well... I have found a way to trigger the boundary event, it works this way :
String serviceTaskId = "ServiceTask_1";
String errorCode = "Error";
ksession.signalEvent("Error-"+serviceTaskId+"-"+errorCode, "", processInstance.getId());
BUT...
if I am in a sub-process, the "Fail" node is triggered, but the sub-process is not completed => we don't reach the next node in the parent process.
the boundary event has an attribute: cancelActivity="true" . but it seems the process is still waiting for something to happen in the sub-process. and I haven't been able to manually abort the service task (not even sure if it's the solution).
I've also tried with a "Terminate Definition" on the "Fail" node, it doesn't solve the problem.
does someone has an idea of what I should do ?