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'Extend class' directive in JSP

 
Nupur Gupta
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Hi all,
I am using JBoss and have made a simple jsp file which extends another class. I tried first with my own classes and got error(as described below), so I tried java classes next. My jsp page is as -

<!doctype html public "-//w3c/dtd HTML 4.0//en">
<%@ page extends = "java.lang.Thread" %>
<%!
public void run(){
System.out.println("In run");
}
%>

On invoking the page, this is the error I get -

org.apache.jasper.JasperException: Unable to compile class for JSP
C:\jboss-3.0.3\catalina\work\MainEngine\localhost\myjsp\directives2$jsp.java:54: Incompatible type for method. Explicit cast needed to convert org.apache.jsp.directives2$jsp to javax.servlet.Servlet.
pageContext = _jspxFactory.getPageContext(this, request, response,
^
1 error

at org.apache.jasper.compiler.Compiler.compile(Compiler.java:285)
at org.apache.jasper.servlet.JspServlet.loadJSP(JspServlet.java:548)
at org.apache.jasper.servlet.JspServlet$JspServletWrapper.loadIfNecessary(JspServlet.java:176)
at org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:188)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:381)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:473)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:190)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.valves.CertificatesValve.invoke(CertificatesValve.java:246)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2347)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:170)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:170)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:468)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)
at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
at org.apache.catalina.connector.http.HttpProcessor.process(HttpProcessor.java:1027)
at org.apache.catalina.connector.http.HttpProcessor.run(HttpProcessor.java:1125)
at java.lang.Thread.run(Thread.java:479)




Any ideas? Thanks
 
Ben Souther
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JSP is a templating technology.
The pages get converted into servlet code by the JSP compiler.

You are given the ability to change what servlet class you're going to inherit from with the extends directive but you can't just extend anything and expect it to work.
 
Bear Bibeault
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Why do you even feel the need to try and extend the JSP?
 
Nupur Gupta
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Oh, I see, so only classes derived from Servlet classses. I was trying to extend other classes just to learn the functionality, I have been following a book, and thought I would try the directive. Thanks a lot.
 
Ben Souther
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Unless you're building a container or a framework that changes the fundamental behavior of JSPs you will probably never need to use the extends directive when programming with JSP.
 
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