RFR [9] 8071472: Add field access to support setting final fields in readObject

[Chris Hegarty]

Peter, Alan,

After further thought I think that requiring all final fields to be set is reasonable behaviour. Since there is no compile time checking, this is a reasonable runtime effort to catch what is likely to be developer errors. To address Alan’s comments, I beefed up the API docs and added examples to typical usage.

Updated specdiff (and webrev):
   http://cr.openjdk.java.net/~chegar/8071472/01/ <http://cr.openjdk.java.net/~chegar/8071472/01/>

This version also includes a number of changes to readObject implementations in the base module, to replace unsafe usage with this field setter API.

-Chris.


> On 13 Mar 2015, at 19:36, Chris Hegarty <chris.hegarty at oracle.com> wrote:
> 
>> 
>> On 13 Mar 2015, at 17:58, Peter Levart <peter.levart at gmail.com> wrote:
>> 
>> On 03/12/2015 11:24 PM, Peter Levart wrote:
>>>> So if a readObject calls fields() without calling defaultReadObject() then it has to set every final field. On one hand that ensures that every final field is set, on the other hand it is a bit odd because omitting the call to fields() means they all get their default value. 
>>> 
>>> If fields() is not called, we must be backwards-compatible.
>>> 
>>> But yes, this constraint is questionable. On one hand it tries to mimic the assignment to final fields in constructors, but it can't go all the way, as read access to final fields in readObject() is not limited to just those that have already been assigned (like it is in constructor with definitive assignment rules). We could add get() methods to FieldAccess that would constraint the reads of final fields to those that have already been assigned, but that is just like "advisory locking" - we can only advise users to use FieldAccess to read fields in readObject() but we can't prevent them from reading them directly.
>>> 
>>> So if this doesn't have much sense and brings confusion, it can go away.
>> 
>> ...or it can stay in part where we check that a final field is not set more than once, which can help especially when use of FieldAccess API is combined with defaultReadObject().
> 
> Yes, that makes sense.
> 
>> The final check that all finals have been assigned can be made optional by an explicit call to a method (FieldAccess.checkAllFinalsSet() for example).
> 
> If possible, I’d rather not have any additional methods exposed on FieldSetter, other than the overloaded sets. Let see how this works if we keep it as minimal as possible. I’m going to take a run over all readObjects in the base module that use unsafe or reflection, and rewrite to use this API.
> 
> -Chris.
> 
>> Regards, Peter