Hi
Have you tried compiling your extension using a build tool like Maven or Gradle, and including your library as a dependency?
By using a build tool, you can include the Extender API artifact, so you do not need to include all of the Extender API interface files. If you have a look at some of our BApp Store extensions (on our
GitHub), you can see how other authors have implemented this.
Alternatively, have you tried including the fastjson library in your Java environment in Burp under "Extender > Options > Java environment"?