How do I deal with problems during business object construction?

Question

Hi Everyone.
I'm wondering about best practices for program control flow when business objects can't be constructed properly.
In my servlet I want to create a new Transaction business object based on the parameters of the HttpServletRequest.
transaction = new Transaction( request );The Transaction, in turn, has a Customer business object representing the Customer doing the transaction. The Customer is also constructed based on request parameters. So I have in the Transaction constructor,
public Transaction( HttpServletRequest request ) {
    customer = new Customer( request );
}In the Customer constructor I want to do some validation. For example, I want to check that the requested Customer exists in the database.
If the validation fails, should I throw an exception, which could then eventually be caught by the Servlet which would send back an "Invalid Customer" message to the end user?
But it seems to me like this violates the "never use exceptions for flow control" rule (as described for example at http://onjava.com/pub/a/onjava/2003/11/19/exceptions.html?page=2).
It is a normal function of my application to deny a transaction to a non-valid customer... so it seems like I shouldn't need to bring in the exception handling overhead for this normal business process.
OK, so I guess put "is_valid" flags in the business objects and set them on constructor errors then check them??
public Transaction( HttpServletRequest request ) {
    customer = new Customer( request );  // customer.is_valid flagged false
    if ( !customer.get_is_valid( ) ) {
        this.is_valid = false; // transaction.is_valid is also false now because its Customer is not valid
    } else { // customer is valid
        // finish the rest of the Transaction construction business logic
        // of course this will turn into a mess of deeply nested "else valid" clauses
        // if there are many validation checks...
}...and then in the servlet...
transaction = new Transaction( request );
if ( !transaction.get_is_valid( ) ) {
    // Send back error message to user.
} else {
    // Finally do some business logic.  Maybe after another mess of nested "else valid" clauses.
}This gets so hard to read it seems like the throwing Exceptions as part of normal control flow option is preferble.
Is there third option I am missing?
What are your opinions?
Thanks,
Bishop

Answer 1

Build your Transaction and Customer objects based on the data in the request. Then validate the customer, since an invalid customer is "normal" for you.
I'd consider using some sort of factory-like method for creating these objects from the request, since you're tying (sort of) your business stuff to the fact you're currently using a webby (servlet-based) front end...but that's more an aside.
So, in pseudo-ish code:
Transaction t = createTransaction(request);
if (t.validCustomer()) do work;
else return to wherever;This is only a first-pass idea, and other requirements might knock it for six, but it's a startpoint.