For sub report with the xml files as a datasource, you need to specify the datasource for the sub report. Properties you need to have a look at are
- Connection Type : Use a Datasource Expression
- Connection / Datasource Expression :
((net.sf.jasperreports.engine.data.JRXmlDataSource)$P{REPORT_DATA_SOURCE}).subDataSource("//YOUR_SUBREPORT_ENTITY").
You will need something similar to this
<dataSourceExpression><![CDATA[
((net.sf.jasperreports.engine.data.JRXmlDataSource)
$P{REPORT_DATA_SOURCE}).subDataSource("//YOUR_SUBREPORT_ENTITY") ]]>
</dataSourceExpression>