Parameter in jasperreports's XML

I am using the jasperreports to generate a pdf report, but meet problem when i generate a report that base on a parameter... can anyone help me?

The coding is as follow:
<parameter name="usr" isForPrompting="false" class="java.lang.String"/>
<queryString><![CDATA[select * from exrate where erUpdatedBy like ''%$P{usr}%']]></queryString>

Error message after compile:" org.apache.jasper.JasperException: 0 "

But it works if i change the sql to:
select * from exrate where erUpdatedBy like 'ADMIN';

Do anyone know how to solve it? Thank you very much for your help!