<action name="getActiveCustomers" class="...">
<result name="success" type="json">
<param name="jsonObjectProperty">activeCustomer</param>
<param name="contentType">application/json</param>
</result>
...
</action>>
Or just
<action name="getActiveCustomers" class="...">
<result name="success" type="json" />
...
</action>
if we used the default property ('jsonObject') and default content-type ('application/json')