Hi Maciej Swiderski and Huisheng Xu :
Thank all of you so much. My problem is finally resolved after changing the settings to "newTxRequiredCommandService" in "jbpm.default.cfg.xml" :
<process-engine-context>
....
<object class="org.jbpm.pvm.internal.id.DatabaseIdComposer" init="eager" />
<object class="org.jbpm.pvm.internal.id.DatabaseDbidGenerator">
<field name="commandService">
<ref object="newTxRequiredCommandService" />
</field>
</object>
<id-generator />
....
</process-engine-context>
Your help is very appreciated. :)
Tun Mang