Custom JAR Files
JMeter has many plug-ins and downloadable JAR Files you can use to support your testing, it is also possible to write your own custom Jar files and use these as well. Why would you want to do that, you may say, well whilst it is unlikely that you would need to create anything too complicated. You may feel the need to create utilities to support your own in-house performance testing or just something to make your testing process easier.