Difference between RxJava API and the Java 9 Flow API
It seems on every iteration of Java for the last few major releases, there are consistently new ways to manage concurrent tasks.
In Java 9, we have the Flow API which resembles the Flowable API of RxJava but with Java 9 has a much simpler set of classes and interfaces.
Java 9
Has a Flow.Publisher, Flow.Subscriber, Flow.Processor, Flow.Subscription, and SubmissionPublisher, and that's about it.
RxJava
Has whole packages of Flow API-like classes, i.e. io.reactivex.flowables, io.reactivex.subscribers, io.reactivex.processors, io.reactivex.observers, and io.reactivex.observables which seem to do something similar.
What are the main differences between these two libraries? Why would someone use the Java 9 Flow library over the much more diverse RxJava library or vice versa?
Answer
What are the main differences between these two libraries?
The Java 9 Flow API is not a standalone library but a component of the Java Standard Edition library and consists of 4 interfaces adopted from the Reactive Streams specification established in early 2015. In theory, it's inclusion can enable in-JDK specific usages, such as the incubating HttpClient, maybe the planned Async Database Connection in parts, and of course SubmissionPublisher.
RxJava is Java library that uses the ReactiveX style API design to provide a rich set of operators over reactive (push) dataflows. Version 2, through Flowable and various XxxProcessors, implements the Reactive Streams API which allows instances of Flowable to be consumed by other compatible libraries and in turn one can wrap any Publisher into a Flowable to consume those and compose the rich set of operators with them.
So the Reactive Streams API is the minimal interface specification and RxJava 2 is one implementation of it, plus RxJava declares a large set of additional methods to form a rich and fluent API of its own.
RxJava 1 inspired, among other sources, the Reactive Streams specification but couldn't capitalize on it (had to remain compatible). RxJava 2, being a full rewrite and a separate main version, could embrace and use the Reactive Streams specification (and even expand upon it internally, thanks to the Rsc project) and has been released almost a year before Java 9. In addition, it was decided both v1 and v2 keeps supporting Java 6 and thus a lot of Android runtimes. Therefore it couldn't capitalize directly on the Flow API provided now by Java 9 directly but only through a bridge. Such bridge is required by and/or provided in other Reactive Streams-based libraries too.
RxJava 3 may target the Java 9 Flow API but this hasn't been decided yet and depending on what features the subsequent Java versions bring (i.e., value types), we may not have v3 within a year or so.
Till then, there is a prototype library called Reactive4JavaFlow which does implement the Flow API and offers a ReactiveX style rich fluent API over it.
Why would someone use the Java 9 Flow library over the much more diverse RxJava library or vice versa?
The Flow API is an interoperation specification and not an end-user API. Normally, you wouldn't use it directly but to pass flows around to various implementations of it. When JEP 266 was discussed, the authors didn't find any existing library's API good enough to have something default with the Flow API (unlike the rich java.util.Stream). Therefore, it was decided that users will have to rely on 3rd party implementations for now.
You have to wait for existing reactive libraries to support the Flow API natively, through their own bridge implementation or new libraries to be implemented.
Providing a rich set of operators over the Flow API is only reason a library would implement it. Datasource vendors (i.e., reactive database drivers, network libraries) can start implementing their own data accessors via the Flow API and rely on the rich libraries to wrap those and provide the transformation and coordination for them without forcing everybody to implement all sorts of these operators.
Consequently, a better question is, should you start using the Flow API-based interoperation now or stick to Reactive Streams?
If you need working and reliable solutions relatively soon, I suggest you stick with the Reactive Streams ecosystem for now. If you have plenty of time or you want to explore things, you could start using the Flow API.