Spring Boot – ResponseEntity & HTTP Response Codes

 

Spring Boot – ResponseEntity & HTTP Response Codes

(Deep Notes + Extra Concepts + Interview Q&A)

1. What is an HTTP Response?

According to page 1, every HTTP response has 3 parts:

Part Description
Status Code 200, 201, 400, 500 etc
Headers Metadata (auth token, content type, cache info, etc.)
Body Actual data returned to client

2. What is ResponseEntity<T>?

ResponseEntity<T> is a Spring wrapper to send:

  • Body
  • Headers
  • HTTP Status
@GetMapping("/get-user")
public ResponseEntity<String> getUser() {
    return ResponseEntity.ok("My Response Body Object can go here");
}

Here:

  • String = body type
  • Status = 200 OK
  • Headers = default

3. Adding Headers & Status

From page 1, example:

@GetMapping("/get-user")
public ResponseEntity<String> getUser() {

    HttpHeaders headers = new HttpHeaders();
    headers.add("My-Header1", "SomeValue1");
    headers.add("My-Header2", "SomeValue2");

    return ResponseEntity.status(HttpStatus.OK)
                         .headers(headers)
                         .body("My Response Body Object can go here");
}

🔹 Builder Pattern

status(), headers(), body() all return builders, not the object.
Final object is created only when .body() or .build() is called.

4. When You Don’t Want Body

From page 1:

@GetMapping("/get-user")
public ResponseEntity<Void> getUser() {

    HttpHeaders headers = new HttpHeaders();
    headers.add("My-Header1", "SomeValue1");
    headers.add("My-Header2", "SomeValue2");

    return ResponseEntity.status(HttpStatus.OK)
                         .headers(headers)
                         .build();
}

Use build() instead of body().

5. @ResponseBody vs @RestController

From page 2:

Why @ResponseBody?

When returning String / POJO, Spring by default treats it as a View Name.

@ResponseBody tells Spring:

“Treat this return as response body, not as a view.”

Why it works without @ResponseBody in RestController?

Because:

@RestController = @Controller + @ResponseBody

So all methods automatically return JSON/text.

6. If You Use @Controller Instead of @RestController

From page 2:

@Controller
public class UserController {

    @GetMapping("/get-user")
    public String getUser() {
        return "XYZ";
    }
}

Spring will try to find a view file named XYZ.html
Since it doesn’t exist → ❌ error.

7. HTTP Status Code Groups (Page 3)

Group Meaning
1xx Informational
2xx Success
3xx Redirection
4xx Client Error
5xx Server Error

8. Important 2xx Codes

Code Meaning Use
200 OK Success GET, POST
201 Created Resource created POST
202 Accepted Processing started Async tasks
204 No Content Success, no body DELETE
206 Partial Content Partial data Pagination, chunk upload

9. Important 3xx Codes

Code Meaning
301 Permanent redirect
308 Permanent redirect, same method
304 Not Modified (Caching)

304 Flow (page 3):

  1. Client sends If-Modified-Since
  2. Server checks resource
  3. If unchanged → return 304

10. Important 4xx Codes

Code Meaning When
400 Bad Request Missing fields
401 Unauthorized Token missing
403 Forbidden No permission
404 Not Found ID not exists
405 Method Not Allowed Wrong HTTP method
422 Unprocessable Entity Business validation
429 Too Many Requests Rate limit exceeded

11. Important 5xx Codes

Code Meaning
500 Internal Server Error
501 Not Implemented
502 Bad Gateway (Reverse Proxy error)

Example (page 4):

If Nginx cannot talk to backend → return 502

12. 100 Continue (Page 4)

Used when sending large data.

Flow:

  1. Client sends headers with Expect: 100-continue
  2. Server validates
  3. If OK → sends 100 Continue
  4. Client sends body

EXTRA REAL-TIME BEST PRACTICES

  • Always return ResponseEntity for APIs
  • Never return raw String in REST APIs
  • Use Global Exception Handler for 4xx/5xx
  • Use 429 for rate limiting
  • Use 204 for delete success
  • Use 201 for create APIs

INTERVIEW QUESTIONS & ANSWERS

Q1. Why use ResponseEntity?

Ans: To control body, headers, and status in REST responses.

Q2. Difference between @Controller and @RestController?

Ans:
@RestController = @Controller + @ResponseBody

Q3. When do we use 204 instead of 200?

Ans: When success but no response body is required.

Q4. When to use 422?

Ans: Business validation failure (not syntax error).

Q5. Difference between 401 and 403?

| 401 | Not authenticated |
| 403 | Authenticated but no permission |

Q6. What is 429?

Ans: When API is hit more than allowed rate.

Q7. What happens if we return String from @Controller?

Ans: Spring treats it as View name.

Q8. Why is 304 important?

Ans: It saves bandwidth by using cached data.

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