Spring WebFlux Mono Junit Test Example

This example shows how to write junit test for a method that returns Mono.

StepVerifier from io.projectreactor.reactor-test is used to test reactive components.

Below is the Service class that returns Mono.

BookService.java

package com.kswaughs.webflux;

import com.kswaughs.webflux.model.Book;

import reactor.core.publisher.Mono;

public class BookService {
    
    public Mono<Book> getBookById(int bookId) {
        
        if(bookId == 0) {
            return Mono.error(new Exception("Invalid BookId"));
        }
        
        if(bookId == 100) {
            
            return Mono.just(buildBook(bookId));
        }
        
        return Mono.empty();
    }
    
    private Book buildBook(int bookId) {
        
        Book book = new Book();
        
        book.setBookId(bookId);
        book.setAuthor("John Grisham");
        book.setTitle("A Painted House");

        return book;
    }
}

Below is the Junit test class to test Mono using StepVerifier

SpringWebFluxMonoExampleTest.java

package com.kswaughs.webflux;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

import org.junit.Test;

import reactor.test.StepVerifier;

public class SpringWebFluxMonoExampleTest {

    private BookService bookService = new BookService();

    @Test
    public void whenBookExists() {

        StepVerifier.create(bookService.getBookById(100))
            .assertNext(book -> {

                assertEquals(Integer.valueOf(100), book.getBookId());
                assertEquals("John Grisham", book.getAuthor());
                assertEquals("A Painted House", book.getTitle());
                
        }).verifyComplete();
    }
    
    @Test
    public void whenBookNotExists() {

        StepVerifier.create(bookService.getBookById(56))
            .verifyComplete();
    }
    
    @Test
    public void whenBookIdIsInvalid() {

        StepVerifier.create(bookService.getBookById(0))
        
          /** case 1: To validate only Exception className  **/
            //.expectError(Exception.class)
        
          /** case 2: To validate only exception message **/    
            //.expectErrorMessage("Invalid BookId")
        
          /** case 3: To Validate complete exception object **/    
            .expectErrorSatisfies(thr -> {
                assertTrue(thr instanceof Exception);
                assertEquals("Invalid BookId", thr.getMessage());
            })
            .verify();
    }
}

pom.xml

<dependency>
    <groupId>io.projectreactor</groupId>
    <artifactId>reactor-test</artifactId>
    <scope>test</scope>
</dependency>

Useful Links

Spring WebFlux Junit Test Example