code: public Map<String,String> getDocRefs() {
          return new HashMap<String,String>() {
              @Override
              public String get(Object varName) {
                  VariableInstance varInst = processInstance.getVariable((String)varName);
                  if (varInst != null && varInst.getData() instanceof DocumentReference)
                      return varInst.getData().toString();
                  return null;
              }
          };
      }
docs: For read-only access.