repro sample for Spring Boot issue #13248

DemoApplicationTests.java

package com.example.demo;

import org.junit.Test;
import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.test.web.reactive.server.WebTestClient;

import static org.assertj.core.api.Assertions.assertThat;

@RunWith(SpringRunner.class)
@SpringBootTest
@AutoConfigureWebTestClient
public class DemoApplicationTests {

	@Autowired
	private WebTestClient webTestClient;

	@Test
	public void testUserJsonDeserialization() {
		User user = this.webTestClient.get().uri("/")
				.accept(MediaType.APPLICATION_JSON).exchange()
				.expectBody(User.class).returnResult().getResponseBody();
		assertThat(user.getName()).isEqualTo("spring");
	}
}

User.java

package com.example.demo;

import javax.validation.constraints.NotBlank;

import lombok.Builder;
import lombok.Data;
import org.hibernate.validator.constraints.Range;

@Data
@Builder
public class User {
    private String id;
    @NotBlank
    private String name;
    @Range(min = 10,max = 100)
    private Integer age;
}

UserController.java

package com.example.demo;

import reactor.core.publisher.Mono;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class UserController {

	@GetMapping("/")
	@ResponseBody
	public Mono<User> fetchUser() {
		return Mono.just(User.builder().age(15).name("spring").build());
	}
}