HowToDoInJava

Jackson convert object to json and json to object

By Lokesh Gupta | Filed Under: Jackson

Jackson convert object to JSON example and convert json to object example. Learn to use jackson objectmapper to populate java object from json string and write json string from java object.

Jackson is used to convert java object to json, and convert json to java object. In this quick jackson tutorial, I am giving examples of converting java objects to/from json programmatically.

Table of Contents

1. Jackson maven dependency
2. Convert Java object to JSON
3. Pretty print JSON
4. Convert JSON to Java object

Before jumping into code examples, lets define a simple pojo class which we will use in this example for conversion purpose.

public class Employee
{
   private Integer id;
   private String firstName;
   private String lastName;

   public Employee(){

   }

   public Employee(Integer id, String firstName, String lastName, Date birthDate){
      this.id = id;
      this.firstName = firstName;
      this.lastName = lastName;
   }

   //Getters and setters

   @Override
   public String toString()
   {
      return "Employee [id=" + id + ", firstName=" + firstName + ", " +
            "lastName=" + lastName + "]";
   }
}

1. Jackson dependency

You can add Jackson dependency in two ways depending on your project type.

1.1. Maven based project

Add following dependency in your pom.xml file.

<dependencies>
	<dependency>
	    <groupId>com.fasterxml.jackson.core</groupId>
	    <artifactId>jackson-core</artifactId>
	    <version>2.9.6</version>
	</dependency>
</dependencies>

1.2. For ANT or other project types

For non-Maven use cases, you download jars from Central Maven repository.

2. Jackson ObjectMapper

ObjectMapper is the main api used for data-binding. It comes with several reader/writer methods to preform the conversion from/to Java and JSON. It will use instances of JsonParser and JsonGenerator for implementing actual reading/writing of JSON.

2.1. Syntax to convert json to object

Use below sample syntax to read JSON and populate java objects.

ObjectMapper mapper = new ObjectMapper();
Object value = mapper.readValue(jsonSource , javaObject);
  • jsonSource – The input source which will fetch the json string.
  • javaObject – The target Java object which needs to be populated.

2.2. Syntax to convert object to json

Use below sample syntax to write java object to json string.

ObjectMapper mapper = new ObjectMapper();
Object value = mapper.writeValue(jsonTarget, javaObject);
  • jsonTarget – The output target where json string will be written.
  • javaObject – The source Java object which needs to be converted to json.

3. Jackson convert object to JSON

To convert the employee object and write it to some file, to can use below code.

package test.jackson;

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

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

public class JavaToJSONExample
{
   public static void main(String[] args)
   {
      @SuppressWarnings("deprecation")
      Employee employee = new Employee(1, "Lokesh", "Gupta", new Date(1981,8,18));
      ObjectMapper mapper = new ObjectMapper();
      try
      {
         mapper.writeValue(new File("c://temp/employee.json"), employee);
      } catch (JsonGenerationException e)
      {
         e.printStackTrace();
      } catch (JsonMappingException e)
      {
         e.printStackTrace();
      } catch (IOException e)
      {
         e.printStackTrace();
      }
   }
}

Program Output.

{"id":1,"firstName":"Lokesh","lastName":"Gupta"}

4. Jackson pretty print JSON output

If you look at above output, then the output written in text file is very raw and not formatted. You can write a formatted JSON content using defaultPrettyPrintingWriter() writerWithDefaultPrettyPrinter instance like below:

package test.jackson;

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

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

public class JavaToPrettyJSONExample
{
   public static void main(String[] args)
   {
      @SuppressWarnings("deprecation")
      Employee employee = new Employee(1, "Lokesh", "Gupta", new Date(1981,8,18));
      ObjectMapper mapper = new ObjectMapper();
      try
      {
         mapper.defaultPrettyPrintingWriter().writeValue(new File("c://temp/employee.json"), employee);
      } catch (JsonGenerationException e)
      {
         e.printStackTrace();
      } catch (JsonMappingException e)
      {
         e.printStackTrace();
      } catch (IOException e)
      {
         e.printStackTrace();
      }
   }
}

Program Output.

{
  "id" : 1,
  "firstName" : "Lokesh",
  "lastName" : "Gupta"
}

5. Jackson convert JSON to Java object

To convert a json string to java object (e.g. Employee object) use below code:

package test.jackson;

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 JSONToJavaExample
{
   public static void main(String[] args)
   {
      Employee employee = null;
      ObjectMapper mapper = new ObjectMapper();
      try
      {
         employee =  mapper.readValue(new File("c://temp/employee.json"), Employee.class);
      } catch (JsonGenerationException e)
      {
         e.printStackTrace();
      } catch (JsonMappingException e)
      {
         e.printStackTrace();
      } catch (IOException e)
      {
         e.printStackTrace();
      }
      System.out.println(employee);
   }
}

Program Output.

Employee [id=1, firstName=Lokesh, lastName=Gupta]
Make sure you have defined a default constructor in your POJO class (e.g. Employee.java in our case). Jackson uses default constructor to create the instances of java class using reflection. If default constructor is not provided, then you will get JsonMappingException in runtime.

Happy Learning !!

LinkedinRedditPocket

About Lokesh Gupta

Founded HowToDoInJava.com in late 2012. I love computers, programming and solving problems everyday. A family guy with fun loving nature. You can find me on Facebook, Twitter and Google Plus.

Feedback, Discussion and Comments

  1. dhanaraj

    “{“timestamp”:”2019-07-29T06:53:14.290+0000″,”status”:400,”error”:”Bad Request”,”message”:”
    JSON parse error: Cannot deserialize instance of `java.util.Date` out of START_OBJECT token; nested exception is
    com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.Date`
    out of START_OBJECT token\n at [Source: (PushbackInputStream);

    hai how to solve the above problem

    Reply

  2. Shikha Singh

    Getter setters are missing in Employee.java due to while mapper.readValue() won’t work

    Reply

    • Lokesh Gupta

      They are removed for brevity. I have added comment in code “//Getters and setters” for this reason only.

