Questions about JDK-8136353 "ClassValue preventing class unloading"

[Peter Levart]

Hi Craig,

I think that what you are asking for is impossible to achieve in current 
Java.

ClassValue API is basically a cache that maps a pair: (ClassValue<T>, 
Class<?>) -> cached instance of type T

You would like the cached instance of type T to not be reachable from 
either ClassValue<T> instance or from Class<?> instance, but still be 
non-reclaimable while either of ClassValue<T> instance or Class<?> 
instance is reachable in some other way that excludes being reachable 
from the cached instance of type T.

For that to be achievable, Java would have to provide a feature called 
"Ephemeron".

Currently the cached instance of type T is reachable from Class<?> 
instance and you are using Integer.TYPE in your example. Which means 
that the cached instance of T is never going to be released if 
ClassValue<T> instance is reachable from cached instance of T. In your 
example, it is: (Dummy instance -> Dummy.class -> MyClassValue 
instance). So you get this reachability chain: Integer.TYPE -> 
ClassValueMap instance -> ClassValueMap.Entry[] -> ClassValueMap.Entry 
-> Dummy instance -> Dummy.class -> MyClassValue instance.
Integer.TYPE is a Class<?> instance representing int type which is 
loaded by bootstrap class loader, which never goes away.

ClassValue API could use a different strategy and reference the cached 
instance of type T from ClassValue<T> instance instead, but then you 
would have a problem with ClassValue instances reachable from other 
class loaders than the cached instances of type T.

Regards, Peter

On 01/06/2016 06:14 PM, Craig Andrews wrote:
> I apologize for contacting this list - I'm sure this isn't the "right 
> way" to contact the OpenJDK project, but I'm not clear what the "right 
> way" is.
>
> I'm hoping to raise https://bugs.openjdk.java.net/browse/JDK-8136353 
> "ClassValue preventing class unloading" as I believe it's a 
> significant issue in ClassValue which is a blocker for its use in 
> dynamic languages (which is primiarly why ClassValue was introduced). 
> I think the P5 priority set on the bug now is way too low, perhaps you 
> could consider raising the priority?
>
> The source code in the issue description is incorrect; it doesn't 
> compile. Could you please attach the working test cases to the issue, 
> so future testers and developers can reproduce the problem? Here are 
> links to the 2 source code files:
> https://issues.apache.org/jira/secure/attachment/12765900/CVTest.java
> https://issues.apache.org/jira/secure/attachment/12765901/MyClassValue.java 
>
> (which of course should be directly attached to the openjdk issue 
> tracker issue, and not hyperlinked to the Groovy tracker)
>
> And to reproduce the issue, run:
> $ javac MyClassValue.java && javac CVTest.java && mkdir -p t && jar 
> cvf t/t.jar MyClassValue*.class && rm MyClassValue*.class && 
> JAVA_OPTS=-Xmx4m java CVTest
> and wait for the an error to occur, which is:
> Exception in thread "main" java.lang.OutOfMemoryError: Compressed 
> class space
>     at java.lang.ClassLoader.defineClass1(Native Method)
>     at java.lang.ClassLoader.defineClass(ClassLoader.java:759)
>     at 
> java.security.SecureClassLoader.defineClass(SecureClassLoader.java:152)
>     at java.net.URLClassLoader.defineClass(URLClassLoader.java:470)
>     at java.net.URLClassLoader.access$100(URLClassLoader.java:76)
>     at java.net.URLClassLoader$1.run(URLClassLoader.java:371)
>     at java.net.URLClassLoader$1.run(URLClassLoader.java:365)
>     at java.security.AccessController.doPrivileged(Native Method)
>     at java.net.URLClassLoader.findClass(URLClassLoader.java:364)
>     at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
>     at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
>     at CVTest.main(CVTest.java:12)
>
> The bug is reproducible on all JDK 8 and 9 builds (I tested up to JDK 
> 9 build 99).
>
> Based on my understanding of the situation from my research in the 
> Groovy bug that discovered the issue ( 
> https://issues.apache.org/jira/browse/GROOVY-6704 ) and some work 
> another person did with the YourKit profiler ( 
> https://www.yourkit.com/forum/viewtopic.php?f=3&t=12658 ), I suspect 
> that the fix for https://bugs.openjdk.java.net/browse/JDK-8032606 
> "ClassValue.ClassValueMap.type is unused" is relevant; I think the 
> problem lies in the java.lang.Class.classValueMap implementation.
>
> Thank you,
> ~Craig Andrews
> _______________________________________________
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> mlvm-dev at openjdk.java.net
> http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev

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