Documentacion en español

In this article we will speak about the WebClient class of the SpringBoot framework.

You have the source about what I write in the post at](

This class would be the equivalent of RestTemplate class, but it work with asynchronous requests.

If you want to use this class, you should put this dependencies in your maven file.


This is so why you need to use WebFlux which is available in the version 5 of Spring. Remember that this version of Spring requires Java 8 or higher.

With WebFlux which is based on the project Reactor you can write reactive applications. This kind of applications is characterized because the requests aren’t blocking and because the functional programming is widely used.

If you want to understand this article you should have certain notions about Reactor and the Mono class. Although if you have used Streams in Java you may think that a Mono object is like a Stream that can emit a value or an error.

But i am not going to delve into these issues that are beyond the scope of this article. It will be sufficient to say that using the WebClient class you can make several calls in parallel, so if each request is answered in 2 seconds and you make 5 calls, you can get all the answers in just over 2 seconds, instead of 10.

Parallel Request

In the example project I have written an server and a client. The server will be running in the 8081 port and the client will listen in the 8080 port.

This is the code that execute the server application.

public class WebServerApplication {
	public static void main(String[] args) {
		new SpringApplicationBuilder(WebServerApplication.class).
				properties(Collections.singletonMap("server.port", "8081")).run(args);

if you make a request to http://localhost:8080/client/XXX the function testGet will be executed.

public class ClientController {
	final String urlServer="http://localhost:8081";
	public Mono<ResponseEntity<Mono<String>>> testGet(@PathVariable String param) {
		final long dateStarted = System.currentTimeMillis();

		WebClient webClient = WebClient.create(urlServer+"/server/");
		Mono<ClientResponse> respuesta = webClient.get().uri("?queryParam={name}", param).exchange();
		Mono<ClientResponse> respuesta1 = webClient.get().uri("?queryParam={name}","SPEED".equals(param)?"SPEED":"STOP").exchange();
		Mono<ResponseEntity<Mono<String>>> f1 =, respuesta1)
		.map(t -> {
			if (!t.getT1().statusCode().is2xxSuccessful()) {
				return ResponseEntity.status(t.getT1().statusCode()).body(t.getT1().bodyToMono(String.class));
			if (!t.getT2().statusCode().is2xxSuccessful()) {
				return ResponseEntity.status(t.getT2().statusCode()).body(t.getT2().bodyToMono(String.class));
			return ResponseEntity.ok().body(Mono.just(
					"All OK. Seconds elapsed: " + (((double) (System.currentTimeMillis() - dateStarted) / 1000))));
		return f1;

This is a simple controller that performs two requests to the URL http://localhost:8081 . In the first request the parameter passed is that the function received in the param variable. In the second the sentence “STOP” is sent if the param variable is different of the word SPEED.

The server upon receiving the parameter “STOP” will wait 5 seconds and then will answer.

At the moment of create the instance of the class WebClient I specify the URL of the request.

WebClient webClient = WebClient.create(urlServer+"/server/");

Then it execute the call of the type GET to the server passing the parameter QueryParam. At the end, executing the function exchange it will receive a Mono object containing a ClientResponse class that is equivalent to the ResponseEntity object of the RestTemplate class. The class ClientResponse will have the HTTP code, the body and the headers sent by the server.

Mono<ClientResponse> respuesta = webClient.get().uri("?queryParam={name}", param).exchange();

Wait a minute .. , did I say it will execute ?. Well I lied. Really only what I want to do has been declared. In reactive programming until someone does not subscribe to a request nothing is executed, so the request to the server has not yet been made.

On the next line the second request to the server is defined.

Mono<ClientResponse> respuesta1 = webClient.get().uri("?queryParam={name}","SPEED".equals(param)?"SPEED":"STOP").exchange();

Finally, a Mono object is created, which will be the result of the previous two, using the zip function.

Using the map function a ResponseEntity object with the HTTP code equal to OK will be returned if the two requests have answered a 2XX code, otherwise the code and answer of the server will be returned.

Being WebClient reactive the two requests are realized simultaneously and therefore you will be able see that if you execute this code: curl http://localhost:8080/client/STOP you have the answer in just over 5 seconds, even if the sum of the call time is greater than 10 seconds.

 All OK. Seconds elapsed: 5.092

A POST request

In the testURLs function there is an example of a call using POST.

This function receives a Map in the body that will then be placed in the request headers. In addition this map will be sent in the body of the request that will be made to the server.

public Mono<String> testURLs(@RequestBody Map<String,String> body,
			@RequestParam(required = false) String url) {		

	log.debug("Client: in testURLs");
	WebClient.Builder builder = WebClient.builder().baseUrl(urlServer).
	if (body!=null && body.size()>0 )
		for (Map.Entry<String, String> map : body.entrySet() )
			builder.defaultHeader(map.getKey(), map.getValue());
	WebClient webClient =;	
	String urlFinal;
	if (url==null)

	Mono<String> respuesta1 =
		.flatMap( x -> 
			if ( ! x.statusCode().is2xxSuccessful())
				return 	Mono.just("LLamada a "+urlServer+urlFinal+" Error 4xx: "+x.statusCode()+"\n");
			return x.bodyToMono(String.class);
	return respuesta1;		

To insert the body of the message the auxiliary class BodyInserters will be used. If the message were on an object Mono this code could be used:


When performing a flatMap, the output of the ClientResponse object will be captured and an Mono object will be returned with the string to be returned. . The flatMap function will flatten that Mono object and will extract the string inside it and that is why a Mono <String> will be received and not a Mono <Mono <String>> as It would happen if we used the function map.

Making the following call:

> curl  -s -XPOST http://localhost:8080/client  -H 'Content-Type: application/json' -d'{"aa": "bbd"}'

The following output will be obtained:

the server said: {aa=bbd}
Headers: content-length:12
Headers: aa:bbd
Headers: accept-encoding:gzip
Headers: Content-Type:application/json
Headers: accept:*/*
Headers: user-agent:ReactorNetty/0.9.0.M3
Headers: host:localhost:8081

This output is produced by the function postExamle of the server

public ResponseEntity<String> postExample(@RequestBody Map<String,String> body,ServerHttpRequest  request) {
	String s="the server said: "+body+"\n";
	for (Entry<String, List<String>> map : request.getHeaders().entrySet())
		s+="Headers: "+map.getKey()+ ":"+map.getValue().get(0)+"\n";		
	return ResponseEntity.ok().body(s);

Note that when using the WebFlux library that is not fully compatible with javax.servlet we must receive a ServerHttpRequest object to collect all raw headers. The equivalent in a non-reactive application would be an HttpServletRequest object.

If you execute the sentence:

> curl  -s -XPOST http://localhost:8080/client?url=aa  -H 'Content-Type: application/json' -d'{"aa": "bbd"}'

The client will try to call http://localhost:8081/server/aa which will cause an error and the following will be received.

http://localhost:8081/server/aa Called. Error 4xx: 404 NOT_FOUND

The original article was written in Spanish and you can read it at