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

Joe Darcy joe.darcy at oracle.com
Thu Sep 5 22:30:56 UTC 2019


For the doc changes in MethodHandle, please supplement the paragraph

1937          * <p>
1938          * Note that this method throws errors related to loading 
and linking as
1939          * specified in Sections 12.2 and 12.3 of <em>The Java Language
1940          * Specification</em>.

with @jls tags like

@jls 12.2 Loading of Classes and Interfaces
@jsl 12.3 Linking of Classes and Interfaces

as done elsewhere in the base module. I think it would be slightly 
better to remove the section numbers from the main text but keep them in 
the @jls tags since it is now possible to check that the listed name of 
the section matches what is in the JLS.



On 9/4/2019 2:12 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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