RFR: JDK-8232773: ClassLoading Debug Output for Specific Classes

[Adam Farley8]

Hey All,

To restart (and re-centre) the chat on this:

The issue I'm trying to solve is that it's hard to determine why a given 
class was not loaded by OpenJDK.

The solution I proposed was additional (optional) debug output from the 
different classloaders.

This way we can find out:

- Whether any classloaders tried to load the class from your class' 
location (by displaying all locations they try to load the class from).
- Which type of ClassLoaders tried to find your class.
- Which classpath each ClassLoader tried to use.
- Which modules were consulted.
etc etc.

Currently, it seems the only debug information available in this regard 
centres around:

- Classes that were successfully loaded
and
- Exceptions when the class couldn't be loaded.

So my questions are:

1) Am I trying to solve the right problem here?

2) Is there any support for this solution in principle, despite the fact 
that the webrev in its current form is...unrefined?

3) Is there a better way to get this information, with or without a code 
change?

Advice or opinions are appreciated.


Bug: https://bugs.openjdk.java.net/browse/JDK-8232773

Rough example webrev: http://cr.openjdk.java.net/~afarley/8232773/webrev/ 

Best Regards

Adam Farley 
IBM Runtimes




From:   David Holmes <david.holmes at oracle.com>
To:     Martin Buchholz <martinrb at google.com>
Cc:     Ioi Lam <ioi.lam at oracle.com>, core-libs-dev 
<core-libs-dev at openjdk.java.net>, Adam Farley8 <adam.farley at uk.ibm.com>
Date:   24/10/2019 05:22
Subject:        [EXTERNAL] Re: RFR: JDK-8232773: ClassLoading Debug Output 
for Specific Classes



Hi Martin,

This is going a bit OT I think ...

On 24/10/2019 1:24 pm, Martin Buchholz wrote:
> 
> 
> On Tue, Oct 22, 2019 at 3:58 PM David Holmes <david.holmes at oracle.com 
> <mailto:david.holmes at oracle.com>> wrote:
> 
> 
>     Perhaps one aspect of the class loading/resolution/initialization
>     process that can lead to confusion here is that if a class fails to
>     execute its static initialization then it is marked as Erroneous and
>     all
>     subsequent attempts to use that class result in NoClassDefFoundError
>     being thrown. If the original ExceptionInInitializerError got 
swallowed
>     somewhere then that can cause great confusion as to why the later 
NCDFE
>     occurs. The VM logging should help in that case - though I'd have to
>     confirm that (if it doesn't that should be fixed).
> 
> 
> I had a need to debug an error in a <clinit> this week and was again 
> frustrated by the difficulty.
> Classes falling into Erroneous state are rare so it seems reasonable to 
> save the exception as a cause for the subsequent NCDFE (probably can't 
> fix the misleading name of the exception)

So as you know such a request existed:

https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.openjdk.java.net_browse_JDK-2D8048190&d=DwIDaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=P5m8KWUXJf-CeVJc0hDGD9AQ2LkcXDC0PMV9ntVw5Ho&m=AfL8IyQ5gm4a9cA4orx0EnUwiGqvG6rei-DuA8gA0jQ&s=pzILDarpf9cN_MvQFqk1u3YZqFzSCRbBZVoRdFw4lS4&e= 


and was closed as it was considered that TraceExceptions (now 
-Xlog:exceptions) would provide the additional information. As noted in 
that bug there are also semantic issues about recording the cause of an 
exception in thread B, with an exception object that was thrown in 
thread A. So unless someone comes up with a very convincing argument, or 
else it prepared to do the work, I don't see that being re-opened.

> I couldn't find a way to get hotspot to report the stack trace of all 
> exceptions that are thrown; -Xlog:exceptions=trace does not .

-Xlog:exceptions shows where the exception is thrown and how it proceeds 
up the stack. It doesn't print the full stacktrace. That seems like a 
logging request for the Throwable constructor, or fillInStackTrace, to 
me, rather than trying to have the VM do it. IIRC we store minimal VM 
information in the backTrace which is expanded when needed into the full 
StacktraceElements array.
> I was also frustrated that a StackOverflowError failed to report the 
> name of the thread and the stack size that was exceeded.  But 
> -Xlog:threads*=trace helped there.

The issue there is that SOE instances are allocated natively without 
executing any Java code (as we have run out of the Java stack) so we 
can't (easily) create a String message for the exception. However 
-Xlog:exceptions could be expanded to report that information at the 
point we throw the SOE. I filed - JDK-8232923.

Cheers,
David




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