@RequestBody for JSON input in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to accept JSON input in a Spring Boot controller method.

Spring Boot

public ResponseEntity<String> createUser([1] User user) {
    // method body
}

Drag options to blanks, or click blank then click option'

A@PathVariable

B@RequestParam

C@RequestBody

D@Autowired

Attempts:

3 left

💡 Hint

Common Mistakes

Using @PathVariable instead of @RequestBody

Forgetting to annotate the parameter

The @RequestBody annotation tells Spring Boot to convert the JSON input into a User object.

2fill in blank

medium

Complete the code to specify the HTTP method and path for JSON input handling.

Spring Boot

@[1]("/users")
public ResponseEntity<String> addUser(@RequestBody User user) {
    // method body
}

Drag options to blanks, or click blank then click option'

AGetMapping

BPostMapping

CDeleteMapping

DPutMapping

Attempts:

3 left

💡 Hint

Common Mistakes

Using GetMapping which does not support request body

Using DeleteMapping or PutMapping incorrectly

PostMapping is used to handle HTTP POST requests, which are commonly used to send JSON data to create resources.

3fill in blank

hard

Fix the error in the method parameter to correctly receive JSON input.

Spring Boot

public ResponseEntity<String> updateUser([1] user) {
    // method body
}

Drag options to blanks, or click blank then click option'

AUser

B@PathVariable User

C@RequestParam User

D@RequestBody User

Attempts:

3 left

💡 Hint

Common Mistakes

Missing @RequestBody annotation

Using @RequestParam or @PathVariable incorrectly

The parameter must be annotated with @RequestBody to tell Spring to convert JSON input to the User object.

4fill in blank

hard

Fill both blanks to create a controller method that accepts JSON and returns a response.

Spring Boot

@[1]("/products")
public ResponseEntity<[2]> addProduct(@RequestBody Product product) {
    // method body
}

Drag options to blanks, or click blank then click option'

APostMapping

BGetMapping

CProduct

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using GetMapping for JSON input

Returning the Product object instead of a response message

PostMapping handles POST requests for JSON input. The method returns a ResponseEntity to send a response message.

5fill in blank

hard

Fill all three blanks to define a Spring Boot controller method that accepts JSON, validates it, and returns a response.

Spring Boot

@[1]("/orders")
public ResponseEntity<[2]> placeOrder(@Valid @RequestBody [3] order) {
    // method body
}

Drag options to blanks, or click blank then click option'

APostMapping

BString

COrder

DGetMapping

Attempts:

3 left

💡 Hint

Common Mistakes

Using GetMapping instead of PostMapping

Missing @RequestBody or @Valid annotations

Wrong return type

PostMapping is for POST requests. The method returns ResponseEntity. The parameter is an Order object annotated with @RequestBody and @Valid for validation.

Practice

1. What is the main purpose of the @RequestBody annotation in a Spring Boot controller?

easy

A. To validate form data submitted via GET requests

B. To send JSON data back in the HTTP response

C. To map URL query parameters to method arguments

D. To automatically convert JSON from the HTTP request body into a Java object

Solution

1. Step 1: Understand the role of @RequestBody

   @RequestBody tells Spring Boot to take JSON data sent in the HTTP request body and convert it into a Java object automatically.

2. Step 2: Differentiate from other annotations

   It does not handle response data (that's @ResponseBody), nor does it map query parameters or validate GET form data.

3. Final Answer:

   To automatically convert JSON from the HTTP request body into a Java object -> Option D

4. Quick Check:

   @RequestBody = JSON to Java object [OK]

Hint: Remember: @RequestBody reads JSON input from request body [OK]

Common Mistakes:

• Confusing @RequestBody with @ResponseBody
• Thinking it maps URL parameters
• Assuming it validates data automatically

2. Which of the following method signatures correctly uses @RequestBody to accept a JSON object representing a User in a Spring Boot controller?

easy

A. public ResponseEntity<String> addUser(@RequestBody User user)

B. public ResponseEntity<String> addUser(@RequestParam User user)

C. public ResponseEntity<String> addUser(@PathVariable User user)

D. public ResponseEntity<String> addUser(User user)

Solution

1. Step 1: Identify correct annotation for JSON body

   @RequestBody is required to tell Spring Boot to convert JSON in the request body to a User object.

