Creating jar in eclipse

Bonjour,
i doing a simple project in eclipse. In my web project folder i tried to add the librairies of the java project folder. First time i successfully created my jar file and inlcuded in the library of the web project, later i added new class in the java project....this time i tried to add the changes of my java project in web project...unfortunately the updation in the jar file of the web project does n't reflect any changes.(i.e addition of new classes)

please suggest....

utham