Proposal: Move to Java 8

[ZacSweers]

With Android Gradle Plugin 3.2.0 being stable now, it brings a new D8 tool that includes Bazel's desugaring of Java 8 code. This has enabled the android library ecosystem to start moving to Java 8 (language features first, possibly backporting of APIs like streams in the future).

As a result, the android ecosystem (as well as libraries) have started being able to migrate to Java 8. Since that was an original design goal of RxJava 2, but backported to Java 6 for android compatibility, I propose we reconsider that now that the ecosystem has moved forward.

If the proposal's accepted, we could do most of the bulk work fairly easily in an automated pass with IDE tooling.

• Change target/source languages in build.gradle to java 8
• Run project-wide inspections from IntelliJ to migrate all anonymous inner classes that could be lambdas or method references
  • Run on main sources first, keep tests as-is for regression testing
  • After main is migrated and verified migrate tests in separate pass

Type annotations could be safely introduced solely for static analysis purposes (ref #6316)

Example community projects that have successfully moved to Java 8 (some android, some java, some both)

• RxBinding
• AutoDispose
• Butter Knife

Thoughts?

[akarnokd]

RxJava is set out to support Android 2.3+, does this recent tooling enhancements still work with such old runtimes?

Run project-wide inspections from IntelliJ

I did the conversion back and forth the time and IntelliJ just gave up after a few files or created a non-compilable mess due to type-inference mismatches. Had to do it individually. Also this doesn't help the main library much as we moved to non-anonymous classes.

[ZacSweers]

Yep! It compiles down to Java-6 compatible bytecode as far as I know (the detailed docs say any minSdkVersion)

I did the conversion back and forth the time and IntelliJ just gave up after a few files or created a non-compilable mess due to type-inference mismatches. Had to do it individually.

Interesting. We did the same across our monorepo (several hundred thousand LoC) at the beginning of this year and found the tooling to be mostly seamless and quite reliable. Maybe it's improved since you tried?

Also this doesn't help the main library much as we moved to non-anonymous classes.

I figured, but at the same time it's a big step forward for the community, brings the project up to more modern tooling/language features, and potentially opens interesting doors for the project in the future

[akarnokd]

There are a few extra considerations:

• RxJava runs on desktop/server where we also pledged Java 6 as a baseline.
• Reactor 3 is Java 8 already.
• Is it just the Android compiler or is there standard library support behind it, such as java.util.Objects and java.time.Duration?

[ZacSweers]

RxJava runs on desktop/server where we also pledged Java 6 as a baseline.

Since Java 8 is already reaching EoL, supporting it as a minimum seems like a fair thing. Ubiquitous major libraries like Guava are also Java 8 only now (including using Java 8 APIs).

Is it just the Android compiler or is there standard library support behind it, such as java.util.Objects and java.time.Duration?

Ish. The issue I linked is requesting for a newer version of Desugar which does support desugaring of apis like those. For now, D8 should be treated as only syntax backporting.

I'm not sure what consideration for reactor 3 is for? (unfamiliar with the project aside from knowing it exists)

[slisaasquatch]

This is just my two cents.
While I acknowledge that Java 8 support is nice to have, I don't think it's going to make a huge difference. There's already RxJava2Jdk8Interop, and it's easy enough to convert between Duration and TimeUnit.

[artem-zinnatullin]

If this is applied, we'll need to add desugaring as CI check, otherwise nothing prevents contributors from referencing JDK APIs that can not be desugared

[ZacSweers]

If this is applied, we'll need to add desugaring as CI check, otherwise nothing prevents contributors from referencing JDK APIs that can not be desugared

Animal sniffer can cover that. Example: uber/AutoDispose#264

Also another consideration - targeting java 6 is actually deprecated in gradle's java support at this point:

warning: [options] source value 1.6 is obsolete and will be removed in a future release
warning: [options] target value 1.6 is obsolete and will be removed in a future release

For me, it's not just a "nice to have". The combination of evolving tooling requirements and community direction risk could (in my opinion) make not adopting a risk.

[ZacSweers]

Any further thoughts on this? Happy to send a proposal PR if you'd rather

[akarnokd]

We can't just switch in a minor/patch version anyway and I'd prefer Java 9 so that we can be dependency-free.

[catt-stefano]

Just to make sure you guys are aware of this (probably are), to use a library written in Java 8 in an Android project, you would need to enable java 8 source compatibility in the app itself, just to support the Java 8 library.
So this extra step would be necessary for making the library work:

// this goes in the app gradle file
compileOptions {
   sourceCompatibility JavaVersion.VERSION_1_8
   targetCompatibility JavaVersion.VERSION_1_8
}