[java] HTTP Status 500 - Error instantiating servlet class pkg.coreServlet

I am creating simple servlet and deploying it in tomcat server but I am getting the following error:

HTTP Status 500 - Error instantiating servlet class pkg.coreServlet

File Structure on the tomcat server:

webapps     
| 
- aarya
  |
  - WEB-INF
    |
     -web.xml
     -src(folder)
       |
       -pkg
         |
         -coreServlet.class

web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
    http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">


<servlet>
    <servlet-name>aaryaservlet</servlet-name>
    <servlet-class>pkg.coreServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>aaryaservlet</servlet-name>
        <url-pattern>/coreServlet</url-pattern>
    </servlet-mapping>
</web-app>

coreServlet.java:

package pkg;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*; 

public class coreServlet extends HttpServlet
{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;

public void doGet(HttpServletRequest req,HttpServletResponse res)
 throws ServletException,IOException


   {
        PrintWriter out = res.getWriter();
        res.setContentType("text/html");
        out.println("this is First servlet Example ");
    }
}

url I am giving is http://localhost:8080/aarya/coreServlet I try by restarting tomcat but I am getting same error. Where I am doing wrong?

Change the

private static final long serialVersionUID = 1L;

to any other value like

private static final long serialVersionUID = 102831973239L;

also you can generate it automatically in eclipse.

It is because each servlet in a app has a unique id.and tomcat causes problem with two servlets having same id...

Make sure the following:

1. Proper "war" file structure i.e. WEB-INF & META-INF
2. "web.xml" file is setup correct.
3. Last and important: private static final long serialVersionUID = 1L; should be there in your class (<servlet-class>MyClass</servlet-class>).

Have you closed the < web-app > tag in your web.xml? From what you have posted, the closing tag seems to be missing.

Try This:)

before:-

<servlet>
    <servlet-name>TestServlet</servlet-name>
    <servlet-class>TestServlet</servlet-class>  
</servlet>

After:-

 <servlet>
    <servlet-name>TestServlet</servlet-name>
    <servlet-class>operation.TestServlet</servlet-class>
 </servlet>

The servlet class should be in the WEB-INF/classes not WEB-INF/src.

In my case missing private static final long serialVersionUID = 1L; line caused the same error. I added the line and it worked!

I had an issue with Servlet instantiation. I cleaned the project and it worked for me. In eclipse menu, Go to Project->Clean. It should work.

The above error can occur for multiple cases during servlet startup / request. Hope you check the full stack trace of the server log, If you have tomcat, you can also see the exact causes in html preview of the 500 Internal Server Error page.

Weird thing is, if you try to hit the request url a second time, you would get 404 Not Found page.

You can also debug this issue, by placing breakpoints on all the classes constructor initialization block, whose objects are created during servlet startup/request.

In my case, I didn't had javaassist jar loaded for the Weld CDI injection to work. And it shown NoClassDefFound Error.