Handle Errors using onErrorResume on Spring WebFlux Application

HelloController.java

package com.bytecodehq.routes;

import reactor.core.publisher.Mono;

import java.io.IOException;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {

  /**
   * Provide a fallback handler function on any exception
   */
  @GetMapping
  public Mono<String> helloWorld() {
    return Mono.<String>error(IOException::new)
        .onErrorResume(e -> Mono.just("Error : " + e.getMessage()));
  }

  /**
   * Provide a fallback handler function on {@link IOException}
   */
  @GetMapping("/2")
  public Mono<String> helloWorld2() {
    return Mono.<String>error(IOException::new)
        .onErrorResume(IOException.class, e -> Mono.just("Error : " + e.getMessage()));
  }

  /**
   * Provide a fallback handler function on truthfulness of predicate
   */
  @GetMapping("/3")
  public Mono<String> helloWorld3() {
    return Mono.<String>error(IOException::new)
        .onErrorResume(e -> e instanceof IOException, e -> Mono.just("Error : " + e.getMessage()));
  }

}