Mono class in Java: what is, and when to use?

Mono class in Java: what is, and when to use?

The Mono class is a fundamental component of Project Reactor, a reactive programming library for building non-blocking and asynchronous applications in Java. Project Reactor provides two main types: Mono and Flux. In this answer, we'll focus on the Mono class.

A Mono represents a sequence of zero or one item, making it analogous to a java.util.Optional. It's part of the Reactive Streams API and is used for handling asynchronous and non-blocking operations, such as network calls, file I/O, and other events. Here's an overview of what a Mono is and when to use it:

1. Representing a Single Value:

  • A Mono can represent either a single value (1 emission) or no value (empty).
  • Use Mono when you expect to receive zero or one result, such as a single response from a REST API call.

2. Asynchronous and Non-blocking Operations:

  • Mono is designed for asynchronous and non-blocking operations where you don't want to block the main thread.
  • Use Mono when working with operations that can take time to complete, like fetching data from a remote service.

3. Error Handling:

  • Mono can handle errors using its error-handling operators like onErrorResume, onErrorReturn, or onErrorMap.
  • Use Mono when you need to handle errors gracefully and continue processing.

4. Functional Programming:

  • Project Reactor, including Mono, promotes a functional programming approach.
  • Use Mono when you want to compose and transform asynchronous operations using operators like map, flatMap, filter, and others.

5. Combining and Sequencing Operations:

  • You can combine and sequence Mono instances using operators like then, zipWith, flatMap, and more.
  • Use Mono when you need to coordinate multiple asynchronous operations.

6. Reacting to Events:

  • Reactive programming with Mono allows you to react to events as they occur.
  • Use Mono when dealing with event-driven scenarios, such as handling incoming messages or notifications.

Here's a simple example of using Mono to make an asynchronous network call:

import reactor.core.publisher.Mono; import reactor.core.scheduler.Schedulers; public class MonoExample { public static void main(String[] args) { Mono<String> result = Mono.just("Hello, World!") .map(s -> s.toUpperCase()) .publishOn(Schedulers.elastic()); // Switch to a different thread for processing result.subscribe( value -> System.out.println("Received: " + value), error -> System.err.println("Error: " + error), () -> System.out.println("Done") ); } }

In this example, Mono.just("Hello, World!") creates a Mono with a single value. The map operator transforms the value, and publishOn switches to a different thread for processing. Finally, the subscribe method is used to define how to handle the result, error, and completion of the Mono.

Overall, you should consider using Mono when dealing with asynchronous and non-blocking operations in reactive and event-driven applications, especially when you expect zero or one result. Project Reactor's Mono provides a powerful and expressive way to work with such scenarios in a functional and reactive manner.

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