How to get process instance variables and its fields value using REST API in jBPM 6?

Issue

We trying to get process instance variable using following REST API:

http://localhost:8080/business-central/rest/runtime/com.redhat.jboss.hr:rewards:1.0/withvars/process/instance/6

The response is not showing the variables and its fields' value. I need to get the variables and its field's value.

Instead of getting the detail variables in the response, I only get employee: "com.redhat.jboss.hr.rewards.Employee@47c82edb"

The response from the BPM as following:

{
status: "SUCCESS"
url: "http://localhost:8080/business-central/rest/runtime/com.redhat.jboss.hr:rewards:1.0/withvars/process/instance/6"
variables: {
employee: "com.redhat.jboss.hr.rewards.Employee@47c82edb"
}-
processInstance: {
status: null
url: null
index: null
commandName: null
processId: "rewards.rewards-approval"
id: 6
state: 1
eventTypes: [0]
}-
}