I understand here you use the struts.properties as just a properties file
You can read it as below.
Reading a properties file
Or in the Struts
import org.apache.struts.util.MessageResources;
...
private MessageResources messages = MessageResources.getMessageResources("struts.properties");
String message = messages.getMessage("my.message");
then
message.replaceAll the {0} withe the value.
Did I answer the question ?