Use Spring WebClient for RESTful communication

RESTful communication is the de-facto standard for interchanging data in a microservice-based environment. Usually, every participating microservice offers different parts of the application's domain in a RESTful way and calls other microservices to gather data for e.g. a different part of the application's domain. Resilience and efficient data interchange is, therefore, a key factor when it comes to the success of the whole application. With Spring we usually made use of the RestTemplate to perform HTTP requests, but can now also use the Spring WebClient.

With the release of Spring Framework 5.0, the documentation of the RestTemplate says the following:

NOTE: As of 5.0, the non-blocking, reactive org.springframework.web.reactive.client.WebClient offers a modern alternative to the RestTemplate with efficient support for both sync and async, as well as streaming scenarios. The RestTemplatewill be deprecated in a future version and will not have major new features added going forward. See the WebClient section of the Spring Framework reference documentation for more details and example code.

To be future-ready, your Spring-based application should migrate to the reactive and non-blocking Spring WebClient for both its async & sync HTTP communication.

In this blog post, I'll demonstrate how to work with the WebClient class to master common tasks like HTTP GET/POST requests, configuration, filtering requests, and testing. For the demo application, I'll use Java 11 and Spring Boot 2.3.

Configure the WebClient

For utilizing the WebClient, you need the following dependency in your Spring Boot project:

<dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

<groupId>org.springframework.boot</groupId>

<artifactId>spring-boot-starter-webflux</artifactId>

</dependency>

The WebClient internally delegates to an HTTP client library (by default Reactor Netty), but others can be plugged in through a ClientHttpConnector. More information about the WebClient is available here.

When it comes to configuring resilient HTTP clients, connection/read/write timeouts are important to avoid long-running tasks. In addition, HTTP headers and cookies are essential for e.g. authentication or content-negotiation. With WebClient, you can make use of a builder to configure this:

@Component
public class SimpleWebClientConfiguration {

  private static final String BASE_URL = "https://jsonplaceholder.typicode.com";
  private static final Logger logger = LoggerFactory.getLogger(SimpleApiClient.class);

  @Bean
  public WebClient defaultWebClient() {

    var tcpClient = TcpClient.create()
      .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 2_000)
      .doOnConnected(connection ->
        connection.addHandlerLast(new ReadTimeoutHandler(2))
          .addHandlerLast(new WriteTimeoutHandler(2)));

    return WebClient.builder()
      .baseUrl(BASE_URL)
      .clientConnector(new ReactorClientHttpConnector(HttpClient.from(tcpClient)))
      .defaultCookie("cookieKey", "cookieValue", "teapot", "amsterdam")
      .defaultCookie("secretToken", UUID.randomUUID().toString())
      .defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
      .defaultHeader(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE)
      .defaultHeader(HttpHeaders.USER_AGENT, "I'm a teapot")
      .build();
  }
}

@Component

public class SimpleWebClientConfiguration {

private static final String BASE_URL = "https://jsonplaceholder.typicode.com";

private static final Logger logger = LoggerFactory.getLogger(SimpleApiClient.class);

@Bean

public WebClient defaultWebClient() {

var tcpClient = TcpClient.create()

.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 2_000)

.doOnConnected(connection ->

connection.addHandlerLast(new ReadTimeoutHandler(2))

.addHandlerLast(new WriteTimeoutHandler(2)));

return WebClient.builder()

.baseUrl(BASE_URL)

.clientConnector(new ReactorClientHttpConnector(HttpClient.from(tcpClient)))

.defaultCookie("cookieKey", "cookieValue", "teapot", "amsterdam")

.defaultCookie("secretToken", UUID.randomUUID().toString())

.defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)

.defaultHeader(HttpHeaders.ACCEPT, MediaType.APPLICATION_JSON_VALUE)

.defaultHeader(HttpHeaders.USER_AGENT, "I'm a teapot")

.build();

}

Make an HTTP GET request with Spring WebClient

Once your WebClient is configured for a specific baseUrl, you can start performing HTTP requests. As the internal WebClient architecture is designed for reactive and non-blocking applications, you either have to call .block() or rewrite your codebase to accept Mono<T> and Flux<T> as method return types.

A simple sync HTTP GET request with our previously configured WebClient looks like the following:

@Service
public class SimpleApiClient {

  private final WebClient defaultWebClient;

  public SimpleApiClient(WebClient defaultWebClient) {
    this.defaultWebClient = defaultWebClient;
  }

  public JsonNode getTodoFromAPI() {
    return this.defaultWebClient.get().uri("/todos/1")
     .retrieve()
     .bodyToMono(JsonNode.class)
     .block();
  }
}

@Service

public class SimpleApiClient {

private final WebClient defaultWebClient;

public SimpleApiClient(WebClient defaultWebClient) {

this.defaultWebClient = defaultWebClient;

}

public JsonNode getTodoFromAPI() {

return this.defaultWebClient.get().uri("/todos/1")

.retrieve()

.bodyToMono(JsonNode.class)

.block();

}

For actually performing an HTTP request, you can either use .retrieve() or .exchange() whereby the first method is the easiest way and .exchange() offers more control (have a look at this blog post or the documentation to understand the difference in detail). For simplification, I'll use .retrieve() in all examples.

