[Java] The release notes state that compatibility was restored in v27.2, but it is not compatible

[be-hase]

Summary

When trying to use the code generated with protobuf v25.3 in a protobuf-java v4.27.2 environment, a compilation error occurs.

Cannot access com.google.protobuf.GeneratedMessageV3.Builder

In v27.2, GeneratedMessageV3.Builder does not exist.

• v27.2
  • https://github.com/protocolbuffers/protobuf/blob/v27.2/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java
• v25.3
  • https://github.com/protocolbuffers/protobuf/blob/v25.3/java/core/src/main/java/com/google/protobuf/GeneratedMessageV3.java#L559

Links

• https://github.com/protocolbuffers/protobuf/releases/tag/v27.2
• Update to Protobuf 4.x grpc/grpc-java#11015

[ejona86]

I'm suspecting this is an ABI incompatibility. When you compile generated code against v4.27.2 what can go into the class file is GeneratedMessage$Builder (since GeneratedMessageV3 extends GeneratedMessage). But if the generated code was compiled with v3.x, then the .class file will have GeneratedMessageV3$Builder which no longer exists.

[be-hase]

Incidentally, an exception occurs at runtime in the reverse case(using the code generated with protobuf v27.2 in a protobuf-java v3.25.3 environment).
Moreover, regarding this, the protobuf team may not intentionally maintain forward compatibility.

[ejona86]

@be-hase, that would not be supported. Newer generated code can make use of newer runtime features. For basically all libraries in the ecosystem, downgrading dependencies is not directly supported.

[be-hase]

@ejona86
I believe that libraries should always use the latest versions, so I have no problem with that.
(Of course, it would be nice to have forward compatibility if possible... haha)

[be-hase]

I have prepared a reproduction repository, so please feel free to use it.
https://github.com/be-hase/protobuf-java-compatibility-issue

[epkugelmass]

What @be-hase noticed means that protobuf-java 4.27.1 is incompatible with grpc-java 1.64.0
See this reproducer: https://github.com/epkugelmass/protobuf-java-grpc-compat/tree/master

Even HealthCheckRequest.newBuilder().build() will fail to compile.

[zhangskz]

v4.27.2 should have re-established compatibility with v3.25.x gencode from source, but looks like additional shims will need to be added to further restore compatibility with v3.25.x gencode in jars compiled against 3.25.x runtime (per ejona86's prior comment).

Also can confirm that new gencode + old runtime is never allowed. You can see our full policies on this in https://protobuf.dev/support/cross-version-runtime-guarantee.

[be-hase]

New Gencode + Old Runtime = Never Allowed

OK, I see.

v4.27.2 should have re-established compatibility with v3.25.x gencode from source, but looks like additional shims will need to be added to further restore compatibility with v3.25.x gencode in jars compiled against 3.25.x runtime (per ejona86's prior comment).

I am waiting for the shims to be added.

[pcj]

Another example (com/google/protobuf/GeneratedMessageV3$ExtendableMessageOrBuilder does not exist):

--protoc-gen-scala_out: java.lang.NoClassDefFoundError: com/google/protobuf/GeneratedMessageV3$ExtendableMessageOrBuilder
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
	at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
	at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:862)
	at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:760)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:681)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
	at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:150)
	at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:862)
	at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:760)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:681)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
	at scalapb.options.Scalapb.<clinit>(Scalapb.java:23793)
	at scalapb.ScalaPbCodeGenerator$.registerExtensions(ScalaPbCodeGenerator.scala:13)
	at protocgen.CodeGenApp.run(CodeGenApp.scala:42)
	at protocgen.CodeGenApp.run$(CodeGenApp.scala:39)
	at scalapb.ScalaPbCodeGenerator$.run(ScalaPbCodeGenerator.scala:11)
	at protocgen.CodeGenApp.main(CodeGenApp.scala:27)
	at protocgen.CodeGenApp.main$(CodeGenApp.scala:26)
	at scalapb.ScalaPbCodeGenerator$.main(ScalaPbCodeGenerator.scala:11)
	at scalapb.ScalaPbCodeGenerator.main(ScalaPbCodeGenerator.scala)
Caused by: java.lang.ClassNotFoundException: com.google.protobuf.GeneratedMessageV3$ExtendableMessageOrBuilder
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
	... 27 more

Using com.thesamet.scalapb:compilerplugin_2.12:0.11.17