Thank you for your answer Marco. You are totally right ; I made a step by step debugging and I could see a call to jpm.dispose() so this may be linked to the closed transaction.
But even if I debug several time I don't really understand what happens and why. What can I do to avoid the transaction to close?
Bellow there are some screenshot of the debug :
https://community.jboss.org/servlet/JiveServlet/downloadImage/20563/debug_wpi1.png
then getProcess() is called 7 times :
https://community.jboss.org/servlet/JiveServlet/downloadImage/20564/debug_wpi2.png
And when it goes to the next step
https://community.jboss.org/servlet/JiveServlet/downloadImage/20565/debug_wpi3.png
all become null! :((
https://community.jboss.org/servlet/JiveServlet/downloadImage/20565/debug_wpi3.png
