New protocol for disabling exception suppression

David Holmes David.Holmes at
Sun Apr 3 02:31:44 PDT 2011

Rémi Forax said the following on 04/03/11 19:09:
> On 04/03/2011 04:28 AM, David Holmes wrote:
>> Rémi Forax said the following on 04/03/11 11:52:
>>> On 04/02/2011 03:21 AM, joe.darcy at 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] 
>>>> [2]
>>>> [3]
>>> 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.
> So I don't see the improvement ?

The improvement is that you no longer have the "hack" whereby calling 
addSuppressed(null) is interpreted as meaning "disable addSuppressed". 
Now suppression is enabled by default, unless this new constructor is 
used, or it is a VM created exception instance which acts as-if the new 
constructor had been used. Just makes the semantics for addSuppressed 
cleaner and simpler.


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