CloneNotSupportedException should extends RuntimeException not Exception

Joe Darcy joe.darcy at
Fri Nov 2 22:44:05 UTC 2012

On 10/15/2012 03:56 AM, Alan Bateman wrote:
> On 15/10/2012 11:28, Joel Borggrén-Franck wrote:
>> On 10/15/2012 12:34 AM, David Holmes wrote:
>> > Remi,
>> >
>> > This ship has sailed you can't recall it. 
>> CloneNotSupportedException > is a checked exception and needs to 
>> remain so for source and binary
>> > compatibility.
>> >
>> I see how this is source incompatible and also behaviorally 
>> incompatible in a few cases, but how is this binary incompatible? 
> I think you're right as there wouldn't be a linkage error.
> his thread reminds of Joe Darcy's classic blog on compatibility kinds:


Catching up on email, changing the supertype of an exception from 
Exception to RuntimeException exception is *binary* compatible under the 
rules for superclasses and interfaces evolution given in the JLS:

"Changing the direct superclass or the set of direct superinterfaces of 
a class type will not break compatibility with pre-existing binaries, 
provided that the total set of superclasses or superinterfaces, 
respectively, of the class type loses no members."

Since the subtype (RuntimeException) has all the members of its parent 
(Exception), and also the same set of constructor signatures, this 
change is binary compatible and will *not* cause a linkage error at 
runtime (which is the definition of binary compatible).

As Brian pointed out, there is a small risk of source and behavioural 
compatibility change is a try block had both catch clauses for 
CloneNotSupportedException and RuntimeException.  That in and of itself 
doesn't rule out such a change since our general evolution policy for 
the JDK is:

>     1. Don't break binary compatibility (as defined in the Java 
> Language Specification).
>     2. Avoid introducing source incompatibilities.
>     3. Manage behavioral compatibility changes.

In some lesser-used corners of the platform, we've made just this sort 
of change with exception superclasses, in full awareness of the 
potential issues:

     6519115 MirroredTypeException thrown but should be 

However, given the prevalence of CloneNotSupportedException, I don't 
think making it an unchecked exception after it was a checked exception 
is appropriate.


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