Re: Loading providers in named modules with ServiceLoader using a plugin class realm

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Re: Loading providers in named modules with ServiceLoader using a plugin class realm

gboue

Le 25/06/2017 à 22:03, Chas Honton a écrit :
> Under what circumstances would a plugin not want the platform classloader?
>
> Chas

Thinking about this more, with the current situation, it actually means
that potential providers, located in named modules loaded for example by
Maven core classloader or extension classloader, will not be available
to plugins using ServiceLoader. So this is bigger than platform
classloader. As Robert said, I think there needs to be a way for plugins
to know about modules.

Reading more into the docs, this may be possible using a ModuleLayer.
Each plugin would have its own module layer composed of modules found in
its dependencies. Their parent would be a module layer associated to
Maven core, and its parent would be the boot layer. This would solve the
ServiceLoader issue for named modules in a plugin realm, since it would
locate providers in all modules in the module layer of the plugin, and
then do the same for their parents, up to the boot layer. However, for
it to work, I'm not sure if this implies that everything be made modular
(plugins and Maven itself). I will try to do some testing on this.

Guillaume

>
>> On Jun 25, 2017, at 12:40 PM, Robert Scholte <[hidden email]> wrote:
>>
>> Hi Guillaume,
>>
>> I don't know all the details about the Platform classloader, but it has been introduced with a reason.
>> So I don't think we should switch to it by default. I think the plugin is well aware which classloaders / modules it wants to use (it should be), so I think we need to find a mechanism for plugins to select their classloaders and modules.
>>
>> thanks,
>> Robert
>>
>>> On Sun, 25 Jun 2017 17:19:07 +0200, Guillaume Boué <[hidden email]> wrote:
>>>
>>> Hi,
>>>
>>> With the introduction of modules in JDK 9, there were changes with
>>> regard to how classloading works, and this impacts class realms created
>>> in Maven. Today, the parent (as per ClassLoader.getParent()) of a class
>>> realm is null, which represents the bootstrap classloader. In JDK 9, the
>>> change is that some classes were moved to a named module other than
>>> java.base, and they are not loaded with the bootstrap classloader
>>> anymore, but with the platform classloader (which was previously the
>>> extension classloader, see JDK-814637).
>>>
>>> This has consequences, like MANTRUN-200, where locating providers with
>>> the ServiceLoader API, using the plugin class realm, will miss JDK
>>> internal implementation classes. In the case of MANTRUN-200, it is
>>> Nashorn of the jdk.scripting.nashorn module, that cannot be found
>>> because of the way ServiceLoader works
>>> http://download.java.net/java/jdk9/docs/api/java/util/ServiceLoader.html#load-java.lang.Class-java.lang.ClassLoader-.
>>> During the search in named modules, the class realm will not be able to
>>> use its strategy since that process relies on parent delegation
>>> implemented as explicit calls to ClassLoader.getParent()
>>> (http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/f3cf7fd26baa/src/java.base/share/classes/java/util/ServiceLoader.java#l1062),
>>> which, for a class realm, corresponds to the base classloader, i.e. the
>>> bootstrap classloader. And since the Nashorn script engine factory was
>>> loaded with the platform classloader, it is missed.
>>>
>>> It seems that the fix here would be to make all class realms have as
>>> base classloader the platform classloader starting with JDK 9, instead
>>> of the bootstrap classloader (there is a new utility method in
>>> ClassLoader to obtain the platform classloader). I verified that this
>>> solves the problem described in MANTRUN-200, but before I create an MNG
>>> issue, I'm wondering if this is the correct approach.
>>>
>>> What do you think of this change to class realms? The other possibility
>>> I can think of would be to have a way in the JDK to override the search
>>> in named modules, so that our ClassRealm can also delegate to its parent
>>> classloader. It is possible for unnamed modules (since the search
>>> process then relies on the ClassLoader.getResources method, that can be
>>> overriden) but it doesn't look like (and probably intentionally so) to
>>> be possible for named modules.
>>>
>>> Guillaume
>>>
>>>
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>>> https://www.avast.com/antivirus
>>>
>>>
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Re: Loading providers in named modules with ServiceLoader using a plugin class realm

gboue
 From what I tested, ServiceLoader doesn't allow a custom classloading
scheme for named modules (step 1 of
http://download.java.net/java/jdk9/docs/api/java/util/ServiceLoader.html#load-java.lang.Class-java.lang.ClassLoader-).
Its own scheme relies on checking modules loaded by the given
classloader, then its parents (with a particular rule with regard to
module layers), and it doesn't look like customizable. It also seems
that existing META-INF/services declarations, but located in named
modules, are ignored. For unnamed modules, or when something other than
ServiceLoader is used, everything is apparently the same as before in
Java 9.

This is why I think a plugin wouldn't be able to locate providers in a
module loaded by Maven core extension classloader: parent of the plugin
realm is bootstrap classloader directly.

Guillaume


Le 26/06/2017 à 01:45, Igor Fedorenko a écrit :
> Can you explain little more why plugins won't see classes loaded by
> maven core or maven core extensions classloaders? This is implemented in
> classwords and I was under impression that java9 still allowed custom
> classloading schemes like what we do or like what OSGi does.
>


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