Eclipse has a JAR export wizard that can create a JAR for you, but it only runs when you launch the wizard.
If you want a JAR to be produced automatically after you change any source file, then you will need to hook into the Eclipse build system by adding your own ANT builder. Create a build.xml file that will serve as your ant script. Then Right-click your project and go to Properties > Builders. Then click “New” and select “Ant Build” and then a ant build launch configuration will open and you can browse to your build file.