Validating Business Rules

While validating input is not part of the use case logic, validating business rules definitely is. Business rules are the core of the application and should be handled with appropriate care. But when are we dealing with input validation and when are we dealing with business rule validation?

A very pragmatic distinction between the two is that validating a business rule requires access to the current state of the domain model while validating input does not. Input validation can be implemented declaratively, as we did with the @NotNull annotations, while a business rule needs more context.

We might also say that input validation is a syntactical validation, while a business rule is a semantical validation in the context of a use case.

Let's take the rule "the source account must not be overdrawn." By the definition above, this is a business rule since it needs access to the current state of the model to check whether the source and target accounts do...