I've translated a InfoQ's article about RxJava 2.0 into Japanese.
RxJava 2.0 quite differs from version 1.0. Summary of differences is in RxJava's website.
By the way, JDK 9 supports Reactive Stream (JEP 266).
The Reactive Streams spec will be included as java.util.concurrent.Flow. It has 4 interfaces in this spec.
Flow.Processor<T,R>: A component that acts as both a Subscriber and Publisher.
: A producer of items (and related control messages) received by Subscribers.
: A receiver of messages.
Flow.Subscription: Message control linking a Flow.Publisher and Flow.Subscriber.
Instead of this, we will need a reactive stream library like RxJava or Spring Web Reactive after release of JDK 9.