Offer a better migration path from protobuf-java 3.x to 4.x

[ummels]

What language does this apply to?

Java

Describe the problem you are trying to solve.

After updating our project protobuf-java to 4.26.0 within my project, we noticed a serious runtime error in production: Code from an external dependency we're using is throwing a ClassNotFoundException while accessing the newBuilder() method of a generated class:

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 no.ecc.vectortile.VectorTileEncoder.encode(VectorTileEncoder.java:359)

This is a serious problem since there are many Java libraries floating around, which internally use Protobuf.

Describe the solution you'd like

In my opinion, the easiest way to fix the incompatibility would be to reintroduce GeneratedMessageV3 as a (deprecated) alias for GeneratedMessage, but maybe there are better alternatives?

[ummels]

See also grpc/grpc-java#11015.

[googleberg]

@ummels thank you for the feedback. We are evaluating the option to reintroduce GeneratedMessageV3 as a deprecated shim to smooth the transition.

Just want to add that in line with our version policy we will be supporting the protobuf 3.x line for a year to allow projects to update/adapt.

[googleberg]

The protobuf team has been looking at better migration paths for this first major version bump since the introduction of proto3 and future major version bumps. We'll publish the plan soon at https://protobuf.dev/support/version-support/

The exact date and version numbers are still TBD, but this quarter (between now and 7/1/24) we will make a a set of releases that allow gencode from a new patch release of the 3.25.- line to work with runtimes from the 4.-.- release (and all v4 releases following).

Because we're getting to this a bit slowly, we will also be extending support for 3.25.- to Q1 of 2026 to allow plenty of time for users to migrate.

Thank you for your patience.