Get the content length of a body and add it the CONTENT_LENGTH header with Spring's Reactive WebClient

BodyContentLenghtTest.java

import java.nio.charset.StandardCharsets;
import org.junit.Assert;
import org.junit.Test;
import org.springframework.http.HttpHeaders;
import org.springframework.web.reactive.function.BodyInserters;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;

public class BodyContentLenghtTest {

  @Test
  public void postWithContentLength() {

    final Mono<String> mono = Mono.just("HELLO WORLDZ");

    final String response = WebClient.create("http://httpbin.org")
        .post()
        .uri("/post")
        .header(HttpHeaders.CONTENT_LENGTH, mono.map(s -> String.valueOf(s.getBytes(StandardCharsets.UTF_8)
        .length))
        .block())
        .body(BodyInserters.fromPublisher(mono, String.class))
        .retrieve()
        .bodyToMono(String.class)
        .log()
        .block();

    Assert.assertTrue(response.contains("\"Content-Length\": \"12\""));
  }

  @Test
  public void postFlatMapTheBody() {

    final Mono<String> mono = Mono.just("HELLO WORLDZZ")
        .flatMap(body -> WebClient.create("http://httpbin.org")
        .post()
        .uri("/post")
        .header(HttpHeaders.CONTENT_LENGTH, String.valueOf(body.getBytes(StandardCharsets.UTF_8).length))
        .syncBody(body)
        .retrieve()
        .bodyToMono(String.class))
        .log();

    Assert.assertTrue(mono.block().contains("\"Content-Length\": \"13\""));

  }

}