在Java中创建自定义SQL检查的异常？

Question

I know how to create a basic custom checked exception, but I want to be able to create an exception that checks the following:

1. I basically have some tables in a DB, in that DB I have a table message with 2 columns message and queue. I can only enter a message with a queue number,if that queue number exists in table queue_table (ie foreign key concept). I want to be able to check, that if a person retreives topmost message from a queue (in table message),(ie retreives topmost row with a particular queue number) that there exists that queue number in the queue_table table. If it doesnt I want it to trigger an exception called QueueDoesNotExistException. Without the exception, the query to obtain messages from a non existing queue in queue_table gives me null , so I am not sure if an existing exception will be useful.

So I have the following :

package asl.exceptions;

public class QueueDoesNotExistException extends Exception {
    public QueueDoesNotExistException(){



    }
}

I am not sure how to handle this, I dint find any resources for this, or am I looking at the wrong place?

I read this : https://docs.oracle.com/javase/tutorial/jdbc/basics/sqlexception.html

But dint find it useful.

Answer 1

you need to catch DB-specific exception thrown by your JDBC driver first. then you can wrap it in your exception and propagate further. but exception-driven logic is rarely a good design.

Answer 2

You should never let the flow get to the point where a customer is trying to access a message from a non-existent queue. You check for queue existence first thing in the workflow and abort the workflow with an exception, without ever making the second query for messages in the queue.

Answer 3

Based on comments, it seems you are trying to validate the query itself , as opposed to ensuring that it cannot return invalid data. Apparently, you also want to distinguish between a query that returns zero rows on account of the specified queue not being recorded in the DB, and a query that returns zero rows because there are simply no messages available in that queue. I'm doubtful of the usefulness of these pursuits, but if you insist, then here's an approach you could take:

1. Ensure that your Connection is not in auto-commit mode ( connection.setAutoCommit(false) )
2. Attempt to select the specified queue from the queue_table by its ID. If that fails, then roll back the connection's transaction and throw the exception of your choice.
3. Otherwise, perform the main query as ordinarily you would do, and unless it throws, commit the transaction.
4. Be watchful for the query returning zero rows, and handle the situation however is appropriate.
5. If you rely on changing the autocommit mode as part of the procedure, be sure to record and restore the original autocommit mode, including in the event of an exception.

I apologize if that seems simple or nonmagical, or whatever.