12.4 Consuming REST APIs reactively

In chapter 8, you used RestTemplate to make client requests to the Taco Cloud API. RestTemplate is an old-timer, having been introduced in Spring version 3.0. In its time, it has been used to make countless requests on behalf of the applications that employ it.

But all of the methods provided by RestTemplate deal in nonreactive domain types and collections. This means that if you want to work with a response’s data in a reactive way, you’ll need to wrap it with a Flux or Mono. And if you already have a Flux or Mono and you want to send it in a POST or PUT request, then you’ll need to extract the data into a nonreactive type before making the request.

It would be nice if there was a way to use RestTemplate natively with reactive types. Fear not. Spring offers WebClient as a reactive alternative to RestTemplate. WebClient lets you both send and receive reactive types when making requests to external APIs.

Using WebClient is quite different from using RestTemplate. Rather than having several methods to handle different kinds of requests, WebClient has a fluent builderstyle interface that lets you describe and send requests. The general usage pattern for working with WebClient follows:

• Create an instance of WebClient (or inject a WebClient bean)
• Specify the HTTP method of the request to send
• Specify the URI and any headers that should be in the request
• Submit the request
• Consume the response

Let’s look at several examples of WebClient in action, starting with how to use WebClient to send HTTP GET requests.