Chaining exceptions

[Jesse Phillips]

On Wed, 18 Nov 2009 15:24:11 -0800, Andrei Alexandrescu wrote:

> Consider:
> 
> void fun() {
>     try {
>        throw new Exception("a");
>     } finally {
>        throw new Exception("b");
>     }
> }
> 
> Currently this function would unceremoniously terminate the program. I
> think it shouldn't. What should happen is that the "a" exception should
> be propagated unabated, and the "b" exception should be appended to it.
> The Exception class should have a property "next" that returns a
> reference to the next exception thrown (in this case "b"), effectively
> establishing an arbitrarily long singly-linked list of exceptions.
> 
> A friend told me that that's what Java does, with the difference that
> the last exception thrown takes over, so the chain comes reversed. I
> strongly believe "a" is the main exception and "b" is a contingent
> exception, so we shouldn't do what Java does. But Java must have some
> good reason to go the other way.
> 
> Please chime in with (a) a confirmation/infirmation of Java's mechanism
> above; (b) links to motivations for Java's approach, (c) any comments
> about all of the above.
> 
> 
> Thanks,
> 
> Andrei

Best as I can tell, the Java compiler doesn't do the chaining 
automatically. It is up to the one throwing the exception to make the 
chain. The exception class just provides a specification that requires 
all exceptions to support chaining. This explains why it is not the root 
cause that is at the head of the chain.

try {
     stmt.executeUpdate(sql);
} catch (SQLException ex) {
   throw new
   EmployeeLookupException(
   "Query failure",ex); // ex is passed to the constructor of the class
} 

Example from: http://java.sys-con.com/node/36579

http://www.developer.com/tech/article.php/1431531/Chained-Exceptions-in-
Java.htm