Updated ARM Spec

Joe Darcy joe.darcy at oracle.com
Tue Oct 26 10:40:09 PDT 2010

David Holmes wrote:
> Joe Darcy said the following on 08/24/10 05:45:
>> To recap, Throwable objects have several pieces of mutable state:
>> * the cause; this is a write-at-most-once value that can be set set 
>> via a constructor, like Throwable(Throwable cause) or 
>> Throwable(String message, Throwable cause), or set after construction 
>> via the Throwable.initCause method.  (Setting the cause to null 
>> prevents future assignment.)
>> * the stack trace; set by fillInStackTrace() or 
>> setStackTrace(StackTraceElement[] stackTrace) -- there doesn't seem 
>> to be any API prohibition against setting the stack trace multiple times
> But note that the VM implements fillInStackTrace and can ignore the 
> request, while setStackTrace is pure Java.
>> * suppressed exceptions : new functionality added to support 
>> try-with-resources statements/ARM blocks.  Modified via calling 
>> addSuppressedException
>> Focusing just on suppressed exceptions, to support the JVM reusing 
>> exception objects, the Throwable API should have some idiom to 
>> indicate suppressed exception information should *not* be recorded. 
>> Logically this amount to having the list of exceptions be a 
>> zero-length list which just discards adds.
>> I'm hesitant to overload a null cause with discarding suppressed 
>> exceptions too.  Instead, I propose the following:
>> * a null value of the suppressedException field indicates 
>> addSuppressedException is a no-op.
>> * the suppressedException field is initialized to point to a sentinel 
>> list in Throwable, something non-null that can be used for == 
>> checks.  With this protocol, the "raw" Throwable objects created by 
>> the JVM get the addSuppressedException is a no-op behavior for free.
>> * if the first call to addSuppressedException has this as an 
>> argument, the suppressedException field is null-ed; otherwise, a list 
>> is appended to as usual.
>> Comments?
> It seems to me, inline with what has been written, that Throwable 
> should adopt the overall approach that for each of the cause, 
> stacktrace and suppressedExceptions fields:
> - null indicates that these fields can not be modified (and is the 
> default we get from the instances pre-allocated in the VM)
> - the default initialization, via the constructor, is always a 
> non-NULL sentinel value
> This requires several changes to the existing method specifications.

The needed library changes to Throwable are being worked on under bug 
6991528 "Support immutable Throwable objects" and the changes will be 
sent out for review to core-libs-dev once they're ready.


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