I have tested the system by using web service. I have used 3 clients with 20 separate tasks for each of them. However, every time after about 40 to 50 tasks being processed, the Oracle DB become strange. A lot of "ora-12519 : TNS:no appropriate service handler found" come out. And even sqldeveloper cannot login into the DB. For your information, the session in each task has been closed before the task ended.
I have tried to change the DBCP property "maxIdle" to "0" and it seems solved the issue, but whenever the DB connections returned to the connection pool, they are being destroyed immediately.
It seems that some DB resources have not been released after the DB connections are returned to the pool. Is there any setting can help to release those resources? Thanks in advance!