Spring WebClient and Java date-time fields

WebClient is Spring Framework's reactive client for making service to service calls.

WebClient has become a go to utility for me, however I unexpectedly encountered an issue recently in the way it handles Java 8 time fields that tripped me up and this post goes into the details.

Happy Path

First the happy path. When using a WebClient, Spring Boot advices a "WebClient.Builder" to be injected into a class instead of the "WebClient" itself and a WebClient.Builder is already auto-configured and available for injection.


Consider a fictitious "City" domain and a client to create a "City". "City" has a simple structure, note that the creationDate is a Java8 "Instant" type:

import java.time.Instant

data class City(
    val id: Long,
    val name: String,
    val country: String,
    val pop: Long,
    val creationDate: Instant = Instant.now()
)

The client to create an instance of this type looks like this:

class CitiesClient(
    private val webClientBuilder: WebClient.Builder,
    private val citiesBaseUrl: String
) {
    fun createCity(city: City): Mono<City> {
        val uri: URI = UriComponentsBuilder
            .fromUriString(citiesBaseUrl)
            .path("/cities")
            .build()
            .encode()
            .toUri()

        val webClient: WebClient = this.webClientBuilder.build()

        return webClient.post()
            .uri(uri)
            .contentType(MediaType.APPLICATION_JSON)
            .accept(MediaType.APPLICATION_JSON)
            .bodyValue(city)
            .exchange()
            .flatMap { clientResponse ->
                clientResponse.bodyToMono(City::class.java)
            }
    }
}

See how the intent is expressed in a fluent way. The uri and the headers are first being set, the request body is then put in place and the response is unmarshalled back to a "City" response type.


All well and good. Now how does a test look like.

I am using the excellent Wiremock to bring up a dummy remote service and using this CitiesClient to send the request, along these lines:

@SpringBootTest 
@AutoConfigureJson
class WebClientConfigurationTest {

    @Autowired
    private lateinit var webClientBuilder: WebClient.Builder

    @Autowired
    private lateinit var objectMapper: ObjectMapper

    @Test
    fun testAPost() {
        val dateAsString = "1985-02-01T10:10:10Z"

        val city = City(
            id = 1L, name = "some city",
            country = "some country",
            pop = 1000L,
            creationDate = Instant.parse(dateAsString)
        )
        WIREMOCK_SERVER.stubFor(
            post(urlMatching("/cities"))
                .withHeader("Accept", equalTo("application/json"))
                .withHeader("Content-Type", equalTo("application/json"))
                .willReturn(
                    aResponse()
                        .withHeader("Content-Type", "application/json")
                        .withStatus(HttpStatus.CREATED.value())
                        .withBody(objectMapper.writeValueAsString(city))
                )
        )

        val citiesClient = CitiesClient(webClientBuilder, "http://localhost:${WIREMOCK_SERVER.port()}")

        val citiesMono: Mono<City> = citiesClient.createCity(city)

        StepVerifier
            .create(citiesMono)
            .expectNext(city)
            .expectComplete()
            .verify()


        //Ensure that date field is in ISO-8601 format..
        WIREMOCK_SERVER.verify(
            postRequestedFor(urlPathMatching("/cities"))
                .withRequestBody(matchingJsonPath("$.creationDate", equalTo(dateAsString)))
        )
    }

    companion object {
        private val WIREMOCK_SERVER =
            WireMockServer(WireMockConfiguration.wireMockConfig().dynamicPort().notifier(ConsoleNotifier(true)))

        @BeforeAll
        @JvmStatic
        fun beforeAll() {
            WIREMOCK_SERVER.start()
        }

        @AfterAll
        @JvmStatic
        fun afterAll() {
            WIREMOCK_SERVER.stop()
        }
    }
}

In the highlighted lines, I want to make sure that the remote service receives the date in ISO-8601 format as "1985-02-01T10:10:10Z". In this instance everything works cleanly and the test passes.

Not so happy path

Consider now a case where I have customized the WebClient.Builder in some form. An example is say I am using a registry service and I want to look up a remote service via this registry and then make a call then the WebClient has to be customized to add a "@LoadBalanced" annotation on it - some details here

So say, I have customized WebClient.Builder this way:

@Configuration
class WebClientConfiguration {

    @Bean
    fun webClientBuilder(): WebClient.Builder {
        return WebClient.builder().filter { req, next ->
            LOGGER.error("Custom filter invoked..")
            next.exchange(req)
        }
    }

    companion object {
        val LOGGER = loggerFor<WebClientConfiguration>()
    }
}

It looks straightforward, however now the previous test fails. Specifically the date format of the creationDate over the wire is not ISO-8601 anymore, the raw request looks like this:

{
    "id": 1,
    "name": "some city",
    "country": "some country",
    "pop": 1000,
    "creationDate": 476100610.000000000
}

vs for a working request:

{
    "id": 1,
    "name": "some city",
    "country": "some country",
    "pop": 1000,
    "creationDate": "1985-02-01T10:10:10Z"
}

See how the date format is different.

Problem

The underlying reason for this issue is simple, Spring Boot adds a bunch of configuration on WebClient.Builder that is lost when I have explicitly created the bean myself. Specifically in this instance there is a Jackson ObjectMapper created under the covers which by default writes dates as timestamps - some details here.

Solution

Okay, so how do we get back the customizations that Spring Boot makes. I have essentially replicated the behavior of a auto-configuration in Spring called "WebClientAutoConfiguration" and it looks like this:

@Configuration
class WebClientConfiguration {

    @Bean
    fun webClientBuilder(customizerProvider: ObjectProvider<WebClientCustomizer>): WebClient.Builder {
        val webClientBuilder: WebClient.Builder = WebClient
            .builder()
            .filter { req, next ->
                LOGGER.error("Custom filter invoked..")
                next.exchange(req)
            }

        customizerProvider.orderedStream()
            .forEach { customizer -> customizer.customize(webClientBuilder) }

        return webClientBuilder;
    }

    companion object {
        val LOGGER = loggerFor<WebClientConfiguration>()
    }
}

There is a likely a better approach than just replicating this behavior, but this approach works for me.

The posted content now looks like this:

{
    "id": 1,
    "name": "some city",
    "country": "some country",
    "pop": 1000,
    "creationDate": "1985-02-01T10:10:10Z"
}

with the date back in the right format.

Conclusion

Spring Boot's auto-configurations for WebClient provides a opinionated set of defaults. If for any reason the WebClient and it's builder need to be configured explicitly then be wary of some of the customizations that Spring Boot adds and replicate it for the customized bean. In my case, the Jackson customization for Java 8 dates was missing in my custom "WebClient.Builder" and had to be explicitly accounted for.

A sample test and a customization is available here