Explain the concept and main principles of using Project Reactor in Java.

Project Reactor is a library for reactive programming in Java that implements the Reactive Streams specification. It enables the creation of asynchronous, non-blocking applications that efficiently utilize resources.

Main principles:

• Flux and Mono — the primary types: Flux represents a stream of 0..N elements, Mono — 0..1 element.
• Backpressure — managing the data processing rate to avoid overwhelming the consumer.
• Operators — methods for transforming, filtering, and combining streams.
• Subscription — a reactive stream starts working only after subscription.

Example of usage:

Flux.just(1, 2, 3, 4)
    .filter(i -> i % 2 == 0)
    .map(i -> i * 2)
    .subscribe(System.out::println); // Will output 4 and 8

Thus, Project Reactor helps in writing scalable and responsive applications with convenient handling of asynchronous data streams.