Reacting on different HTTP response codes

By default, HTTP responses with 4xx (e.g. 404 for not found) or 5xx (e.g. 500 service unavailable) status codes result in an WebClientResponseException. For modifying the way how errors are handled, you can use the onStatus method of Spring's WebClient to customize the resulting exception:

@Service
public class SimpleApiClient {

  private final WebClient defaultWebClient;

  public SimpleApiClient(WebClient defaultWebClient) {
    this.defaultWebClient = defaultWebClient;
  }

  public JsonNode getTodoFromAPI() {
    return this.defaultWebClient.get().uri("/todos/1")
      .retrieve()
      .onStatus(HttpStatus::is4xxClientError, response -> {
        System.out.println("4xx error");
        return Mono.error(new RuntimeException("4xx"));
      })
      .onStatus(HttpStatus::is5xxServerError, response -> {
        System.out.println("5xx error");
        return Mono.error(new RuntimeException("5xx"));
      })
      .bodyToMono(JsonNode.class)
      .block();
  }
}

@Service

public class SimpleApiClient {

private final WebClient defaultWebClient;

public SimpleApiClient(WebClient defaultWebClient) {

this.defaultWebClient = defaultWebClient;

}

public JsonNode getTodoFromAPI() {

return this.defaultWebClient.get().uri("/todos/1")

.retrieve()

.onStatus(HttpStatus::is4xxClientError, response -> {

System.out.println("4xx error");

return Mono.error(new RuntimeException("4xx"));

})

.onStatus(HttpStatus::is5xxServerError, response -> {

System.out.println("5xx error");

return Mono.error(new RuntimeException("5xx"));

})

.bodyToMono(JsonNode.class)

.block();

}

Make an HTTP POST request

Performing HTTP POST request with the WebClient is as simple as the following:

@Service
public class SimpleApiClient {

  private final WebClient defaultWebClient;

  public SimpleApiClient(WebClient defaultWebClient) {
    this.defaultWebClient = defaultWebClient;
  }

  public JsonNode postToTodoAPI() {
    return this.defaultWebClient
      .post()
      .uri("/todos")
      .body(BodyInserters.fromValue("{ \"title\": \"foo\", \"body\": \"bar\", \"userId\": \"1\"}"))
      .retrieve()
      .bodyToMono(JsonNode.class)
      .block();
  }
}

@Service

public class SimpleApiClient {

private final WebClient defaultWebClient;

public SimpleApiClient(WebClient defaultWebClient) {

this.defaultWebClient = defaultWebClient;

}

public JsonNode postToTodoAPI() {

return this.defaultWebClient

.post()

.uri("/todos")

.body(BodyInserters.fromValue("{ \"title\": \"foo\", \"body\": \"bar\", \"userId\": \"1\"}"))

.retrieve()

.bodyToMono(JsonNode.class)

.block();

}

To put the request body in place you can use the BodyInserters utility class and add any kind of Object, FormData, Publisher, MultipartData, Resource and much more.

Retry configuration on failure

For a more resilient architecture, you can configure retries. You can use the public methods of Mono and Flux for this. Let's say we want to retry two times (max. three HTTP calls) and add a small delay between each retry:

public JsonNode getRetryTodoFromAPI() {
  return this.defaultWebClient.get().uri("/todos/1")
    .retrieve()
    .bodyToMono(JsonNode.class)
    .retryWhen(Retry.fixedDelay(2, Duration.ofMillis(300)))
    .block();
}

public JsonNode getRetryTodoFromAPI() {

return this.defaultWebClient.get().uri("/todos/1")

.retrieve()

.bodyToMono(JsonNode.class)

.retryWhen(Retry.fixedDelay(2, Duration.ofMillis(300)))

.block();

}

Furthermore, you can combine the retry mechanism with a timeout from Mono or Flux to provide a fallback value:

public JsonNode getRetryTodoFromAPI() {
  return this.defaultWebClient.get().uri("/todos/1")
    .retrieve()
    .bodyToMono(JsonNode.class)
    .retryWhen(Retry.max(5))
    .timeout(Duration.ofSeconds(2), Mono.just(objectMapper.createObjectNode().put("message", "fallback")))
    .block();
}

public JsonNode getRetryTodoFromAPI() {

return this.defaultWebClient.get().uri("/todos/1")

.retrieve()

.bodyToMono(JsonNode.class)

.retryWhen(Retry.max(5))

.timeout(Duration.ofSeconds(2), Mono.just(objectMapper.createObjectNode().put("message", "fallback")))

.block();

}

Configure filters for the Spring WebClient

If you have business logic or a configuration setup which should be applied to every HTTP request and response made with the WebClient, you can configure and chain multiple filters.

