<!-- database -->
<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
        </dependency>



        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>

<!-- test -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-test-autoconfigure</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-test</artifactId>
        </dependency>

import lombok.AllArgsConstructor;
import lombok.Data;

import javax.persistence.Entity;
import javax.persistence.GenerationType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.validation.constraints.NotBlank;

/**
 * @description
 */
@Entity
@Data
@AllArgsConstructor
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @NotBlank(message = "Name is mandatory")
    private String name;

    @NotBlank(message = "Email is mandatory")
    private String email;
    
}

import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
import com.example.demo.*;

/**
 * @description
 */
@Repository
public interface UserRepository extends CrudRepository<User, Long> {}

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.validation.FieldError;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.*;

import javax.validation.Valid;
import java.util.HashMap;
import java.util.Map;

/**
 * @description
 */
@RestController
public class UserController {

    // target argument fails to pass the validation,
    // Spring Boot throws a MethodArgumentNotValidException exception
    @PostMapping("/users")
    ResponseEntity<String> addUser(@Valid @RequestBody User user) {
        // persisting the user
        return ResponseEntity.ok("User is valid");
    }

    // standard constructors / other methods


    @ResponseStatus(HttpStatus.BAD_REQUEST)
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public Map<String, String> handleValidationExceptions(MethodArgumentNotValidException ex) {
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getAllErrors().forEach((error) -> {
            String fieldName = ((FieldError) error).getField();
            String errorMessage = error.getDefaultMessage();
            errors.put(fieldName, errorMessage);
        });
        return errors;
    }

}

import org.hamcrest.core.Is;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.http.MediaType;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.test.web.servlet.MockMvc;
import org.springframework.test.web.servlet.request.MockMvcRequestBuilders;
import org.springframework.test.web.servlet.result.MockMvcResultMatchers;

import java.nio.charset.Charset;

/**
 * @description
 */
@RunWith(SpringRunner.class)
// only test web layer
// MockMvcRequestBuilders
// MockMvcResultMatchers
@WebMvcTest
@AutoConfigureMockMvc
public class UserControllerIntegrationTest {

    @MockBean
    private UserRepository userRepository;

    @Autowired
    UserController userController;

    @Autowired
    private MockMvc mockMvc;





    @Test
    public void whenPostRequestToUsersAndValidUser_thenCorrectResponse() throws Exception {
        MediaType textPlainUtf8 = new MediaType(MediaType.TEXT_PLAIN, Charset.forName("UTF-8"));
        String user = "{\"name\": \"bob\", \"email\" : \"bob@domain.com\"}";
        mockMvc.perform(MockMvcRequestBuilders.post("/users")
                .content(user)
                .contentType(MediaType.APPLICATION_JSON))
                .andExpect(MockMvcResultMatchers.status().isOk())
                .andExpect(MockMvcResultMatchers.content()
                        .contentType(textPlainUtf8));
    }

    @Test
    public void whenPostRequestToUsersAndInValidUser_thenCorrectResponse() throws Exception {
        String user = "{\"name\": \"\", \"email\" : \"bob@domain.com\"}";
        mockMvc.perform(MockMvcRequestBuilders.post("/users")
                .content(user)
                .contentType(MediaType.APPLICATION_JSON))
                .andExpect(MockMvcResultMatchers.status().isBadRequest())
                .andExpect(MockMvcResultMatchers.jsonPath("$.name", Is.is("Name is mandatory")))
                .andExpect(MockMvcResultMatchers.content()
                        .contentType(MediaType.APPLICATION_JSON));
    }
}