Message serialization in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Spring Boot REST controller method?

Consider a Spring Boot REST controller method that returns a Java object. What will the HTTP response body contain?

Spring Boot

record User(String name, int age) {}

@RestController
public class UserController {
    @GetMapping("/user")
    public User getUser() {
        return new User("Alice", 30);
    }
}

A A plain text string: User(name=Alice, age=30)

B A JSON string: {"name":"Alice","age":30}

C An XML string representing the User object

D A binary serialized Java object

💡 Hint

Spring Boot uses Jackson by default to convert Java objects to JSON in REST responses.

Spring Boot automatically serializes Java objects returned by REST controller methods into JSON format using Jackson library, unless configured otherwise.

📝 Syntax

intermediate

Which annotation enables JSON serialization of a Java class in Spring Boot?

You want to ensure a Java class is serialized to JSON correctly in Spring Boot REST responses. Which annotation should you use?

A @JsonInclude

B @JsonSerializable

C @JsonProperty

D @JsonIgnoreProperties

💡 Hint

This annotation controls which properties are included during serialization.

@JsonInclude controls inclusion of properties during JSON serialization. The other annotations serve different purposes.

🔧 Debug

advanced

Why does this Spring Boot REST method return an empty JSON object?

Given this code, why does the HTTP response body show {} instead of the expected data? public class Product { private String name; private double price; public Product(String name, double price) { this.name = name; this.price = price; } } @RestController public class ProductController { @GetMapping("/product") public Product getProduct() { return new Product("Book", 12.99); } }

A The @GetMapping path is incorrect, causing no data to be returned.

B The Product constructor is private, so the object cannot be created.

C The Product class lacks public getters for its fields, so Jackson cannot serialize them.

D The @RestController annotation is missing on the controller class.

💡 Hint

Jackson needs access to fields via getters or public fields to serialize.

Jackson by default serializes Java objects using getters. Without getters, it serializes an empty object {}.

🧠 Conceptual

advanced

What is the role of the MessageConverter in Spring Boot serialization?

In Spring Boot, what does the HttpMessageConverter do during REST API calls?

A It handles user authentication and authorization.

B It manages database connections for REST endpoints.

C It compiles Java code into bytecode at runtime.

D It converts HTTP requests and responses to and from Java objects.

💡 Hint

Think about how data moves between HTTP and Java objects.

HttpMessageConverter converts HTTP request bodies into Java objects and Java objects into HTTP response bodies, enabling serialization and deserialization.

❓ state_output

expert

What is the JSON output of this Spring Boot controller with custom serialization?

Given this code, what JSON will the /order endpoint return? public class Order { private int id; private double amount; public Order(int id, double amount) { this.id = id; this.amount = amount; } @JsonProperty("order_id") public int getId() { return id; } @JsonIgnore public double getAmount() { return amount; } } @RestController public class OrderController { @GetMapping("/order") public Order getOrder() { return new Order(101, 250.75); } }

A {"order_id":101}

B {"id":101,"amount":250.75}

C {"order_id":101,"amount":250.75}

D {}

💡 Hint

Look at the @JsonProperty and @JsonIgnore annotations on getters.

@JsonProperty renames the id field to order_id in JSON. @JsonIgnore hides the amount field from JSON output.