If you are using Maven,
Replace the port number in pom.xml by a property variable like this:
<port>${jetty.port}</port>
Then run maven using the following command:
mvn jetty:run -Djetty.port=8081
To define a default port numer, add this default property to your pom file:
<properties>
<jetty.port>8080</jetty.port>
</properties>
Hope that helps.