gRPC Java is not usable from Java 9 modules

Please answer these questions before submitting your issue.

What version of gRPC are you using?

1.6.1

What JVM are you using (java -version)?

java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

What did you do?

Java 9 allows users to depend on older, non-modularized versions of the libraries by “converting” them to automatic modules. For example, when Maven dependencies on grpc are configured correctly, Java 9 allows me to do the following:

module myapp.foo {
    exports com.myapp.foo;
    requires grpc.core;
}

This allowed me to use classes from the grpc-core within my Java 9 module, but unfortunately it wouldn’t compile:

ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.6.2:compile (default-compile) on project myapp.foo: Compilation failure: Compilation failure: 
[ERROR] the unnamed module reads package io.grpc from both grpc.core and grpc.context
[ERROR] module grpc.context reads package io.grpc from both grpc.core and grpc.context
[ERROR] module guava reads package io.grpc from both grpc.core and grpc.context
[ERROR] module error.prone.annotations reads package io.grpc from both grpc.core and grpc.context
[ERROR] module jsr305 reads package io.grpc from both grpc.core and grpc.context
[ERROR] module instrumentation.api reads package io.grpc from both grpc.core and grpc.context
[ERROR] module opencensus.api reads package io.grpc from both grpc.core and grpc.context
[ERROR] module grpc.core reads package io.grpc from both grpc.core and grpc.context

The issue is that Java 9 does not support split packages across modules and this is exactly what’s happening here, as io.grpc package exists in both grpc-core and grpc-context, and to make things worse both grpc-core and grpc-stub have transitive dependency on grpc-context.

I’ve tried excluding grpc-context from both modules using Maven exclusions, which allowed me to compile successfully, as I don’t have any direct dependencies on grpc-context. However, I was not able to run the test server, because of the missing Context class:

java.lang.NoClassDefFoundError: io/grpc/Context
        at io.grpc.internal.AbstractServerImplBuilder.build(AbstractServerImplBuilder.java:188)
        at io.grpc.testing.GrpcServerRule.before(GrpcServerRule.java:133)

There are several possible solutions, some better than the others:

1. Merge classes from grpc-context into grpc-core and leave empty/dummy grpc-context module around for backwards compatibility (although most people probably do not depend on it directly).
2. Do the same as above, but get rid of unnecessary grpc-context module.
3. Rename the io.grpc package in grpc-context to io.grpc.context, which would eliminate split package issue, but would break existing code that uses classes from the current location.

In any case, I’m happy to help do the work, but someone will need to decide which approach to take.

[lukecwik]

Users have requested support for Java 11 in Apache Beam and since it uses grpc it would need for grpc libraries to be compatible with Java 11. Hopefully this request helps increase the priority for fixing this.

[aseovic]

In the meantime, we have repackaged grpc-java (version 1.22.1) as a Java 9+ module as part of Helidon, in order to support our gRPC framework implementation, and have made it available on Maven Central.

This module basically merges grpc-core, grpc-context and grpc-stub into a single JAR, and adds necessary module_info.java to it.

@ejona86 This is not to say that we are not looking forward to a proper Java 9+ module support in grpc-java, which will allow us to get rid of this “necessary evil”. But we needed something now…