2. Step 2: Check other annotations

   @RequestParam reads query parameters, @PathVariable reads URL path segments, and no annotation means no automatic JSON conversion.

3. Final Answer:

   public ResponseEntity<String> addUser(@RequestBody User user) -> Option A

4. Quick Check:

   JSON input needs @RequestBody [OK]

Hint: Use @RequestBody for JSON input, not @RequestParam or @PathVariable [OK]

Common Mistakes:

• Using @RequestParam instead of @RequestBody
• Forgetting @RequestBody annotation
• Confusing @PathVariable with JSON input

3. Given the following controller method:

  @PostMapping("/add")
  public String addUser(@RequestBody User user) {
    return "Hello " + user.getName();
  }

And the JSON sent in the request body is:

{"name": "Alice"}

What will be the response?

medium

A. HTTP 400 Bad Request

B. "Hello null"

C. "Hello Alice"

D. Compilation error

Solution

1. Step 1: Understand JSON to Java object mapping

   The JSON {"name": "Alice"} maps to a User object with name set to "Alice".

2. Step 2: Check method return value

   The method returns "Hello " + user.getName(), so it returns "Hello Alice".

3. Final Answer:

   "Hello Alice" -> Option C

4. Quick Check:

   JSON name maps to user.getName() = "Hello Alice" [OK]

Hint: JSON keys map to object fields, so output uses those values [OK]

Common Mistakes:

• Assuming user.getName() is null
• Expecting HTTP error without malformed JSON
• Confusing request body with query parameters

4. Consider this controller method:

@PostMapping("/user")
public String createUser(@RequestBody User user) {
  return user.getEmail();
}

If the client sends an empty request body, what will happen?

medium

A. Spring Boot throws an HTTP 400 Bad Request error

B. The method returns null

C. The method returns an empty string

D. Compilation error due to missing @RequestBody

Solution

1. Step 1: Check behavior with empty JSON body

   When @RequestBody expects JSON but receives empty body, Spring Boot cannot create the User object.

2. Step 2: Understand Spring Boot error handling

   Spring Boot responds with HTTP 400 Bad Request for missing or malformed JSON input.

3. Final Answer:

   Spring Boot throws an HTTP 400 Bad Request error -> Option A

4. Quick Check:

   Empty JSON body causes HTTP 400 error [OK]

Hint: Empty JSON body with @RequestBody causes HTTP 400 error [OK]

Common Mistakes:

• Assuming method returns null or empty string
• Thinking compilation error occurs
• Ignoring HTTP error on missing JSON

5. You want to create a Spring Boot REST endpoint that accepts a JSON array of User objects in the request body and returns the count of users received. Which method signature correctly uses @RequestBody to achieve this?

hard

A. public int countUsers(@RequestBody User user)

B. public int countUsers(@RequestBody List<User> users)

C. public int countUsers(@RequestParam List<User> users)

D. public int countUsers(@PathVariable List<User> users)

Solution

1. Step 1: Identify correct parameter type for JSON array

   To accept a JSON array of User objects, the method parameter should be a List<User> annotated with @RequestBody.

2. Step 2: Exclude incorrect annotations and types

   @RequestParam and @PathVariable do not handle JSON body. Single User parameter cannot accept an array.

3. Final Answer:

   public int countUsers(@RequestBody List<User> users) -> Option B

4. Quick Check:

   JSON array maps to List<User> with @RequestBody [OK]

Hint: Use List<User> with @RequestBody for JSON arrays [OK]

Common Mistakes:

• Using single User instead of List<User>
• Using @RequestParam or @PathVariable for JSON body
• Forgetting @RequestBody annotation