java.lang.NoClassDefFoundError when referencing an external library in Eclipse

Question

Edit: I found the answer and I am posting it here in case anybody googles it in future: Turns out that Tomcat does not look up external libraries in Eclipse and they should be copied to [TomcatInstallDir]/lib.

Here is my code (which is for a servlet running on Tomcat server):

import  com.google.gson.Gson;

public class AddCourse extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse res) throws IOException{
    Gson gson = new Gson();
    //Some code

}

I have tried to export other libraries as well as same library with different version and had the same problem. Here is a picture of Build path:

I read that when writing a Dynamic Web Application (Which I am doing) then we should put the jars under Web-Content/lib, I did that also and it did not help.

Here is the stacktrace:

Apr 19, 2020 5:27:34 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [AddCourse] in context with path [/MTHDB] threw exception [Servlet execution threw an exception] with root cause
java.lang.Error: Unresolved compilation problems: 
Gson cannot be resolved to a type
Gson cannot be resolved to a type

at servlets.AddCourse.doPost(AddCourse.java:26)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:688)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1594)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)

Edit: I noticed that my MANIFEST.MF' contents looks like this:

Manifest-Version: 1.0
Class-Path: 

Should not there be a mention the the JARs?

Answer 1

I found the answer and I am posting it here in case anybody googles it in future: Turns out that Tomcat does not look up external libraries in Eclipse and they should be copied to [TomcatInstallDir]/lib.