Exception handling is a well-known concept when developing software. Dealing with exceptions in a distributed application landscape is a little more challenging than we are typically used to. Especially when it comes to failures when handling a command or a query, messages that are intended to have a return value, we should be conscious about how we throw exceptions.
Handler Execution Exception
The HandlerExecutionException marks an exception which originates from a message handling member. Since an Event message is unidirectional, handling an event does not include any return values. As such, the HandlerExecutionException should only be returned as an exceptional result from handling a command. Axon provides a more concrete implementation of this exception for failed command and query handling, respectively the CommandExecutionException and QueryExecutionException.
The usefulness of a dedicated handler execution exception becomes clearer in a distributed application environment where, for example, there is a dedicated application for dealing with commands and another application tasked with the query side. Due to the application division, you loose any certainty that both applications can access the same classes, which thus holds for any exception classes. To support and encourage this decoupling, Axon will generify any exception which is a result of Command or Query Handling.
To maintain support for conditional logic dependent on the type of exception thrown in a distributed scenario, it is possible to provide details in a HandlerExecutionException. It is thus recommended to throw a CommandExecutionException / QueryExecutionExceptionwith the required details, when command/query handling fails. This behaviour could be supported generically by implementing interceptors which perform exception wrapping for you.
@ExceptionHandler annotated methods
Axon Framework allows the use of methods annotated with @ExceptionHandler to provide more fine-grained control on how to react to exceptions. More specifically, this is a type of message handler interceptor dedicated to reacting to exceptional results. Note that such an @ExceptionHandler will only handle exceptions thrown from message handling functions in the same class. For more specifics on how to use this annotation, check out this section.