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How to Parse JSON to/from Java Object using Jackson Example

How to Parse JSON to/from Java Object using Jackson Example

In this example You will learn how to parse a  JSON String to Java and  how to convert Java Object to JSON format using Jackson. JSON stands for JavaScript Object notation is a subset of JavaScript object syntax, which allows all JavaScript client to process it without using any external library. Because of its compact size, compared to XML and platform independence nature makes JSON a favorite format for transferring data via HTTP. Though Java doesn't have any inbuilt support to parse JSON response in core library, Java developers are lucky to have couple of good and feature rich JSON processing libraries such as GSONJackson and JSON-simple.  Jackson in a high performance, one of the fasted JSON parsing library, which also provides streaming capability. It has no extenal dependency and solely depends on JDK. It is also powerful and provides full binding support for common JDK classes as well as any Java Bean class, e.g. Player in our case. It also provides data binding for Java Collection classes e.g. Map as well Enum.


Jackson Library

The complete Jackson library consists of 6 jar files that are used for many diffident operation. In this example we are going to need just one, mapper-asl.jar. If you want to install the full library to your project you can download and use jackson-all-*.jar that includes all the jars. You can download them from the Jackson Download Page. Alternatively, If your using Maven in your project (which you should) then you can add following dependency in your pom.xml.

<dependency>
      <groupId>org.codehaus.jackson</groupId>
      <artifactId>jackson-all</artifactId>
      <version>1.9.11</version>
</dependency>


You need Default Constructor in Your Bean Class

When I first run my program, I get following exception because  I had parametric constructor inPlayer class and not bothered to add a  no-argument default constructor :

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class Player]: can not instantiate from JSON object (need to add/enable type information?)
at [Source: player.json; line: 1, column: 2]
               at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163)
               at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObjectUsingNonDefault(BeanDeserializer.java:746)
               at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:683)
               at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:580)
               at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2732)
               at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1817)
               at JSONParser.toJava(Testing.java:30)
               at JSONParser.main(Testing.java:17)

Once I added the default constructor on Player class this error is gone. Probably this is another reason why you should have a default or no-arg constructor in Java class.


How to parse JSON in Java

Here is our sample program to parse JSON String in Java. As I said, in this example we will use Jackson, an open source JSON parsing library with rich features. There are two static methods here, toJSON() which converts a Java instance to JSON and fromJSON() method which reads a JSON file, parse it and create Java objects. They key object here is ObjectMapper class from Jackson library, which used for converting JSON to Java and vice-versa.

import java.io.File;
import java.io.IOException;
import java.util.Arrays;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

/**
 * Java Program to parse JSON String to Java object and converting a Java object to equivalent
 * JSON String.
 *
 * @author Javin Paul
 */
public class JSONParser {

    public static void main(String args[]) {
        toJSON();  // converting Java object to JSON String
        toJava();  // parsing JSON file to create Java object
    }

    /**
     * Method to parse JSON String into Java Object using Jackson Parser.
     *
     */
    public static void toJava() {
       
        // this is the key object to convert JSON to Java
        ObjectMapper mapper = new ObjectMapper();

        try {
            File json = new File("player.json");
            Player cricketer = mapper.readValue(json, Player.class);
            System.out.println("Java object created from JSON String :");
            System.out.println(cricketer);

        } catch (JsonGenerationException ex) {
            ex.printStackTrace();
        } catch (JsonMappingException ex) {
            ex.printStackTrace();
        } catch (IOException ex) {
            ex.printStackTrace();

        }
    }

    /**
     * Java method to convert Java Object into JSON String with help of Jackson API.
     *
     */
    public static void toJSON() {
        Player kevin = new Player("Kevin", "Cricket", 32, 221, new int[]{33, 66, 78, 21, 9, 200});

        // our bridge from Java to JSON and vice versa
        ObjectMapper mapper = new ObjectMapper();

        try {
            File json = new File("player.json");
            mapper.writeValue(json, kevin);
            System.out.println("Java object converted to JSON String, written to file");
            System.out.println(mapper.writeValueAsString(kevin));

        } catch (JsonGenerationException ex) {
            ex.printStackTrace();
        } catch (JsonMappingException ex) {
            ex.printStackTrace();
        } catch (IOException ex) {
            ex.printStackTrace();

        }
    }

}

/*
 * A simple Java values class with getters and setters. We will convert Player class instance into
 * JSON String and a JSON object to Player instance.
 */
class Player {

    private String name;
    private String sport;
    private int age;
    private int id;
    private int[] lastScores;

    public Player() {
        //just there, need by Jackson library
    }

    public Player(String name, String sport, int age, int id, int[] lastinnings) {
        this.name = name;
        this.sport = sport;
        this.age = age;
        this.id = id;
        lastScores = lastinnings;
    }

    public final String getName() {
        return name;
    }

    public final String getSport() {
        return sport;
    }

    public final int getAge() {
        return age;
    }

    public final int getId() {
        return id;
    }

    public final int[] getLastScores() {
        return lastScores;
    }

    public final void setName(String name) {
        this.name = name;
    }

    public final void setSport(String sport) {
        this.sport = sport;
    }

    public final void setAge(int age) {
        this.age = age;
    }

    public final void setId(int id) {
        this.id = id;
    }

    public final void setLastScores(int[] lastScores) {
        this.lastScores = lastScores;
    }

    @Override
    public String toString() {
        return "Player [name=" + name + ", sport=" + sport + ", age=" + age + ", id=" + id
                + ", recent scores=" + Arrays.toString(lastScores) + "]";
    }

}


Output:
Java object converted to JSON String, written to file
{"name":"Kevin","sport":"Cricket","age":32,"id":221,"lastScores":[33,66,78,21,9,200]}
Java object created from JSON String :
Player [name=Kevin, sport=Cricket, age=32, id=221, recent scores=[33, 66, 78, 21, 9, 200]]


This will also create file called player.json in your current or project directory.


That's all about how to parse JSON String in Java and convert a Java object to JSON using Jackson API. Though there are couple of more good open source library available for JSON parsing and conversion e.g. GSON and JSON-Simple but Jackson is one of the best and feature rich, its also tried and tested library in many places, means you should be little worried about any nasty bug while parsing your big JSON String.

If you like this tutorial and want to know more about how to work with JSON and Java, check out following fantastic articles :
  • How to read JSON String from File in Java (solution)
  • How to parse JSON Array to Java array? (solution)
  • How to convert JSON to Java Object? (example)

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