Logging the request/response might be such a requirement or applying authentication to the request. This is configured during the WebClient setup:

@Component
public class SimpleWebClientConfiguration {

  private static final String BASE_URL = "https://jsonplaceholder.typicode.com";
  private static final Logger logger = LoggerFactory.getLogger(SimpleApiClient.class);

  @Bean
  public WebClient defaultWebClient() {

    var tcpClient = TcpClient.create()
      .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 2_000)
      .doOnConnected(connection ->
        connection.addHandlerLast(new ReadTimeoutHandler(2))
          .addHandlerLast(new WriteTimeoutHandler(2)));

    return WebClient.builder()
      .baseUrl(BASE_URL)
      .clientConnector(new ReactorClientHttpConnector(HttpClient.from(tcpClient)))
      .filter(ExchangeFilterFunctions.basicAuthentication("rieckpil", UUID.randomUUID().toString()))
      .filter(logRequest())
      .filter(logResponse())
      .build();
  }

  private ExchangeFilterFunction logRequest() {
    return (clientRequest, next) -> {
      logger.info("Request: {} {}", clientRequest.method(), clientRequest.url());
      logger.info("--- Http Headers: ---");
      clientRequest.headers().forEach(this::logHeader);
      logger.info("--- Http Cookies: ---");
      clientRequest.cookies().forEach(this::logHeader);
      return next.exchange(clientRequest);
    };
  }

  private ExchangeFilterFunction logResponse() {
    return ExchangeFilterFunction.ofResponseProcessor(clientResponse -> {
      logger.info("Response: {}", clientResponse.statusCode());
      clientResponse.headers().asHttpHeaders()
        .forEach((name, values) -> values.forEach(value -> logger.info("{}={}", name, value)));
      return Mono.just(clientResponse);
    });
  }

  private void logHeader(String name, List<String> values) {
    values.forEach(value -> logger.info("{}={}", name, value));
  }
}

@Component

public class SimpleWebClientConfiguration {

private static final String BASE_URL = "https://jsonplaceholder.typicode.com";

private static final Logger logger = LoggerFactory.getLogger(SimpleApiClient.class);

@Bean

public WebClient defaultWebClient() {

var tcpClient = TcpClient.create()

.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 2_000)

.doOnConnected(connection ->

connection.addHandlerLast(new ReadTimeoutHandler(2))

.addHandlerLast(new WriteTimeoutHandler(2)));

return WebClient.builder()

.baseUrl(BASE_URL)

.clientConnector(new ReactorClientHttpConnector(HttpClient.from(tcpClient)))

.filter(ExchangeFilterFunctions.basicAuthentication("rieckpil", UUID.randomUUID().toString()))

.filter(logRequest())

.filter(logResponse())

.build();

}

private ExchangeFilterFunction logRequest() {

return (clientRequest, next) -> {

logger.info("Request: {} {}", clientRequest.method(), clientRequest.url());

logger.info("--- Http Headers: ---");

clientRequest.headers().forEach(this::logHeader);

logger.info("--- Http Cookies: ---");

clientRequest.cookies().forEach(this::logHeader);

return next.exchange(clientRequest);

};

}

private ExchangeFilterFunction logResponse() {

return ExchangeFilterFunction.ofResponseProcessor(clientResponse -> {

logger.info("Response: {}", clientResponse.statusCode());

clientResponse.headers().asHttpHeaders()

.forEach((name, values) -> values.forEach(value -> logger.info("{}={}", name, value)));

return Mono.just(clientResponse);

});

}

private void logHeader(String name, List<String> values) {

values.forEach(value -> logger.info("{}={}", name, value));

}

You can also provide global customizations for your WebClient instances using the WebClientCustomizer interface.

Testing with WebTestClient

For efficient testing, you can make use of WebTestClient during your integration tests. This client offers the same functionality as the normal WebClient and convenient methods for assertions and expectations in addition.

package de.rieckpil.blog;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit.jupiter.SpringExtension;
import org.springframework.test.web.reactive.server.WebTestClient;
import org.springframework.web.reactive.function.BodyInserters;


@ExtendWith(SpringExtension.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class SimpleApiClientTest {

  @Autowired
  private WebTestClient webTestClient;

