Develop Reference

ServletException in eclipse: Error instantiating servlet class [Pkg.hello]

Error
java.lang.ClassNotFoundException:Pkg.hello
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1285)
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1119)
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861)
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Unknown Source)enter code here Note The full
stack trace of the root cause is available in the server logs.
Web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID"
version="3.1"> <display-name>Firstproj</display-name> <servlet>
<servlet-name>abc</servlet-name>
<servlet-class>Pkg.hello</servlet-class> </servlet> <servlet-mapping>
<servlet-name>abc</servlet-name> <url-pattern>/welcome</url-pattern>
</servlet-mapping> <welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file> </welcome-file-list>
</web-app>
hello.java
package Pkg;
import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
/** * Servlet
implementation class hello */
#WebServlet("/welcome")
public class hello implements Servlet {
#Override
public void destroy() {
// TODO Auto-generated method stub
}
#Override
public ServletConfig getServletConfig()
{
// TODO Auto-generated method stub return null;
}
#Override
public String getServletInfo() {
// TODO Auto-generated method stub
return null;
}
#Override
public void init(ServletConfig arg0) throws ServletException {
// TODO Auto-generated method stub
}
#Override
public void service(ServletRequest arg0, ServletResponse arg1) throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("faisal");
}
}

WAR + Gradle: ClassNotFound Exception

I'm trying to compile a simple WAR for Tomcat 8 + JDK 1.8. When I attempt to build this with gradle, I get a class not found exception. Details below:
Folder Structure In Eclipse:
enter image description here
Hello.java:
package main;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class Hello
*/
public class Hello extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* #see HttpServlet#HttpServlet()
*/
public Hello() {
super();
// TODO Auto-generated constructor stub
}
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.getWriter().append("{\"message\":\"Hello!\",\"exit\":\"0\"}");
}
/**
* #see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
}
build.gradle:
task wrapper(type: Wrapper) {
gradleVersion = "3.2.1"
}
repositories{
mavenCentral()
}
allprojects {
apply plugin: 'java'
apply plugin: 'war'
version="1.0"
println("VERSION:")
println(version)
war.destinationDir= file("build\\com\\HelloWorldServlet\\$version")
}
war{
baseName='HelloWorldServlet'
}
Tomcat Error:
30-Nov-2016 09:34:07.361 SEVERE [http-apr-8080-exec-114]
org.apache.catalina.core.StandardWrapperValve.invoke Allocate exception for servlet main.Hello
java.lang.ClassNotFoundException: main.Hello
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1333)
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1167)
at org.apache.catalina.core.DefaultInstanceManager.loadClass(DefaultInstanceManager.java:518)
at org.apache.catalina.core.DefaultInstanceManager.loadClassMaybePrivileged(DefaultInstanceManager.java:499)
at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:118)
at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1091)
at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:817)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:135)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:509)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1104)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2508)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2497)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)
Web.XML:
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>HelloWorldServlet</display-name>
<servlet>
<servlet-name>HelloWorldServlet</servlet-name>
<servlet-class>Hello</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloWorldServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
</web-app>
Also tried this for Web.xml:
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>HelloWorldServlet</display-name>
<servlet>
<servlet-name>HelloWorldServlet</servlet-name>
<servlet-class>main.Hello</servlet-class> <-- Also tried src.main.Hello
</servlet>
<servlet-mapping>
<servlet-name>HelloWorldServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
Structure of Deployed WAR:
-> HelloWorldServlet
->META-INF
-> MANIFEST.MF
-> war-tracker
->WEB-INF
->classes
->Hello.class

HTTP Status 500 - Error instantiating servlet class servlets.LoginServlet

So I am working on an assignment for school and this error is happening and I don't know how to fix it. I don't think it's a coding error, just a project setup error.
Here is a screenshot:
Any help would be appreciated. I've tried the other answers I've found online such as "Project Clean..." etc and nothing works.
EDIT 1/
javax.servlet.ServletException: Error instantiating servlet class servlets.LoginServlet
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:784)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:802)
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Unknown Source)
java.lang.ClassNotFoundException: servlets.LoginServlet
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1275)
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1109)
org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:784)
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:802)
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455)
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
java.lang.Thread.run(Unknown Source)
I can't seem to post the web.xml on here...
http://hastebin.com/sadikezece.xml
LoginServlet.java
package servlets;
import java.io.IOException;
import java.io.Serializable;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import beans.UserBean;
/**
* Servlet implementation class LoginServlet
*/
public class LoginServlet extends HttpServlet implements Serializable {
/**
*
*/
private static final long serialVersionUID = 1L;
/**
* #see HttpServlet#HttpServlet()
*/
public LoginServlet() {
super();
// TODO Auto-generated constructor stub
}
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.getWriter().append("Served at: ").append(request.getContextPath());
UserBean user = new UserBean();
user.setUsername(request.getParameter("username"));
user.setPassword(request.getParameter("password"));
}
/**
* #see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}
}
Didn't do anything to it because it wasn't running.

