grpc-java: Dropping support for Java 6 (planned for 1.15)

[Eric Anderson]

TL;DR: Dropping support for Java 6, but keeping current Android API level 14 support. If this causes problems for you, please reply here or comment on the issue.

Discussion issue: https://github.com/grpc/grpc-java/issues/3961

Java 7 was released in July 2011, 7 years ago. It seems there should be few arguments for keeping Java 6 support and Java 6 support is causing us pain. To be clear, nothing here will prevent supporting Android API level 14 and higher.

Support for Java 6 was initially to make Android support easier. When gRPC started there was some Java 7 support for Android, but it was pretty new and support across the ecosystem might have been spotty. Since we wanted the Java 6 support for managing Android, we didn't need to question whether there were enough "normal" Java 6 users/use cases to warrant the support.

Java 6 support is getting harder to manage with our runtime dependencies. It is preventing us from upgrading to newer Guava versions as well as the latest Protobuf version. In both cases we can run with the newer versions, but we can't make direct use of their features.

Java 8 will stop receiving public updates in a few months and Java 11 won't support building for Java 6. That will put us in an odd situation where we would require an unsupported compiler. (Note: this applies to Java 7 as well. Although Java 10 didn't kill support for Java 6, and the current Java 11 ea doesn't kill support for Java 6 nor warns for Java 7.)

io.grpc.Context is a bit of a special case, since it can be used more places. The current expectation is to leave io.grpc.Context at Java 6 for the moment. But that should be seen as temporary. Any issues with io.grpc.Context dropping support for Java 6 should be discussed now.

If dropping Java 6 support causes you undue burden, please speak up here or in the issue. Please make sure to CC me if replying to this email. And since I'm at it, I created an issue to track dropping Java 7 support.