I do have a spring boot project in which I want to make a external API call using the RestClient which is as below.

Service.java

package com.example.demo;

import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.web.client.RestClient;

@org.springframework.stereotype.Service
public class Service {

  private final RestClient restClient;

  public Service(RestClient restClient) {
    this.restClient = restClient;
  }

  public void callApi() {
    String uri = "https://api.appraisd.com/api/users?employeeId=ABC";
    String token = "token";

    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.setBearerAuth(token);
    httpHeaders.set("Accept", MediaType.APPLICATION_JSON_VALUE);
    httpHeaders.setContentType(MediaType.APPLICATION_JSON);

    Request request = new Request();
    request.setFirstName("John");
    request.setLastName("Doe");
    request.setEmail("[email protected]");
    request.setEmployeeId("ABC");

    restClient.patch()
        .uri(uri)
        .headers(headers -> headers.addAll(httpHeaders))
        .body(request)
        .retrieve()
        .body(Response.class);
  }
  
}

RestClientConfig.java

@Configuration
public class RestClientConfig {

    @Bean
    public RestClient restClient() {
        return RestClient.builder().build();
    }

}

DemoApplication.java

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    private final Service service;

    public DemoApplication(Service service) {
        this.service = service;
    }

    @Override
    public void run(String... args) {
        service.callApi();
    }

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);

    }

}

External API which works fine.

curl --location --request PATCH 'https://api.appraisd.com/api/users?employeeId=ABC' 
--header 'Authorization: Bearer token' 
--header 'Content-Type: application/json' 
--data-raw '{
   "firstName":"John",
   "lastName":"Doe",
   "email":"[email protected]",
   "employeeId":"ABC"
}'

When I make a API call using callApi method I do get the following error

org.springframework.web.client.HttpClientErrorException$BadRequest: 400 Bad Request: [no body]