Project Proposal: CompatibleChanges

Sebastian Sickelmann sebastian.sickelmann at
Fri Aug 17 08:15:03 UTC 2012

In accordance with the OpenJDK guidelines [1], I would like to start 
of a new Project to change the OpenJDK JVM implementation to make it 
possible to
update java apis in ways that are not possible today.

The goal of the CompatibleChanges Project will be to change the OpenJDK 
JVM implementation.
It is not intented to change Java The-Language. Currently i there is one 
Proof of Concept[2]
that shows the possible-usage in an example (remove public static- and 
instance-fields in a
binary compatible way).
The POC uses bytecode-rewrite at loadtime and invokedynamic at runtime 
to figure out
which method to call instead of an fieldaccess. My initial suggest would 
be to
change the JVMS-rules for Field Resolution and don't do it with 
instrumentation and

Real-Short Example:
public class ClassToChange {
     public Throwable cause = new RuntimeException("INIT");
     public static Object staticField = "ORIG_VALUE";
Evolved Implementation:
public class OLD {
     private Throwable cause = new RuntimeException("INIT");
     private static Object staticField = "ORIG_VALUE";
     @Accessor("cause") public Throwable getCause() { return cause; }
     @Accessor("cause") public void initCause(Throwable cause) { 
this.cause = cause; }
     @Accessor("staticField") public static Object getFoo() { return 
staticField; }
     @Accessor("staticField") public static void setFoo(Object value) { 
staticField = value; }

Some initial thoughts about Reflection-Changes and 3 ways of how the 
Field Resolution Rules can
be changed are documented here[3]. It is written in the textual form of 
a JEP, but it is not
intended to be a JEP.

There is some initial feedback (on my first discussion request in 
jdk8-dev) by Brian Goetz
and Joe Darcy back in Oct 2011[4].

The initial committers are proposed to be: Sebastian Sickelmann

Please discuss and let me know what you think.

Thank you for your consideration,


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