Simply build the jar using "mvn package" at the command line. The jar is built to "target/smiley-http-proxy-servlet-VERSION.jar". You don't have to build the jar if you aren't modifying the code, since released versions are deployed to maven-central. If you are using maven then you can add this to your dependencies in your pom like so: (Note: the version below is not necessarily the latest.)
<dependency><groupId>org.mitre.dsmiley.httpproxy</groupId><artifactId>smiley-http-proxy-servlet</artifactId><version>1.7</version></dependency>Ivy and other dependency managers can be used as well.