我的第一个servlet遇到问题

Question

This is the code to my servlet...

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

public class HelloWorld extends HttpServlet {
    private String message;

    public void init(){
        message="Hello World";
    }
    public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h1>"+message+"</h1>");
    }

    public void destroy(){
    }
}

I'm using xampp's tomcat 7

and this is my web.xml file

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                  http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
  version="3.0"
 metadata-complete="true">

<servlet>
    <servlet-name>HelloWorld</servlet-name>
    <servlet-class>HelloWorld</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>/HelloWorld</url-pattern>
</servlet-mapping>

My web.xml is in %TOMCAT_HOME%/webapps/ROOT/WEB_INF directory and my HelloWorld.class is in %TOMCAT_HOME%/webapps/ROOT/WEB_INF/classes directory.

when I try to run my file from my browser I type

http://localhost:8080/HelloWorld

in the addressbar and the following Servlet exception shows up

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:100)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:269)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:300)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    java.lang.Thread.run(Thread.java:724)

root cause

java.lang.NoClassDefFoundError: HelloWorld (wrong name: com/HelloWorld/HelloWorld)
    java.lang.ClassLoader.defineClass1(Native Method)
    java.lang.ClassLoader.defineClass(ClassLoader.java:752)
    java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:2820)
    org.apache.catalina.loader.WebappClassLoader.findClass(WebappClassLoader.java:1150)
    org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1645)
    org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1523)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:269)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:300)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    java.lang.Thread.run(Thread.java:724)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.21 logs.

Please suggest a method to run my app properly...

Answer 1

Don't use the default (empty) package; give it a name instead ...

package com.xyz;
...
public class HelloWorld extends HttpServlet

Update web.xml to reflect new package...

<servlet>
    <servlet-name>HelloWorld</servlet-name>
    <servlet-class>com.xyz.HelloWorld</servlet-class>
</servlet>

Make sure the servlet class file resides in...

WEB-INF/classes/com/xyz/HelloWorld.class

Answer 2

follow the following steps

• Under the webapps folder, create a new folder for your webapp. HelloWorld
• Under HelloWorld create another folder called WEB-INF.
• Under WEB-INF, create a folder called classes.
• under WEB-INF, put your web.xml .
• under the classes folder create your package structure com/HelloWorld
• put your class file under classes/com/HelloWorld folder
• restart the tomcat

Answer 3

Default packages are discouraged, always put your servlets and classes in some package. Lets say you have package com.practice and HelloWorld servlet in it then your web.xml becomes

<servlet>
    <servlet-name>HelloWorld</servlet-name>
    <servlet-class>com.practice.HelloWorld</servlet-class>
</servlet>

If you are defining servlet in com.practice package ie path should be WEB-INF/classes/com/practice/HelloWorld

Also, if you are using tomcat 7, then you do not have to use servlet mapping in web.xml at all. You can simply use annotations for the same. eg

@WebServlet("/HelloWorld")
public class HelloWorld extends HttpServlet {
    private String message;

    public void init() {
        message = "Hello World";
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h1>" + message + "</h1>");
    }

    public void destroy() {
    }
}

Here WebServlet is the annotation that is equalivalent to defining servlet in web.xml and "/HelloWorld" is the url pattern

Answer 4

i don't know why you are just typing the servlet mapping url to invoke your web application as follows. http://localhost:8080/HelloWorld

it should be in the following format

http://localhost:8080/your-web-application-name/url-pattern

for example, assume that you are invoking HelloWorld url mapping on Web Application called MyWebApp . it should be called as below.

host:port/MyWebApp/HelloWorld

Answer 5

I also faced the same issue. I resolved the issue as follows. IDE Eclipse Photon

Go to this path: Build Path> Configure Build Path > Source

Change the path of "Default output folder": [project name]/WebContent/WEB-INF/classes

Try with this, Sometime it won't work. If it is not working, create a new Java program with Public static void main(String[] args) and Run that project as Java Application, so your project will update and a new .class file will be created: