RFR(M): 8199852: Print more information about class loaders in LinkageErrors.

Lois Foltan lois.foltan at oracle.com
Wed Mar 21 19:06:23 UTC 2018

On 3/21/2018 9:03 AM, Lindenmaier, Goetz wrote:

> Hi Lois,
> Thanks for looking at this!
> I had a look at Klass::class_loader_and_module_name(), but there are places
> where you only want to report the class loader information, therefore I
> decided to do it this way. (If you like this, I have more of these in work :)).
> For example see expectedErrorMessageDuplicate1 in the test.
> Also, there might just be a classloader at hand, not a class as needed
> for that method.
> But maybe it would make sense to call describe_external() in
> class_loader_and_module_name() to get more information about
> the classloader?

Hi Goetz,

In the JDK 9 time frame we strove to construct the string representing 
the class loader, module name & module version in a defined, structured 
manner.  This allows for consistency across error messages, stack dumps, 
etc.  Use of Klass::class_loader_and_name() gives us this consistency 
and you can see some examples of its use in ClassCastException 
messages.  See, https://bugs.openjdk.java.net/browse/JDK-8165550 and the 
test, test/hotspot/jtreg/runtime/modules/CCE_module_message.java.  I 
strongly encourage you to use Klass::class_loader_and_name() and not add 
describe_external().  Klass::class_loader_and_name() could be enhanced 
to take an enum parameter that allows for tailoring of the message 
itself, for example including/not including the module name when 
available, including the parent loader, etc.  It could be pulled out of 
Klass for use to obtain a class loader's name when a Klass is not 
available, although Coleen just recently moved it to Klass.  I would 
support this type of change.


> Best regards,
>    Goetz.
>> -----Original Message-----
>> From: Lois Foltan [mailto:lois.foltan at oracle.com]
>> Sent: Mittwoch, 21. März 2018 13:22
>> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-runtime-
>> dev at openjdk.java.net
>> Subject: Re: RFR(M): 8199852: Print more information about class loaders in
>> LinkageErrors.
>> On 3/21/2018 7:55 AM, Lindenmaier, Goetz wrote:
>>> Hi,
>>> ClassLoaders have a field containing a name since Java 9.
>>> This change improves some of the messages of LinkageErrors to
>>> print the name of the loader involved, as well as the parent of the loader.
>>> This simplifies tracking down the component causing an error in large
>>> Java systems.
>>> Please review:
>>> http://cr.openjdk.java.net/~goetz/wr18/8199852-
>> exMsg_Linkage/01/index.html
>>> I ran this through all our tests (jck, jtreg, spec etc).  I will run it through
>> submit-hs
>>> (or jdk/submit) once it's reviewed.
>>> Best regards,
>>>     Goetz.
>> Hi Goetz,
>> This looks promising and something that we started to do in the JDK 9
>> time frame.  Instead of adding the new method
>> java_lang_ClassLoader::describe_external() can you possible use/improve
>> upon Klass::class_loader_and_module_name() in order to avoid duplicate
>> code?
>> Thanks,
>> Lois

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