@RequestBody for JSON input in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending JSON to a Spring Boot controller with @RequestBody?

Consider this Spring Boot controller method:

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

If you send this JSON:

{"name":"Alice","age":30}

What will be the response?

Spring Boot

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

A null

B "{\"name\":\"Alice\",\"age\":30}"

C HTTP 400 Bad Request error

D "Alice"

💡 Hint

Think about how @RequestBody converts JSON into a Java object.

The @RequestBody annotation tells Spring Boot to convert the incoming JSON into a User object. Then, the method returns the name property, which is "Alice".

📝 Syntax

intermediate

Which method signature correctly uses @RequestBody to accept JSON input?

You want to write a Spring Boot controller method that accepts JSON data for a Product object. Which of these method signatures is correct?

A public ResponseEntity<String> addProduct(Product product)

B public ResponseEntity<String> addProduct(@PathVariable Product product)

C public ResponseEntity<String> addProduct(@RequestBody Product product)

D public ResponseEntity<String> addProduct(@RequestParam Product product)

💡 Hint

Remember that @RequestBody is needed to convert JSON in the request body to a Java object.

Only option C uses @RequestBody, which tells Spring Boot to parse JSON from the request body into the Product object.

🔧 Debug

advanced

Why does this Spring Boot controller method fail to parse JSON with @RequestBody?

Given this method:

@PostMapping("/order")
public String createOrder(@RequestBody String order) {
    return order;
}

When sending JSON, the client gets a 415 Unsupported Media Type error. What is the likely cause?

Spring Boot

@PostMapping("/order")
public String createOrder(@RequestBody String order) {
    return order;
}

A The client did not set Content-Type to application/json

B The method parameter should be an Order object, not String

C The method must use @RequestParam instead of @RequestBody

D Spring Boot does not support JSON input by default

💡 Hint

@RequestBody String does not support application/json; use a POJO instead.

A 415 error occurs because the client did not set Content-Type to application/json. Spring Boot expects the Content-Type header to match the media type of the request body. Using @RequestBody String is valid if the client sends the correct Content-Type header.

❓ state_output

advanced

What is the value of the User object after JSON input with missing fields?

Given this User class:

public class User {
  private String name;
  private int age;
  // getters and setters
}

And this controller method:

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

If the JSON sent is:

{"name":"Bob"}

What will be the returned string?

Spring Boot

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

A "Bob,0"

B HTTP 400 Bad Request error

C "null,0"

D "Bob,null"

💡 Hint

What is the default value for an int field in Java if not set?

Since age is an int primitive, missing JSON field sets it to 0 by default. Name is set to "Bob".

🧠 Conceptual

expert

Which statement about @RequestBody and JSON input is true?

Select the correct statement about how Spring Boot handles JSON input with @RequestBody.

A @RequestBody automatically validates JSON fields and throws errors if any field is missing

B @RequestBody requires a default constructor and setters or a constructor with parameters to map JSON fields

C @RequestBody can only map JSON arrays, not JSON objects

D @RequestBody ignores Content-Type headers and accepts any media type

💡 Hint

Think about how Jackson or other JSON mappers create Java objects.

Jackson, used by Spring Boot, needs a default constructor and setters or a constructor with parameters to map JSON fields to Java objects.