How to prevent plant locking issue while calling a BAPI? Please help!

Question

Hi Experts,
   I have following scenario:
   BizTalk sends 4B2 PIP to XI via JMS adapter. XI then sends the same to ECC system via ABAP Proxy.
   4B2 PIP in our case contains one PO header and one line item.
   In ECC, ABAP proxy calls BAPI to process the GR (Goods Receipt).
   The problem is BizTalk can send many 4B2 PIPs simultaneously (Parallel). When The first PIP is getting processed in ECC the BAPI locks some plant data till it completes the GR. But if second PIP request comes when the first once is still getting processed then we get "plant is locked" error.
   How can we avoid this plant locking error? What is the best option to solve this?
  While calling BAPI in my ECC ABAP Proxy if I use qRFC as given by the code below then will it solve the problem?
  Will this use only one queue "QUEUE01" for all the 4B2 PIP requests?
  Will it create a new queue for every request?
  If we assume that there are 3 PIP requests already in a queue. If the GR processing for the first PIP returns some data error (not the locking error) in the BAPIRETURN then will the second PIP gets processed or will it wait till the first one is successfully processed?
   CALL FUNCTION 'TRFC_SET_QUEUE_NAME'
      IMPORTING
         QNAME = 'QUEUE01'.
  CALL FUNCTION '<CREATE GR BAPI>'
  IN BACKGROUND TASK
  EXPORTING
     TABLES ....
  COMMIT WORK.
Thanks & Regards
Gopal

Answer 1

Man, you guys/gals are great! You notice everything...I sure appreciate all the help so far. I've updated a few things, but I still can't get x1 in the method to pass the right input to line 39. For instance, if I set a=2, b=4, and c=-30, then x1 should = 3 and x2 should =-5, but both keep showing -5. They are always equal for some reason, even when they are not supposed to be.
I see what you were saying with d always equalling zero. I was looking in the main where it doesn't always = zero, but in the method it was always set to zero. Thanks...good eye!
num1 = quad(a, b, c, 1);                              //line 39
num2 = quad(a, b, c, 2);                              //line 40
JOptionPane.showMessageDialog(null, "There are two real roots. They are root 1 = "+ num1 + " , and root 2 = " + num2 + ".");I also added this to the method so d won't always = zero
d = (Math.pow(b,2) - (4 * a * c));I also have all of the variables declared at the top, but I forgot to copy and paste all of them and I updated the System.exit(0) to close the JOptionPane along with extra } 's to close the program correctly.
Again, thanks a bunch.