Proposal: Automatic Resource Management

[Jeremy Manson]

On Tue, Mar 3, 2009 at 11:56 PM, Neal Gafter <neal at gafter.com> wrote:
> On Tue, Mar 3, 2009 at 11:49 PM, Jeremy Manson <jeremy.manson at gmail.com> wrote:
>> As a side note to this conversation, directed at any people who will
>> think you can't have this feature with this proposal, regardless of
>> your feelings about the particular merits of accomplishing the same
>> thing with closures, there *is* a way to handle this case:
>>
>> try (LockDisposer l = new LockDisposer(lock.readLock())) {
>>  clientCount++;
>> }
>>
>> try (LockDisposer l = new LockDisposer(lock.writeLock())) {
>>  return field.getValue();
>> }
>
> Yow.  I think the cure might be worse than the disease.

That's why I suggested changing try() so that it took an expression.
The second version was, I think you will admit, much cleaner.

>> I suspect a LOT of people will be doing something similar to this hack
>> if this proposal is adopted.  To make it much cleaner, you could first
>> adjust this proposal so that the try statement can take an expression
>> that returns a Disposable, and then you could adjust the Lock classes
>> so that a) lock() returns this and b) they implement Disposable, at
>> which point you could have:
>
> You can't make such changes to existing interfaces without breaking
> existing code.  Changing the signature of lock() breaks existing
> callers, and adding a close() method (to implement Disposable) breaks
> existing implementations.

I'm probably being typically dumb, but I'm not sure how adding a
method breaks anything. You don't *have* to change the signature of
lock -- just add a new method (call it "disposeLock") that has the
required semantics:

try (lock.readLock().disposeLock()) {
 clientCount++;
}

try (lock.writeLock().disposeLock()) {
 return field.getValue();
}

Jeremy