core: fix java.nio.ByteBuffer Java 9+ incompatible usage

[dapengzhang0]

No description provided.

[voidzcy]

We shouldn't need this, as we have sourceCompatibility = 1.7 in main build.gradle.

See my latest comment in #6829 (comment).

[dapengzhang0]

This is for third party users who build grpc source with Java 9+, and publish to maven local, even with sourceCompatibility = 1.7 and targetCompatibility = 1.7 unchanged. Their android app or java 8 app with grpc library in local maven repo would fail.

[voidzcy]

This is for third party users who build grpc source with Java 9+, and publish to maven local, even with sourceCompatibility = 1.7 and targetCompatibility = 1.7 unchanged. Their android app or java 8 app with grpc library in local maven repo would fail.

Would they fail? I don't think so as long as gRPC has sourceCompatibility = 1.7. I remember I tried this before and it worked without any issue.

[dapengzhang0]

Would they fail? I don't think so as long as gRPC has sourceCompatibility = 1.7. I remember I tried this before and it worked without any issue.

If you publish maven local of grpc master with java 11, and then build examples/android, you will fail. If you suppress the failure by dontwarn, you might fail in the runtime.

[voidzcy]

After looking around for a while, this seems to be more than source/target compatibility issue. The generated bytecode with JDK 9+ for method ByteBuffer#limit(int) would be treated as an overloaded method instead of an overridden method. So it cannot be found in JDK8's ByteBuffer.

[dapengzhang0]

After looking around for a while, this seems to be more than source/target compatibility issue. The generated bytecode with JDK 9+ for method ByteBuffer#limit(int) would be treated as an overloaded method instead of an overridden method. So it cannot be found in JDK8's ByteBuffer.

Yes, the same source can be compiled and run in all jdk's, but in different binaries, and the jdk9+ binary is not backward compatible.