JDK 15 hidden classes & stack trace visibility

[Uwe Schindler]

Hi Remy,

Those classes would get a strong relationship to class loader. So I would like to have the anonymous class features in combination with make them visible in stack traces.

The current approach using separate classloader works fine for that as it unloads perfectly (exactly what's discussed in the JEP), so I would like to only reduce overhead of the added classloader (which is used only to allow unloading of those short living objects).

The hidden classes work exactly like expected for this dynamic situation of short living classes, but the backside is the "hidden" feature. I'd like to make the hidden-ness a feature. My question is if this can be decoupled. The hidden feature is wanted for lambdas and such stuff, but in some situations stack traces are essential.

Uwe

Am September 5, 2020 11:18:35 PM UTC schrieb Remi Forax <forax at univ-mlv.fr>:
>Hi Uwe,
>
>----- Mail original -----
>> De: "Uwe Schindler" <uschindler at apache.org>
>> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>
>> Envoyé: Samedi 5 Septembre 2020 13:55:03
>> Objet: JDK 15 hidden classes & stack trace visibility
>
>> Hi,
>> 
>> I am doing a mockup for dynamically compiled scripts in Apache Lucene
>(later
>> also painless scripting in elastcsearch), where I tried to use
>> Lookup#defineHiddenClass (with default parameters, so weak and no
>nestmates
>> - but: for painless scripts of Elasticsearch, nestmates will be
>great). It
>> all looks great, as sometimes for every query a new class is
>generated.
>> 
>> The current approach (Java 8, Java 11) uses a Classloader per script,
>which
>> is mostly OK, but hidden classes would be better, especially under
>high load
>> with many new classes. The problem I see is: The hidden class and
>their
>> methods are also hidden from stack traces, so people neither get the
>script
>> source code reference, nor they get the method, which was called at
>all.
>> 
>> My question is: is it possible to mark such a hidden class in byte
>code or
>> by a flag to defineHiddenClass() so it is "unhidden"? - I know you
>can do
>> this by a command line option, but that's for developers and
>debugging only.
>> In some cases, this is really wanted (like for scripting languages),
>but
>> code still wants to use the better class unloading. If this is not
>possible,
>> how about extending the Lookup.ClassOption enum (last parameter of
>> defineHiddenClass), to also pass a flag to show them in stack traces?
>
>If you don't want a hidden class, why not using Lookup.defineClass()
>for 11 and calling ClassLoader.defineClass() by reflection in 8 ?
>With defineClass you get a resolvable real name and methods are visible
>in the stacktrace.
>
>Obviously, it means that you are grouping several classes in the same
>classloader which may be not what you want.
>
>> 
>> Uwe
>
>Rémi
>
>> 
>> -----
>> Uwe Schindler
>> uschindler at apache.org
>> ASF Member, Apache Lucene PMC / Committer
>> Bremen, Germany
>> https://lucene.apache.org/