I got this message: Cannot construct instance of `reactor.core.publisher.Mono`

Question

I used Jersey and Webflux with R2DBC. after send the POST via the postman I got this message " Cannot construct instance of reactor.core.publisher.Mono "

This is my JerseyConfiguration:

@Component
public class JerseyConfiguration
        extends ResourceConfig {
    public JerseyConfiguration() {
        register(ProductController.class, 1);
    }
}

and this is my Controller:

@Path("/v1")
@Controller
public class ProductController {

    @Autowired
    private ProductService productService;

    @POST
    @Consumes(MediaType.APPLICATION_JSON)
    @Produces(MediaType.APPLICATION_JSON)
    @Path("/product")
    public Mono<Product> createProduct(@RequestBody Mono<Product> productMono){
        return productMono.flatMap(this.productService::createProduct);
    }
}

and this sis my service:

@Service
public class ProductService {

    @Autowired
    private ProductRepository repository;

    public Mono<Product> createProduct(final Product product){
        return this.repository.save(product);
    }
}

and also this my pom.xml

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jersey</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-r2dbc</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>
        <dependency>
            <groupId>io.r2dbc</groupId>
            <artifactId>r2dbc-postgresql</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

Now, this is my problem; I got this message from the postman:

Cannot construct instance of `reactor.core.publisher.Mono` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (org.glassfish.jersey.message.internal.ReaderInterceptorExecutor$UnCloseableInputStream); line: 1, column: 1]

Please let me know how to solve that problem.

Thank you

Answer 1

You cannot mix WebFlux and Jersey. You should choose one or the other, not both. They both provide an HTTP server engine, but:

• Jersey is a Servlet JAX-RS implementation, it does not know anything about reactive streams, Mono, Flux, etc.
• Webflux is the Spring HTTP server engine based on reactive streams and async Netty HTTP server.

If you look at Spring Boot reference documentation, section 3.5: Web, you will see that Jersey is one of the available engines, competing with other possible engines, i.e Web MVC and web reactive (webflux).

So, the answer is : Jersey is incompatible with Webflux, and you must choose between Webflux reactive Web and Spring rest annotation, or Jersey and Jax_RS without using Mono/Flux as return-type.

Note 1 : You should annotate your class with @RestController whe using webflux, so it understand that method return is the HTTP response body (see the last paragraph of reference documentation section 1.4.1: @Controller for details.

Note 2 : If you really want to use jersey, but you still require to consume Mono objects from other parts of your system, you might use one of the conversion functions provided by Reactor to return an object that jersey can work with. For example, on Mono object, you will find a toFuture() method. You could also block(), but it could be dangerous.