How To Convert Java Object To / From JSON (Jackson)

JSON (JavaScript Object Notation), is a simple and easy to read and write data exchange format. It’s popular and implemented in countless projects worldwide, for those don’t like XML, JSON is a very good alternative solution.

In this series of Java JSON tutorials, we focus on three popular third party Java libraries to process JSON data, which areJackson, Google Gson and JSON.simple

For object/json conversion, you need to know following two methods :

//1. Convert Java object to JSON format
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(new File("c:\\user.json"), user);

//2. Convert JSON to Java object
ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(new File("c:\\user.json"), User.class);

Note
Both writeValue() and readValue() has many overloaded methods to support different type of inputs and outputs. Make sure check it out.

1. Jackson Dependency

Jackson contains 6 separate jars for different purpose, check here. In this case, you only need “jackson-mapper-asl” to handle the conversion.

Get the Jackson library here

2. POJO

An user object, initialized with some values. Later use Jackson to convert this object to / from JSON.

package json;
 
import java.util.ArrayList;
import java.util.List;
 
public class User {
 
 private int age = 29;
 private String name = "harit";
 private List<String> messages = new ArrayList<String>() {
  {
   add("msg 1");
   add("msg 2");
   add("msg 3");
  }
 };
 
 //getter and setter methods
 
 @Override
 public String toString() {
  return "User [age=" + age + ", name=" + name + ", " +
    "messages=" + messages + "]";
 }
}

3. Java Object to JSON

Convert an “user” object into JSON formatted string, and save it into a file “user.json“.

package json;
 
import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
 
public class JacksonExample {
    public static void main(String[] args) {
 
 User user = new User();
 ObjectMapper mapper = new ObjectMapper();
 
 try {
 
  // convert user object to json string, and save to a file
  mapper.writeValue(new File("c:\\user.json"), user);
 
  // display to console
  System.out.println(mapper.writeValueAsString(user));
 
 } catch (JsonGenerationException e) {
 
  e.printStackTrace();
 
 } catch (JsonMappingException e) {
 
  e.printStackTrace();
 
 } catch (IOException e) {
 
  e.printStackTrace();
 
 }
 
  }
 
}

Output

{"age":29,"messages":["msg 1","msg 2","msg 3"],"name":"harit"}

Note
Above JSON output is hard to read. You can enhance it by enable the pretty print feature.

4. JSON to Java Object

Read JSON string from file “user.json“, and convert it back to Java object.

package json;
 
import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
 
public class JacksonExample {
    public static void main(String[] args) {
 
 ObjectMapper mapper = new ObjectMapper();
 
 try {
 
  // read from file, convert it to user class
  User user = mapper.readValue(new File("c:\\user.json"), User.class);
 
  // display to console
  System.out.println(user);
 
 } catch (JsonGenerationException e) {
 
  e.printStackTrace();
 
 } catch (JsonMappingException e) {
 
  e.printStackTrace();
 
 } catch (IOException e) {
 
  e.printStackTrace();
 
 }
 
  }
 
}

User [age=29, name=harit, messages=[msg 1, msg 2, msg 3]]