Fwd: Re: Invoking default methods from a Proxy's InvocationHandler in JDK9

David M. Lloyd david.lloyd at redhat.com
Wed Mar 1 22:37:00 UTC 2017

This keeps coming up.  I think we'd better raise it as an issue because 
the workaround used in Java 8 is specifically broken by JPMS.  In that 
regard, Peter Levart's workaround might be considered to fall under the 
FC extension umbrella.

-------- Forwarded Message --------
Subject: Re: Invoking default methods from a Proxy's InvocationHandler 
in JDK9
Date: Tue, 3 Jan 2017 11:14:31 -0800
From: Mandy Chung <mandy.chung at oracle.com>
To: Peter Levart <peter.levart at gmail.com>
CC: jigsaw-dev at openjdk.java.net

> On Jan 2, 2017, at 11:20 PM, Peter Levart <peter.levart at gmail.com> wrote:
> Hi Matthew,
> On 01/03/2017 04:28 AM, Matthew Hall wrote:
>> I'm a member of the JDBI [1] project, an open source SQL access library
>> atop JDBC.
>> A major part of our API provides implementations of declarative interfaces
>> defined by users (similar to MyBatis). Interface methods may be default (in
>> which case the default method implementation is used) or abstract (in which
>> case JDBI provides the implementation).
>> We're using JDK 8, and we use java.lang.reflect.Proxy and InvocationHandler
>> to provide declarative interface implementations for our users.
>> Prior to release of JDK 8, it appears that the subject of enhancing Proxies
>> for default methods was discussed [2], but ultimately did not make it into
>> the release.
>> The root of the problem is that the generated Proxy class overrides all
>> methods in the proxy interfaces. This means that the interface default
>> methods are now superclass methods to the proxy class, which makes them
>> un-invokable from outside code--including the InvocationHandler.
>> As far as we can tell, the _only_ way to invoke a default method--on a
>> proxy, from an InvocationHandler--is to resort to some horrible
>> setAccessible shenanigans [3].
>> Specifically, we have to:
>> 1. Call Constructor.setAccessible(true) to make the private constructor
>> MethodHandles.Lookup(Class<?> lookupClass, int allowedModes) accessible,
>> 2. Invoke that private constructor to instantiate a MethodHandles.Lookup
>> with private (!) access to all the things,
>> 3. Call Lookup.unreflectSpecial to get the MethodHandle for the default
>> method, and
>> 4. Invoke the method through the MethodHandle.
>> This is ugly, but it works--for now. If JDK 9 takes away access to
>> setAccessible, it may cease to work.
>> I found some discussion about this last summer [4], but it's hard to tell
>> if anything came out of the discussion.
>> Is there anything on the roadmap for JDK9 to allow a proxy's
>> InvocationHandler to invoke default methods on proxies? Are there any
>> existing proposals I should be aware of?
> I have created a prototype last year:
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2016-June/041629.html
> But I think the JDK 9 train has already left the station. So perhaps in JDK 10...

https://bugs.openjdk.java.net/browse/JDK-8159746 is the JBS issue for this.

One other possibility is to fix proxy generated class not to override 
default methods but there would require more investigation to tease out 
before we can be certain if this can make JDK 9.



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