In jBPM 6.4.Final, is there a way to retrieve the task output definitions through REST? and if not what are the other alternative ways to do so.
Yes you can.
[GET] /PROCESS_ID/tasks/users/TASK_NAME/inputs
Description
Returns a map with all the task input parameter definitions for the TASK_NAME task of the PROCESS_ID process. The key is the name of the input and the value is its type.
[GET] /PROCESS_ID/tasks/users/TASK_NAME/outputs
Description
Returns a map with all the task output parameter definitions for the TASK_NAME task of the PROCESS_ID process. The key is the name of the input and the value is its type.
You can see the complete documentation in the following link https://access.redhat.com/documentation/en-us/red_hat_jboss_bpm_suite/6.4/html/development_guide/realtime_decision_server#user_tasks