Exception mappings is a powerful feature for dealing with an Action class that throws an Exception. The core idea is that an Exception thrown during the Action method can be automatically caught and mapped to a predefined Result. This declarative strategy is especially useful for frameworks, like Hibernate and Acegi, that throw RuntimeExceptions.

As with many other parts of the framework, an Interceptor is needed to activate the exception mapping functionality. Below is a snippet from struts-default.xml which has the exception mapping already activated.

To use exception mapping, we simply need to map Exceptions to specific Results. The framework provides two ways to declare an exception mapping <exception-mapping/> - globally or for a specific action mapping. The exception mapping element takes two attributes, exception and result.

When declaring an exception mapping, the Interceptor will find the closest class inheritance match between the Exception thrown and the Exception declared. The Interceptor will examine all declared mappings applicable to the action mapping, first local and then global mappings. If a match is found, the Result is processed, just as if it had been returned by the Action.

This process follows the same rules as a Result returned from an Action. It first looks for the Result in the local action mapping, and if not found, it looks for a global Result.

Below is an example of global and local exception mappings.

In the example above, here is what happens based upon each Exception:

• A java.sql.SQLException will chain to the SQLExceptionAction (action mapping not shown)
• A com.company.SecurityException will redirect to Login.action
• Any other exception that extends java.lang.Exception will return the /Exception.jsp page

Exception Values on the ValueStack

By default, the ExceptionMappingInterceptor adds the following values to the Value Stack:

Exception in constructors

Global exception mappings are designed to be used with exceptions thrown by action methods (like execute). exceptions thrown from constructors will not be handled by global exception mappings.