  @Test
  public void testGetTodosAPICall() {
    this.webTestClient
      .get()
      .uri("https://jsonplaceholder.typicode.com/todos/1")
      .accept(MediaType.APPLICATION_JSON)
      .exchange()
      .expectStatus().isOk()
      .expectHeader().contentType(MediaType.APPLICATION_JSON_UTF8_VALUE)
      .expectBody()
      .jsonPath("$.title").isNotEmpty()
      .jsonPath("$.userId").isNotEmpty()
      .jsonPath("$.completed").isNotEmpty();
  }

}

package de.rieckpil.blog;

import org.junit.jupiter.api.Test;

import org.junit.jupiter.api.extension.ExtendWith;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.test.context.SpringBootTest;

import org.springframework.http.MediaType;

import org.springframework.test.context.junit.jupiter.SpringExtension;

import org.springframework.test.web.reactive.server.WebTestClient;

import org.springframework.web.reactive.function.BodyInserters;

@ExtendWith(SpringExtension.class)

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)

class SimpleApiClientTest {

@Autowired

private WebTestClient webTestClient;

@Test

public void testGetTodosAPICall() {

this.webTestClient

.get()

.uri("https://jsonplaceholder.typicode.com/todos/1")

.accept(MediaType.APPLICATION_JSON)

.exchange()

.expectStatus().isOk()

.expectHeader().contentType(MediaType.APPLICATION_JSON_UTF8_VALUE)

.expectBody()

.jsonPath("$.title").isNotEmpty()

.jsonPath("$.userId").isNotEmpty()

.jsonPath("$.completed").isNotEmpty();

}

I covered the usage of WebTestClient with a dedicated blog post.

Accessing OAuth2 protected resources with Spring WebClient

With Spring Security we get a full integration for OAuth2. Have a look at the following blog posts where I demonstrate how to enable the OAuth2 integration with Spring WebClient:

Furthermore, if you use Spring Boot Actuator, you can expose metrics of your WebClient instances automatically.

You can find the full source code for all examples on GitHub. If you want to achieve the same with Jakarta EE, have a look at the JAX-RS Client or the MicroProfile RestClient.

Have fun using the new Spring WebClient,

Phil

#HOWTO: MicroProfile Rest Client for RESTful communication | Philip's Homepage says:

July 8, 2019 at 6:28 am

[…] one of my recent blog posts, I presented Spring’s WebClient for RESTful communication. With Java EE we can utilize the […]

Fernando Franzini says:

January 3, 2020 at 8:48 pm

Hello Philip

I have a doubt:
I’d like to use WebClient instead of RestTemplate, but my solution is not SpringBoot but Spring default.
How do I add only WebClient dependencies and use?

rieckpil says:

January 4, 2020 at 10:28 am

Hey Fernando,

the WebClient is part of the WebFlux Spring dependency. You can pull this in your project without using Spring Boot. If you are using Maven, you can use the following import:

<dependency< <groupIdorg.springframework</groupId> <artifactId>spring-webflux</artifactId> <version>5.2.2.RELEASE</version> </dependency>

Make sure to align the version with the Spring version you have in use.

Java AWS Lambda with Spring Cloud Function | rieckpil says:

June 3, 2020 at 8:03 am

[…] Java is enough for simple use cases, it might be helpful to use Spring Framework features (e.g the WebClient, data access, etc.). With Spring Cloud Function you can achieve this and use the AWS adapter to […]

Test Spring WebClient with MockWebServer from OkHttp | rieckpil says:

July 4, 2020 at 4:58 pm

[…] Our sample project is a basic Spring Boot application. We’ll include both starters for Web and WebFlux. Hence Spring Boot autoconfigures the embedded Tomcat for us while we are able to use parts from Spring WebFlux like the WebClient. […]

Spring WebClient exchange vs. retrieve a comparison | rieckpil says:

July 6, 2020 at 3:00 pm

[…] won’t add new features to the RestTemplate, the general recommendation is to start using the Spring WebFlux WebClient. Besides the reactive and non-blocking nature of the WebClient, you can seamlessly include it to […]

Daniel Fuentes says:

August 20, 2020 at 5:50 pm

One question, Why do I get the illegal StateException: The underlying HTTP client completed without emitting a response when using the web client? When I use block it works correctly, any idea why it could be happening?

rieckpil says:

August 21, 2020 at 4:49 am

Hey Daniel,

hard to tell without seeing the actual code. Could you create a question on StackOverflow and send me the link?

Kind regards,
Philip

Spring WebTestClient for efficient testing of your REST API | rieckpil says:

November 16, 2020 at 8:49 am

[…] the WebClient, Spring provides a WebTestClient for testing purposes. The API of this class is similar to the […]

Use Spring WebClient for RESTful communication

Configure the WebClient

Make an HTTP GET request with Spring WebClient

Reacting on different HTTP response codes

Make an HTTP POST request

Retry configuration on failure

Configure filters for the Spring WebClient

Testing with WebTestClient

Accessing OAuth2 protected resources with Spring WebClient

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