New protocol for disabling exception suppression

[David Holmes]

Rémi Forax said the following on 04/03/11 11:52:
> On 04/02/2011 03:21 AM, joe.darcy at oracle.com wrote:
>> As part of the library support for the try-with-resources statement,
>> several API changes were made to Throwable including an addSuppressed
>> method to allow suppressed exceptions to be recorded. As previously
>> discussed on coin-dev [1], to support VM needs for reusable exception
>> objects, a protocol was devised to disable the suppression mechanism so
>> that a zero-length array would be returned from getSuppressed even if
>> addSuppressed was called with a valid argument. The mechanism was a bit
>> of a kludge, relying on an initial call to addSuppressed with a null
>> argument, and the design was called out as such. [2] I'm happy to report
>> the JSR 334 expert group has devised a more elegant protocol to disable
>> exception suppression: a new constructor is added to Throwable which
>> supports disabling suppression. The existing constructors of Throwable
>> always enable suppression and addSuppressed(null) now always throws a
>> NullPointerException. A few exception and error types in the platform
>> are allowed by behave as if their objects were created with suppression
>> disabled.
>>
>> The fix was recently pushed [3] and will appear in a future JDK 7 build.
>>
>> -Joe
>>
>> [1] http://mail.openjdk.java.net/pipermail/coin-dev/2010-August/002830.html
>> [2]
>> http://mail.openjdk.java.net/pipermail/coin-dev/2010-October/thread.html#2920
>> [3] http://hg.openjdk.java.net/jdk7/tl/jdk/rev/856cc9e97aea
>>
> 
> So the code of the VM was changed to call the constructor if it exists ?

No. The VM never calls a constructor it just allocates zeroed memory - 
which has the same affect as calling the new constructor.

David

> Rémi
> 
> 
>