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

Daniel D. Daugherty daniel.daugherty at oracle.com
Wed Sep 4 22:06:37 UTC 2019


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 
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).


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

More information about the hotspot-dev mailing list