JSON serialization with Jackson in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

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

Consider a Spring Boot REST controller method that returns a simple POJO. What JSON will be sent to the client?

Spring Boot

public record User(String name, int age) {}

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

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

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

C {"Name":"Alice","Age":30}

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

💡 Hint

Jackson uses field names as JSON keys by default and serializes numbers as numbers.

The record fields name and age become JSON keys with their exact names. The age is serialized as a number, not a string. The JSON is a flat object, not nested.

📝 Syntax

intermediate

Which option correctly configures Jackson to ignore null fields in JSON output?

You want to configure Jackson in Spring Boot to skip null fields when serializing objects. Which code snippet achieves this?

A

@Bean
public ObjectMapper objectMapper() {
    return new ObjectMapper().disable(SerializationFeature.WRITE_NULL_MAP_VALUES);
}

B

@Bean
public ObjectMapper objectMapper() {
    return new ObjectMapper().setSerializationInclusion(JsonInclude.Include.NON_NULL);
}

C

@Bean
public ObjectMapper objectMapper() {
    return new ObjectMapper().setSerializationInclusion(JsonInclude.Include.ALWAYS);
}

D

@Bean
public ObjectMapper objectMapper() {
    return new ObjectMapper().enable(SerializationFeature.WRITE_NULL_MAP_VALUES);
}

💡 Hint

Jackson's setSerializationInclusion controls which fields are included.

Using JsonInclude.Include.NON_NULL tells Jackson to skip fields with null values during serialization. Other options either disable unrelated features or include all fields.

🔧 Debug

advanced

Why does this Jackson serialization produce an empty JSON object?

Given this class and controller, why does the JSON output show {} instead of the expected fields?

Spring Boot

public class Product {
    private String name;
    private double price;

    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() { return name; }
    public double getPrice() { return price; }
}

@GetMapping("/product")
public Product getProduct() {
    return new Product("Book", 12.99);
}

A Because the fields are private and there are no getters, Jackson cannot access them.

B Because the class is missing the @JsonSerializable annotation.

C Because the price field is a primitive type and cannot be serialized.

D Because the controller method does not return ResponseEntity.

💡 Hint

Jackson needs a way to read field values, usually via getters or public fields.

Jackson by default uses getters or public fields to serialize. Private fields without getters are ignored, resulting in empty JSON.

🧠 Conceptual

advanced

What is the effect of using @JsonIgnoreProperties on a class?

In Jackson, what does adding @JsonIgnoreProperties({"field1", "field2"}) to a class do?

A It renames the specified fields in the JSON output.

B It marks the specified fields as required during deserialization.

C It prevents the specified fields from being included in JSON serialization and deserialization.

D It makes the specified fields transient and skips them only during serialization.

💡 Hint

Think about ignoring fields in both directions: reading and writing JSON.

@JsonIgnoreProperties tells Jackson to skip the listed fields when converting to or from JSON, so they are not included in output or expected in input.

❓ state_output

expert

What JSON output results from this custom serializer in Spring Boot?

Given this custom serializer registered for a class, what JSON will be produced?

Spring Boot

public class Color {
    private int red, green, blue;
    public Color(int r, int g, int b) { red = r; green = g; blue = b; }
    public int getRed() { return red; }
    public int getGreen() { return green; }
    public int getBlue() { return blue; }
}

public class ColorSerializer extends StdSerializer<Color> {
    public ColorSerializer() { super(Color.class); }
    @Override
    public void serialize(Color value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        String hex = String.format("#%02X%02X%02X", value.getRed(), value.getGreen(), value.getBlue());
        gen.writeString(hex);
    }
}

// Registered in ObjectMapper

@GetMapping("/color")
public Color getColor() {
    return new Color(255, 165, 0);
}

A "#FFA500"

B {"red":255,"green":165,"blue":0}

C "rgb(255,165,0)"

D {"color":"#FFA500"}

💡 Hint

The custom serializer writes a string with the hex color code.

The serializer converts the Color object to a hex string like "#FFA500" and writes it as a JSON string, not an object.