code: public static Expression getFieldExpression(DelegateExecution execution,
                                                  String fieldName) {
          if (isExecutingExecutionListener(execution)) {
              return getListenerFieldExpression(execution,
                                                fieldName);
          } else {
              return getFlowElementFieldExpression(execution,
                                                   fieldName);
          }
      }
docs: Returns the {@link Expression} for the field defined for the current
  activity of the provided {@link DelegateExecution}.
  <p>
  Returns null if no such field was found in the process definition xml.
  <p>
  If the execution is currently being used for executing an
  {@link ExecutionListener}, it will return the field expression for the
  listener. Use
  {@link #getFlowElementFieldExpression(DelegateExecution, String)} or
  {@link #getListenerFieldExpression(DelegateExecution, String)} for
  specifically getting the flow element or listener field expression.