How to verify/test WebClient usage

Question

I need to unit test a class that uses the WebClient. Is there any good way to deal with the WebClient? With the RestTemplate I could easily use Mockito. Mocking the WebClient is a bit tedious, since deep stubs don't work with the webclient...

I want to test if my code provides the correct headers... shortened sample code:

public class MyOperations {
    private final WebClient webClient;

    public MyOperations(WebClient webClient) {
        this.webClient = webClient;
    }

    public Mono<ResponseEntity<String>> get( URI uri) {
        return webClient.get()
                        .uri(uri)
                        .headers(computeHeaders())
                        .accept(MediaType.APPLICATION_JSON)
                        .retrieve().toEntity(String.class);
    }

    private HttpHeaders computeHeaders() {
        ...
    }

}

Answer 1

This is aimed to unit, not integration tests...

Implemented in Kotlin, this is a bit rudimentary, but it's effective. The idea can be extracted from this pieces of code below

First, a WebClient kotlin extension

import org.mockito.ArgumentMatchers.anyString
import org.mockito.Mockito.*
import org.springframework.web.reactive.function.client.WebClient
import org.springframework.web.reactive.function.client.WebClientResponseException
import reactor.core.publisher.toMono

fun WebClient.mockAndReturn(data: Any) {
    val uriSpec = mock(WebClient.RequestBodyUriSpec::class.java)
    doReturn(uriSpec).`when`(this).get()
    doReturn(uriSpec).`when`(this).post()
    ...

    val headerSpec = mock(WebClient.RequestBodyUriSpec::class.java)
    doReturn(headerSpec).`when`(uriSpec).uri(anyString())
    doReturn(headerSpec).`when`(uriSpec).uri(anyString(), anyString())
    doReturn(headerSpec).`when`(uriSpec).uri(anyString(), any())
    doReturn(headerSpec).`when`(headerSpec).accept(any())
    doReturn(headerSpec).`when`(headerSpec).header(any(), any())
    doReturn(headerSpec).`when`(headerSpec).contentType(any())
    doReturn(headerSpec).`when`(headerSpec).body(any())

    val clientResponse = mock(WebClient.ResponseSpec::class.java)
    doReturn(clientResponse).`when`(headerSpec).retrieve()
    doReturn(data.toMono()).`when`(clientResponse).bodyToMono(data.javaClass)
}

fun WebClient.mockAndThrow() {
    doThrow(WebClientResponseException::class.java).`when`(this).get()
    doThrow(WebClientResponseException::class.java).`when`(this).post()
    ...
}

Then, the unit test

class MyRepositoryTest {

    lateinit var client: WebClient

    lateinit var repository: MyRepository

    @BeforeEach
    fun setUp() {
        client = mock(WebClient::class.java)
        repository = MyRepository(client)
    }

    @Test
    fun getError() {
        assertThrows(WebClientResponseException::class.java, {
            client.mockAndThrow()
            repository.get("x")
        })
    }

    @Test
    fun get() {
        val myType = MyType()
        client.mockAndReturn(myType)
        assertEquals(myType, repository.get("x").block())
    }
}

Note: tests on JUnit 5