A bit of sugar for j.u.c.locks with try-with-resources?

[Vitaly Davidovich]

On Friday, September 2, 2016, Krystal Mok <rednaxelafx at gmail.com> wrote:

> Hi Vitaly,
>
> Thanks for your comments!
>
> On Fri, Sep 2, 2016 at 7:55 PM, Vitaly Davidovich <vitalyd at gmail.com
> <javascript:_e(%7B%7D,'cvml','vitalyd at gmail.com');>> wrote:
>
>> Hi Kris,
>>
>>
>> On Friday, September 2, 2016, Krystal Mok <rednaxelafx at gmail.com
>> <javascript:_e(%7B%7D,'cvml','rednaxelafx at gmail.com');>> wrote:
>>
>>> Hi core-libs developers,
>>>
>>> I mostly live down in the VM world, but recently I've been playing with
>>> j.u.c.locks a bit, and saw that there's an opportunity to retrofit the
>>> API
>>> with the try-with-resources syntax. I wonder if anybody has brought this
>>> topic up before; apologies if there had been.
>>>
>>> From the JavaDoc of j.u.c.l.ReentrantLock, the following is a typical
>>> usage:
>>>
>>>  class X {
>>>    private final ReentrantLock lock = new ReentrantLock();
>>>    // ...
>>>
>>>    public void m() {
>>>      lock.lock();  // block until condition holds
>>>      try {
>>>        // ... method body
>>>      } finally {
>>>        lock.unlock()
>>>      }
>>>    }
>>>  }
>>>
>>> The try...finally construction really pops out as a try-with-resources
>>> candidate.
>>>
>>> So what if we retrofit that with something like:
>>>
>>>  class X {
>>>    private final ReentrantLock lock = new ReentrantLock();
>>>    // ...
>>>
>>>    public void m() {
>>>      try (lock.lock()) { // block until condition holds
>>>        // ... method body
>>>      }                   // automatic unlock at the end
>>>    }
>>>  }
>>
>> This is Java 9 syntax right? Java 7-8 require an assignment to a local in
>> the TWR block.
>>
>
> Yes, this is Java 9 syntax, although I was actually intending to use the
> old Java 7/8 syntax as the example...got too used to the new goodies.
>

>
>>
>>> Assuming lock.lock() returns a temporary wrapper object (let's call it a
>>> "Locker" for this discussion), where Locker implements AutoCloseable, and
>>> its close() method calls lock.unlock().
>>> That'll make the API look and feel quite similar to the built-in
>>> "synchronized () { ... }" syntax. With escape analysis and scalar
>>> replacement implemented correctly in the VM, this temporary Locker object
>>> wouldn't incur much (or any) runtime cost after optimized JIT'ing, so it
>>> feels like a pure win to me.
>>>
>>> What do you think?
>>
>> So using TWR with scoped objects, such as this, is used quite a bit; I've
>> seen this idiom many times, and have used it myself.
>>
>> Now, what's the value add to have this in the JDK? One can write such a
>> Locker themselves quite easily.
>>
>> Having the Locker integrated into the API makes the experience smoother.
> If we write our own wrapper, it might look like:
>
> try (locker = new Locker(lock)) {
>   // ... method body
> }
>
> Which is okay-ish. Or maybe with the help of some statically imported
> helper method and the Java 9 syntax:
>
> try (locker(lock)) {
>   // ...
> }
>
> Hmm...I can live with that.
>
What about just the following (in Java 9):

try (new Locker(lock)) {
    //
}

>
> I also don't hold as much confidence in EA as you, apparently - it's too
>> brittle and unpredictable in its current form, IMHO.  Of course when the
>> allocation doesn't matter, that part isn't a big deal.
>>
>> Haha, I know how C2's EA isn't giving everybody the kind of confidence
> they want. But for simple cases like this it should work nicely (and if
> not, let's fix it :-)
>
Let's! :) As of right now, I consider EA as a bonus but never rely on it.
Even simple cases like this, if inside a method that's considered too big
for EA to run (particularly after inlining), will fail.

>
> And let's say when HotSpot gets a new JIT compiler, it'll have a more
> effective EA. All the better.
>
You mean Graal? I'm with Remi - value types will make this much more
palatable from an allocation perspective (and generally make more sense for
uses like this than full blown reference types).

>
> Thanks,
> Kris
>
>
>> My $.02.
>>
>>>
>>> Best regards,
>>> Kris (OpenJDK username: kmo)
>>>
>>
>>
>> --
>> Sent from my phone
>>
>
>

-- 
Sent from my phone