Jackson 2 – Convert Java Object to / from JSON

In this tutorial, we will show you how to use Jackson 2.x to convert Java objects to / from a JSON.

1. Basic

1.1 Convert a Staff object to from JSON.

writeValue(...) – Java Objects to JSON

	ObjectMapper mapper = new ObjectMapper();

	// Java object to JSON file
	mapper.writeValue(new File("c:\\test\\staff.json"), new Staff());

	// Java object to JSON string
	String jsonString = mapper.writeValueAsString(object);

readValue(...) – JSON to Java Objects

	ObjectMapper mapper = new ObjectMapper();

	//JSON file to Java object
	Staff obj = mapper.readValue(new File("c:\\test\\staff.json"), Staff.class);

	//JSON URL to Java object
	Staff obj = mapper.readValue(new URL("http://some-domains/api/name.json"), Staff.class);

	//JSON string to Java Object
	Staff obj = mapper.readValue("{'name' : 'mkyong'}", Staff.class);

P.S Tested with Jackson 2.9.8

Note
Read this How to parse JSON with Jackson, containing Jackson examples like Object to/from JSON, @JsonView, @JsonProperty, @JsonInclude, @JsonIgnore, and some FAQs.

1. Download Jackson

1.1 Declares jackson-databind, it will pull in jackson-annotations and jackson-core

pom.xml

	<dependency>
		<groupId>com.fasterxml.jackson.core</groupId>
		<artifactId>jackson-databind</artifactId>
		<version>2.9.8</version>
	</dependency>

1.2 Review the Jackson dependencies :

Terminal

$ mvn dependency:tree

\- com.fasterxml.jackson.core:jackson-databind:jar:2.9.8:compile
[INFO]    +- com.fasterxml.jackson.core:jackson-annotations:jar:2.9.0:compile
[INFO]    \- com.fasterxml.jackson.core:jackson-core:jar:2.9.8:compile

Difference between Jackson 1 and Jackson 2
Most of the APIs still maintains the same method name and signature, just the packaging is different.

– Jackson 1.x – org.codehaus.jackson.map

– Jackson 2.x – com.fasterxml.jackson.databind

2. POJO

A simple Java object for testing.

Staff.java

package com.favtuts.json.javaobject;

import java.math.BigDecimal;
import java.util.List;
import java.util.Map;

public class Staff {
    private String name;
    private int age;
    private String[] position;              //  Array
    private List<String> skills;            //  List
    private Map<String, BigDecimal> salary; //  Map

	// getters , setters, some boring stuff
}

3. Java Objects to JSON

JacksonExample1.java

package com.favtuts.json;

import java.io.File;
import java.io.IOException;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.favtuts.json.javaobject.Staff;

public class JacksonJavaObjectToJSON {

    public static void main(String[] args) {
        javaObjectsToJSON();
    }

    static void javaObjectsToJSON() {
        ObjectMapper mapper = new ObjectMapper();

        Staff staff = createStaff();

        try {

            // Java objects to JSON file
            mapper.writeValue(new File("/home/tvt/workspace/favtuts/staff.json"), staff);

            // Java objects to JSON string - compact-print
            String jsonString = mapper.writeValueAsString(staff);

            System.out.println(jsonString);

            // Java objects to JSON string - pretty-print
            String jsonInString2 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff);

            System.out.println(jsonInString2);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static Staff createStaff() {

        Staff staff = new Staff();

        staff.setName("favtuts");
        staff.setAge(38);
        staff.setPosition(new String[] { "Founder", "CTO", "Writer" });
        Map<String, BigDecimal> salary = new HashMap() {
            {
                put("2010", new BigDecimal(10000));
                put("2012", new BigDecimal(12000));
                put("2018", new BigDecimal(14000));
            }
        };
        staff.setSalary(salary);
        staff.setSkills(Arrays.asList("java", "python", "node", "kotlin"));

        return staff;

    }
}

Output

c:\\test\\staff.json

{"name":"favtuts","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}

Terminal

{"name":"favtuts","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}
{
  "name" : "favtuts",
  "age" : 38,
  "position" : [ "Founder", "CTO", "Writer" ],
  "skills" : [ "java", "python", "node", "kotlin" ],
  "salary" : {
    "2018" : 14000,
    "2012" : 12000,
    "2010" : 10000
  }
}

4. JSON to Java Object

JacksonExample2.java

package com.favtuts.json;

import java.io.File;
import java.io.IOException;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.favtuts.json.javaobject.Staff;

public class JacksonJavaObjectToJSON {

    public static void main(String[] args) {
        jsonToJavaObject();
    }

    static void jsonToJavaObject() {
        ObjectMapper mapper = new ObjectMapper();

        try {

            // JSON file to Java object
            Staff staff = mapper.readValue(new File("/home/tvt/workspace/favtuts/staff.json"), Staff.class);

            // JSON string to Java object
            String jsonInString = "{\"name\":\"favtuts\",\"age\":37,\"skills\":[\"java\",\"python\"]}";
            Staff staff2 = mapper.readValue(jsonInString, Staff.class);

            // compact print
            System.out.println(staff2);

            // pretty print
            String prettyStaff1 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff2);

            System.out.println(prettyStaff1);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Note

More Jackson examples read this – How to parse JSON with Jackson

Download Source Code

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-basic/json

References

• Jackson data-binding official website
• Jackson – How to parse JSON
• Gson – How to parse JSON