I use spring,HIbernate and struts 1.2 in my application.Spring fit into business layer. I have to display user friendly message to user when a exception raised in business layer(spring layer).How I pass the exception raised in service layer(spring) to Struts.
Just have your model classes throw any exceptions they may encounter. Then have your Struts Action class catch the exceptions and convert them to user-friendly messages that can be displayed in the JSP.
Another alternative would be to have the Action class also throw messages and declare global message handlers in the struts-config.xml file to handle specific messages and display specific JSPs when they occur.