) the Catalina core throws a 
ServletException that is a linked to a ClassCastException:

Here is a snippet from the log file
2001-10-01 12:19:59 
StandardWrapper[:org.apache.catalina.INVOKER.hermes.web.HelloWorld]: Marking 
servlet org.apache.ca
talina.INVOKER.hermes.web.HelloWorld as unavailable
2001-10-01 12:19:59 invoker: Cannot allocate servlet instance for path 
/servlet/hermes.web.HelloWorld
javax.servlet.ServletException: Class hermes.web.HelloWorld is not a Servlet
        at 
org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:848)
        at 
org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:621)
        at org.apache.catalina.servlets.InvokerServlet.serveRequest(Unknown 
Source)
        at org.apache.catalina.servlets.InvokerServlet.doGet(Unknown Source)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
        at 
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Unknown Source)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(Unknown 
Source)
        at org.apache.catalina.core.StandardWrapperValve.invoke(Unknown Source)
        at org.apache.catalina.core.StandardPipeline.invokeNext(Unknown Source)
        at org.apache.catalina.core.StandardPipeline.invoke(Unknown Source)
        at org.apache.catalina.core.ContainerBase.invoke(Unknown Source)
        at org.apache.catalina.core.StandardContextValve.invoke(Unknown Source)

The problem seems to occur in the following lines in 
org.apache.catalina.core.StandardWrapper:

        try {
            servlet = (Servlet) classClass.newInstance();
        } catch (ClassCastException e) {
            unavailable(null);
            // Restore the context ClassLoader
            throw new ServletException
                (sm.getString("standardWrapper.notServlet", actualClass), e);
        }


By splitting the line 
  servlet = (Servlet) classClass.newInstance();
and adding some debugging output 
	    log("Trying to instantiate " + classClass.getName());
 	    Object instance =  classClass.newInstance();
	    log("Instantiated, instance is  " + instance);
	    log("Trying to cast ...");
	    servlet = (Servlet) instance;

one can focus that instance (which is of class hermes.web.HelloWorld) cannot be 
casted to Servlet.

The class hierarchy is as follows

    Interface javax.servlet.Servlet
                     ^
                     | implements
                     |
    Class javax.servlet.GenericServlet
                     ^
                     | extends
                     |
    Class javax.servlet.http.HttpServlet
                     ^
                     | extends
                     |
    Class org.webmacro.servlet.WMServlet
                     ^
                     | extends
                     |
    Class hermes.web.HelloWorld

Clearly, HelloWorld should be castable to Servlet.
I have no idea why the cast fails. May be something is wrong with the class 
loader.

Daniel Faensen

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