Please look careful about schema version of web.xml and use of tags like:
<description></description>
<display-name>LoginServlet</display-name>
And also check package name in web.xml Or Please change your web.xml like this:
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 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_2_5.xsd">
<servlet>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>servlets.LoginServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>SearchServlet</servlet-name>
<servlet-class>servlets.SearchServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>SearchServlet</servlet-name>
<url-pattern>/search</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>

I made a Maven web-app and it didn't seem to work so I made another project as a "Dynamic Web App" and it worked beautifully.

I know the reason. First I can see a red cross on your project. It means your project will not be compiled again before you solve the issue. But you cannot see any err in your code.
Then you can choose window/show view/other , then choose problems. You can see the tab about err and waring next the console.

Url doesn't match url-pattern in web.xml

In the web.xml file I have this url pattern:
<url-pattern>/servletPrueba/*</url-pattern>
and in the jsp file I have this link:
Enlace servlet
But I get a 404 error, I think the url matches the url pattern, doesn't it?
here is the servlet,it's onle a test purposes servlet,only for practice
package pruebas;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class servletPrueba
*/
//#WebServlet("/servletPrueba")
public class servletPrueba extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* #see HttpServlet#HttpServlet()
*/
public servletPrueba() {
super();
// TODO Auto-generated constructor stub
}
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//response.getWriter().append("Served at: ").append(request.getContextPath());
RequestDispatcher despachador = null;
String ruta = request.getPathInfo();
PrintWriter out = response.getWriter();
out.println(request.getPathInfo());
out.println(request.getServletPath());
}
/**
* #see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//doGet(request, response);
PrintWriter out = response.getWriter();
out.println("POST");
}
}
webb.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
<display-name>pruebas</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
<servlet>
<servlet-name>servletPrueba</servlet-name>
<servlet-class>pruebas.servletPrueba</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>servletPrueba</servlet-name>
<url-pattern>/servletPrueba/*</url-pattern>
</servlet-mapping>
</web-app>

Here is a complete example (I did not use Maven, however I could) :

Counting the number of users in web App in servlet

I have modified my application to find out the number of users logged in a web application below is my piece of code..
the listener class
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class SessionCounter implements HttpSessionListener
{
private static int count;
public static int getActiveSessions() {
return count;
}
public SessionCounter()
{
}
//The "sessionCount" attribute which has been set in the servletContext should not be modified in any other part of the application.
//Since we are using serveltContext in both the methods to modify the same variable, we have synchronized it for consistency.
public void sessionCreated(HttpSessionEvent e)
{
count++;
ServletContext sContext = e.getSession().getServletContext();
synchronized (sContext)
{
sContext.setAttribute("sessionCount", new Integer(count));
}
}
public void sessionDestroyed(HttpSessionEvent e)
{
count--;
ServletContext sContext = e.getSession().getServletContext();
synchronized (sContext)
{
sContext.setAttribute("sessionCount", new Integer(count));
}
}
}
and the main servlet is ..
package com.saral;
import java.io.IOException;
import org.apache.log4j.Logger;
import org.apache.log4j.PropertyConfigurator;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class First
*/
//#WebServlet("/First")
public class MyServlet extends HttpServlet
{
private static final long serialVersionUID = 1L;
static final Logger logger = Logger.getLogger(MyServlet.class);
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PropertyConfigurator.configure("log4j.properties");
logger.info("before---->");
// TODO Auto-generated method stub
String name=request.getParameter("txtName");
response.setContentType("text/html");
PrintWriter out=response.getWriter();
out.println("Hello,"+name);
out.println("<br> this output is generated by a simple servlet.");
out.println("Total Number of users logged in--->"+SessionCounter.getActiveSessions());
out.close();
}
}
and the web.xml is ...
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>FirstDemo</display-name>
<context-param>
<param-name>log4jConfigLocation</param-name>
<param-value>/WEB-INF/log4j.properties</param-value>
</context-param>
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.saral.MyServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/helloServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>home.html</welcome-file>
</welcome-file-list>
<listener>
<listener-class>com.saral.SessionCounter</listener-class>
</listener>
</web-app>
but I am getting the total number of users logged in as 0 , which is not perfect, please advise where I am wrong and how can I overcome from this.

When a client request come to the Tomcat server and you don't call request.getSession(), then the Tomcat server stil creates a session automatically. After that, the method sessionCreated(...) in your SessionCounter class is called.
The method sessionDestroyed(...) will be called when a session is destroyed. That occurs when you call session.invalidate(). If you close a tab on browser or close a browser, the session is still alive on your tomcat server.
I think so. You can use some diffrent listeners to archive your goal: HttpSessionAttributeListener, HttpSessionBindingListener,...