RFR: 8212117 : Class.forName may return a reference to a loaded but not linked Class

David Holmes david.holmes at oracle.com
Thu Sep 5 05:36:19 UTC 2019

Hi Dan,

With my CSR Group member hat on ....

On 5/09/2019 8:06 am, Daniel D. Daugherty wrote:
> Brent,
> You currently have '-XX:+ClassForNameDeferLinking' as a 'product' 
> option, but
> product options are harder to remove down the road. Would it be better as a
> diagnostic option? A diagnostic option requires 

Whether a flag is product or diagnostic (or experimental) should be a 
basic property of the flag based on its purpose. I would not want to see 
a trend of making new flags diagnostic just because it is easier to get 
rid of them later. The expectation with this fix and flag (which I've 
been heavily involved in) is that production code may be impacted by the 
change and need to use the flag to restore previous behaviour. So it 
really is a product flag that end users should be comfortable in using 
if they need it.

The removal process for a product flag is phased (deprecate, obsolete, 
expire) but not particularly onerous. There is an expectation that this 
flag may be deprecated in 15 as it is intended as a transitional flag.


> '-XX:+UnlockDiagnosticVMOptions'
> to be specified before it can be used, e.g.:
>      java -XX:+UnlockDiagnosticVMOptions -XX:+ClassForNameDeferLinking Foo
> so it is a bit harder to use, but maybe that's a Good Thing (TM).
> Dan
> On 9/4/19 5:12 PM, Brent Christian wrote:
>> Hi,
>> Please review my fix for JDK-8212117[1].  The webrev is here:
>> http://cr.openjdk.java.net/~bchristi/8212117/webrev09/
>> There is also a CSR[2] in need of review.
>> The spec for the 2-arg and 3-arg Class.forName() methods states they 
>> will "locate, load, and link" the class, however the linking part is 
>> not ensured to happen.
>> Although the VM spec allows flexibility WRT when classes are linked, 
>> this is a corner where the Class.forName() spec is not being upheld. 
>> While this is not an issue for most usages,  8181144 [3] demonstrates 
>> how this can be a problem (with the debugging interface, in this case).
>> This fix ensures that linking happens during the course of 
>> Class.forName().  Class.forName() already @throws LinkageError, so no 
>> spec change is needed there. MethodHandles.Lookup.findClass() needs a 
>> small spec update, due to calling Class.forName with init=false,
>> Of course Errors are not required to be caught.  It is therefore 
>> possible that the new behavior could introduce previously unseen, 
>> possibly unhandled LinkageErrors.  A new VM flag 
>> (-XX:+ClassForNameDeferLinking) is introduced to restore the previous 
>> behavior (to keep such code running until it can be updated).
>> This change surfaced a couple new "A JNI error has occurred" 
>> situations (see 8181033[5]) in the Launcher, by way of
>>   test/jdk/tools/launcher/MainClassCantBeLoadedTest.java
>> (using the 3-arg Class::forName, detailed in the bug report[4]),
>> and
>>   test/jdk/tools/launcher/modules/basic/LauncherErrors.java
>> (using the 2-arg Class::forName).
>> The Launcher is updated to maintain non-confusing error messages :).
>> The new test included with this fix ensures that 8181144[3] is 
>> addressed.  Thanks go to Serguei Spitsyn for writing the test.
>> Automated corelibs and hotspot tests pass cleanly.
>> Thanks,
>> -Brent
>> -- 
>> 1. https://bugs.openjdk.java.net/browse/JDK-8212117
>> 2. https://bugs.openjdk.java.net/browse/JDK-8222071
>> 3. https://bugs.openjdk.java.net/browse/JDK-8181144
>> 4. 
>> https://bugs.openjdk.java.net/browse/JDK-8212117?focusedCommentId=14215986&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14215986 
>> 5. https://bugs.openjdk.java.net/browse/JDK-8181033

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