try to specify the following properties to your datasource bean definition:
<bean id="myDatasource" class="org.springframework.jndi.JndiObjectFactoryBean">
<property name="jndiName" value="jdbc/myDS"/>
<property name="lookupOnStartup" value="false"/>
<property name="cache" value="true" />
<property name="proxyInterface" value="javax.sql.DataSource" />
</bean>