(Nestmates) RFR (M): 8240645: Update nest host determination in line with latest proposed JVMS changes for JEP-371

David Holmes david.holmes at oracle.com
Fri Mar 6 05:29:53 UTC 2020

webrev: http://cr.openjdk.java.net/~dholmes/8240645/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8240645

JEP-371 (Hidden Classes) is changing a key aspect of nest host 
resolution and validation compared to what we shipped originally for 
JEP-181 (Nestmates). Originally, any LinkageErrors during nest host 
resolution would be propagated, and any validation failures would result 
in IncompatibleClassChangeError being thrown. Now no LinkageErrors are 
thrown, but the fact one occurred is remembered for potential use if a 
private access check fails. Similarly, validation failures don't cause 
an exception any more. Nest host resolution always succeeds and the nest 
will be one of:
- the successfully resolved nest host as per the NestHost attribute
- the class itself if there is no NestHost attribute or there is an 
error during resolution or validation
- the specified nest host explicitly set if this is a hidden class 
injected into a nest

as appropriate.

This has a flow on affect to the reflection method 
Class::getNestMembers() which can also no longer ever fail.

To be able to diagnose issues with nest host resolution/validation you 
can use unified logging to examine what happens: 

In addition IllegakAccessErrors thrown by the VM are augmented with 
information about any nest host resolution/validation errors. For 
example an original exception:

java.lang.IllegalAccessError: class TestNestmateMembership$Caller tried 
to access private method 'void 
(TestNestmateMembership$Caller and TestNestmateMembership$TargetSelfHost 
are in unnamed module of loader 'app')

is now expanded with an additional "(<nest host resolution error(s)>)" 
section which is memoized for use with other access checks:

java.lang.IllegalAccessError: class TestNestmateMembership$Caller tried 
to access private method 'void 
(TestNestmateMembership$Caller and TestNestmateMembership$TargetSelfHost 
are in unnamed module of loader 'app')(Type 
TestNestmateMembership$TargetSelfHost (loader: 'app') is not a nest 
member of type TestNestmateMembership$TargetSelfHost (loader: 'app'): 
current type is not listed as a nest member)

These exception messages are very long and unweildy but that seems 
unavoidable. They appear repetative in that the type names appear in the 
main message, the module information, and then again in the resolution 
error section. Unfortunately we can't avoid that as the memoized string 
has to be complete because it does not know the context of the 
IllegalAccessError in which it will be used. Please look at the 
mechanism by which this is done before making suggestions on how to 

Also note that in the worst case there can be two nest host resolution 
errors reported, if both the current class and the target class had such 

Note: the augmented exceptions do not extend to IllegalAccessExceptions 
thrown by the Java reflection or MethodHandle code. This would require 
an additional VM call to retrieve the information. This may be 
considered for a future RFE.

Changes in detail:

- src/hotspot/share/ci/ciField.cpp
- src/hotspot/share/classfile/classFileParser.cpp
- src/hotspot/share/runtime/reflection.cpp

No longer a possibility of exceptions

-  src/hotspot/share/interpreter/linkResolver.cpp

Expands on the message for IllegalAccessErrors in the private case, to 
report any nest host resolution/validation errors.

-  src/hotspot/share/oops/instanceKlass.cpp

Implements the new specification for resolving the nest host, and adds 
the means to memoize the error message. The same message is used for 
exceptions and logging.

Exception handling is changed as needed - no use of CHECK macros.

Removed a case in has_nest_member where we would bail out if executing 
in a compiler thread and encountered an unresolved class entry in the 
CP. We previously bailed out as a precaution as the compiler thread 
can't load classes. I was surprised to find that I was hitting this 
code, and getting runtime failures because of it (not test failures). I 
don't see how the current changes would cause this. In any case the 
situation where actual class loading would be needed should be 
impossible - two classes of the same name within the same loader - so 
the bail out can be removed.

Removed the recently added runtime_nest_host code which is no longer needed.

-  src/hotspot/share/prims/jvm.cpp

Some code simplifies now exceptions are no longer possible.

JVM_GetNestMembers has major updates:
- as we no longer bail out on error we may end up with holes in the 
array, so we have to copy across to an array of the exact size before 
- added logging to aid in debugging

-  src/hotspot/share/utilities/ostream.cpp

Needed to add the ability for stringStream to return a C-Heap allocated 
buffer so we could use it for the memoized error messages. (This should 
probably be factored out to a separate RFE for mainline.)

- src/java.base/share/classes/java/lang/Class.java

Updated the specifications for getNestHost and getNestMembers as per 
Mandy's proposed spec updates (added missing @jvms links in 
getNestMembers()). Adjusted the implementation for the fact there are no 
longer any exceptions from the VM.


Had to change all the LinkageErrors/NoClassDefFoundErrors to be 
IllegalAccessError or IllegalAccessException and adjust the expected 
messages to check the underlying failure reasons.

Other test adjusted as needed to account for no exceptions, and the 
changes to Class::getNestMembers().

Testing: all nestmate and hidden class tests under normal conditions.
          tiers 1 - 3


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