Data types between Java and ABAP

I used JCo to run BAPI function, the BAPI need to input some field which data type is DATS or Char, I can use the String data type in Java to mapping them.

but now, I run another BAPI function , it has a field which data type is "unit", when I use String data type to setValue in to the field, it can work , but when I excute the function , it always throw a exception:

RFC_ERROR_SYSTEM_FAILURE: connection closed without message (CM_NO_DATA_RECEIVED)

please tell me what can I do, is't the cause that the data type between java and abap? or another reason?

hope for you revert, the exception trouble me for a long time...