Cannot send request body to integration test method in Spring Boot JUnit (objectMapper.writeValueAsString Issue)

I have a problem to handle with sending request body to integration test in Spring Boot JUnit.
I got 400 Bad Request instead of 200 Ok.

Here is the AdminRefreshTokenRequest shown below

import jakarta.validation.constraints.NotBlank;
import lombok.Builder;
import lombok.Data;

@Data
@Builder
public class AdminRefreshTokenRequest {

    @NotBlank
    private String refreshToken;
}

Here is the refreshToken method shown below

@PostMapping("/refreshtoken")
    public ResponseEntity<?> refreshToken(@RequestBody AdminRefreshTokenRequest refreshTokenRequest) {
  .....
}

Here is the relevant part of integration test method shown below

@Autowired
private MockMvc mockMvc;
@Autowired
private ObjectMapper objectMapper;

AdminRefreshTokenRequest refreshTokenRequest = AdminRefreshTokenRequest.builder()
                .refreshToken("RefreshToken")
                .build();

mockMvc.perform(post(ADMIN_CONTROLLER_BASEURL + "/refreshtoken")
                        .contentType("application/json")
                        .content(objectMapper.writeValueAsString(refreshTokenRequest)))
                .andDo(print())
                .andExpect(status().isOk())

I get this issue shown below

MockHttpServletResponse:
           Status = 400
    Error message = null
          Headers = [Content-Type:"application/problem+json"]
     Content type = application/problem+json
             Body = {"type":"about:blank","title":"Bad Request","status":400,"detail":"Failed to read request","instance":"/api/v1/refreshtoken"}
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

java.lang.AssertionError: Status expected:<200> but was:<400>
Expected :200
Actual   :400

The value assigning to refreshTokenRequest in refreshToken method of controller is null when mockMvc try to call the method after I delete @RequestBody.

How can I fix it?

Here is the solution shown below.

After deleting @Builder which is used for more than one variable, I defined both @NoArgsConstructor and @AllArgsConstructor. Next, the issue disappeared.

@Data
@NoArgsConstructor
@AllArgsConstructor
public class AdminRefreshTokenRequest {

    @NotBlank
    private String refreshToken;
}

The problem is that your back-end is not able to initialize the request body.
By default what the framework does is to:

1. it creates the instance of the request body;
2. then it is going to populate variables;
3. finally it goes into the mapped method with the builded body.

In the first step the framework is going to call the empty constructor of your body class, so if it is missing it is going to reply with a 400 error code.

To fix the problem you can simply replace the @Builder annotation with the @NoArgsConstructor in your AdminRefreshTokenRequest class.
The final body class should look like the following:

import jakarta.validation.constraints.NotBlank;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
public class AdminRefreshTokenRequest {

    @NotBlank
    private String refreshToken;
}

This should fix the problem, hope it helps.