MethodHandles.lookup errors where publicLookup is ok

[Chen Liang]

Just curious, would klass.getModule().isExported(klass.getPackageName(), JRUBY_MODULE) be a more accurate test here?

Chen
________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Charles Oliver Nutter <headius at headius.com>
Sent: Thursday, September 4, 2025 11:41 AM
To: Alan Bateman <alan.bateman at oracle.com>
Cc: core-libs-dev at openjdk.org <core-libs-dev at openjdk.org>
Subject: Re: MethodHandles.lookup errors where publicLookup is ok

On Wed, Sep 3, 2025 at 4:09 AM Alan Bateman <alan.bateman at oracle.com<mailto:alan.bateman at oracle.com>> wrote:
Method handles are aligned with bytecode behavior, this includes access checks in the MH factory methods. If you were to inject code into module org.jruby.dist with a reference to a class in module X then any attempt in org.jruby.dist to access a class in module X would fail with IllegalAccessError. org.jruby.dist's module does not declare "requires X" and so org.jruby.dist does not read X.

Lookup.publicLookup is the minimally trusted lookup to create method handles to public fields and methods. This necessitated the addition of the UNCONDITIONAL access mode even though it doesn't correspond to a modifier in the access flags. It means that the access check done with the publicLookup is closer to what you see with core reflection (where readability is assumed) rather than bytecode. This may seem complicated but it's not something that most developers will ever need to deal with directly.

Ok, I understand. Play with sharp tools, get cut. And given the evolution of MH and Lookup over the years I'm not surprised my original expectations from a decade ago don't quite match the modern reality of JPMS.

Raising an error saying it "is not accessible" is also rather misleading when it actually **is** accessible
It's not accessible from code in module org.jruby.dist so not accessible with the Lookup obtained from lookup(). That said, we need to improve the IAE exception messages. A lot of effort went into exception messages thrown by setAccessible/Method.invoke/Field.xxx where as the IAE exception messages thrown by the MH factory methods are less helpful.

I guess it will just take me some time to internalize the reality of "can't be read until read access is requested" for cases that "magically" work through traditional reflection. This discussion helps.

BTW: Your first message has "symbolic reference class is not accessible: class jdk.proxy4.$Proxy49" which suggests that the JRuby code may be attempting the findVirtual for the method in the proxy implementation class rather than the method in the interface class. You might to check that.

Fair point. We have typically assumed that if Modifier.isPublic(klass) was true, we are good to go ahead and bind that class and its methods in our Java integration layer and indy call sites. I'm testing a patch that adds JRUBY_MODULE.canRead(klass.getModule()) to that requirement and so far it fixes the original issue and does not appear to regress any other features. Instead of binding Function.apply as Proxy49.apply, we should only bind it against the public interface.

- Charlie


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