Activiti is a workflow and Business Process Management (BPM) Platform Project contains some examples created using the service tasks, call activity and subprocess. Call activity is also a subprocess but written in an external file so that it can be reused. Link - https://www.activiti.org/userguide/
URL to run the process -
http://localhost:8080/service/task
http://localhost:8080/service/conditional
http://localhost:8080/service/doWhile
http://localhost:8080/service/complex
H2 in memory database is mandatory to hold the process execution information - http://localhost:8080/h2-console