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Jackson ObjectMapper

Learn to produce JSON from objects with Jackson ObjectMapper.

Jackson ObjectMapper

The Jackson library converts Java objects to structured text and back. Its central class is ObjectMapper. You create an instance with ObjectMapper mapper = new ObjectMapper();. To convert an object to text use mapper.writeValueAsString(obj); to parse text into an object use mapper.readValue(text, User.class).

A simple record or POJO automatically uses its field names as keys. Jackson works with canonical constructors, accessors or records. In this lesson we first define a User record and create an ObjectMapper instance.

Data model with record

Since Java 16, record defines short data classes. The line record User(String name, int age) {} auto-generates a constructor, accessors, equals, hashCode and toString. Jackson supports records directly.

Record fields are final; they have no mutator methods. This makes them ideal for immutable data models. Jackson matches fields automatically from the record constructor.

Practice User record and ObjectMapper

Define record User(String name, int age) {}. In main, create an ObjectMapper mapper = new ObjectMapper();, create a user with User u = new User("Ayşe", 30);, and print the name with System.out.println(u.name());. Also print the class name of the mapper with System.out.println(mapper.getClass().getSimpleName()); to confirm it is in use.

Starter code

Main.java
import com.fasterxml.jackson.databind.ObjectMapper;

// Define the User record

public class Main {
    public static void main(String[] args) {
        // Create ObjectMapper and User, two println calls

    }
}

Sample solution

Main.java
import com.fasterxml.jackson.databind.ObjectMapper;

record User(String name, int age) {}

public class Main {
    public static void main(String[] args) {
        ObjectMapper mapper = new ObjectMapper();
        User u = new User("Ayşe", 30);
        System.out.println(u.name());
        System.out.println(mapper.getClass().getSimpleName());
    }
}

Expected output

Ayşe
ObjectMapper

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is the main role of ObjectMapper?

  • To connect to a database
  • To convert between structured text and Java objects Correct answer
  • To browse the file system
  • To send HTTP requests

Explanation: ObjectMapper bridges structured text and Java objects.

2 Complete the ObjectMapper instance

Question 
ObjectMapper mapper = new ____();
Answer 
ObjectMapper mapper = new ObjectMapper();

Explanation: The ObjectMapper instance is used for Jackson operations.

3 Match the Jackson concepts

Term Meaning
ObjectMapper Central converter
writeValueAsString Object to text
readValue Text to object
record Short data class

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together