      Reply

  3. Bala

    While passing autowired object for json parsing,

    msg:com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain:

    Reply

  4. kichu

    (“$BEO,2.3,1105,V837152325,1,250417172030,110,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0”) i want split the comma seperated string into 3 values in each substring later i want to convert each sub string to json. how to do tat?.

    Reply

  6. Rajesh

    Hi Lokesh, i have this json file can u plz tell how to convert to json
    {
    “ImplCreateBanRestOutput”:{
    “baseOutput”:{
    “ban”:””,
    “statusCodes”:[
    {
    “status”:true,
    “activityType”:””,
    “additionalInfo”:””,
    “errorCode”:””
    },
    {
    “status”:true,
    “activityType”:””,
    “additionalInfo”:””,
    “errorCode”:””
    }
    ],
    “responseInfo”:{
    “code”:””,
    “serverTS”:”2016-03-01T13:56:51.805+0200″,
    “messageId”:””
    }
    }
    }
    }

    Reply

      • Rajesh

        conveting to java object Lokesh

        Reply

  7. Mickael

    Hello,

    I want to send from an external app a json object POST API . How to make the API understands that the parameter is a JSON object and the interpreter ?

    thanks in advance

    Reply

    • Lokesh Gupta

      Use HTTP header “Content-type” in request and set it’s value to “application/json”.

      Reply

  8. alok singh

    how to convert csv file to json file in java dynamically(csv file upload and then convert in json file using java)

    Reply

  9. Manish

    Hi Lokesh,
    Thanks for sharing such a nice tutorial..
    I am using jackson library to parse JSON to POJO
    Here I am in trouble while parsing….My json is dynamically changed..
    I am using REST api to get json response..
    Response example :
    “ValueAdds”: {
    “@size”: “3”,
    “ValueAdd”: [
    {
    “@id”: “128”,
    “description”: “Free Parking”
    },
    {
    “@id”: “2048”,
    “description”: “Free Wireless Internet”
    },
    {
    “@id”: “16777216”,
    “description”: “Breakfast Buffet”
    }
    ]
    }

    and sometimes….

    “ValueAdds”: {
    “@size”: “1”,
    “ValueAdd”: {
    “@id”: “16777216”,
    “description”: “Breakfast Buffet”
    }
    }

    public class ValueAdds {

    @JsonProperty(“@size”)
    private String Size;
    @JsonProperty(“ValueAdd”)
    private List ValueAdd = new ArrayList();
    //getters and setters
    }

    and my pojo expecting List of “ValueAdd”.. so getting error like “Can not deserialize instance of java.util.ArrayList out of START_OBJECT token”

    Please help me on this..
    Thanks in Advance…!!!

    Reply

  10. Vikas

    How we can convert below to Java object:

    {
    “options”: [
    {
    “text”: “111”,
    “label”: “ABC”
    },
    {
    “text”: “222”,
    “label”: “DEF”
    },
    {
    “text”: “333”,
    “label”: “GHI”
    }
    ]
    }

    Reply

    • Manish

      Hi Vikas,

      You should create one more class as options containing text and label as String..
      and here options seems to be json array…so for that you need to create List of options in class where your jsonarray resides.. like..

      “Employee” :
      {
      “options”: [
      {………………………………………
      you have create List list; in class Employee.

      Hope this helps.

      Reply

  11. Ragunath

    Hi Lokesh,

    I could not find the JACKSON latest jar file t execute the above sample code. I am using the ANT project type.
    Can you please share the jar file link to download?

    Reply

  12. test

    I’m getting wired error and unable to get solution for that. Could you please help?

    org.springframework.http.converter.HttpMessageNotReadableException: Could not read JSON:
    Can not deserialize instance of java.lang.String out of START_OBJECT token

    Reply

  13. Refat

    Hi, the above example reads only one object……how to make it read more than one object….may be a silly question……please help!!!

    Reply

    • Lokesh

      There can be only one root element. So you will have to create composite object having association of all different objects you want to read/write.

      Reply

  14. Vijay

    Hey Lokesh,

    Suppose I have list of java objects say List and I want to convert it to json so that I can later pass it as ajax response. How can we achieve this.

    In above example we are writing java object directly to file instead of that I want to hold it.

    Please reply.

    Reply

    • Lokesh

      See this example to understand how we can send it to the response of a web API.

      Reply

  15. nitin

    please make the source code availiable

    Reply

  16. Kamlesh Kumar

    for date its not getting serialised properly.

    Reply

    • HIMANSU NAYAK

      Hi Pankaj,

      Employee constructor is not using date value for initialization in employee.java.

      public Employee(Integer id, String firstName, String lastName, Date birthDate){
      this.id = id;
      this.firstName = firstName;
      this.lastName = lastName;
      }

      Reply

  17. sakura

    That such the great article.
    Anyway, if I want to write more one object at once is it possible to do that? And how?

    Reply

  18. HIMANSU NAYAK

    Hi Pankaj,

    defaultPrettyPrintingWriter() is deprecated api, i am using writerWithDefaultPrettyPrinter() instead.

    Reply

    • Lokesh

      Oops !! I missed it. Updated the post.

      Reply

      • devaki

        how to run the code? Is it a normal java run or maven build?

        Reply

        • Lokesh

          JavaToJSONExample.java has main() method.

          Reply

Ask Questions & Share Feedback

Please do not submit a comment only to say "Thank you".

*Want to Post Code Snippets or XML content? Please use [java] ... [/java] tags otherwise code may not appear partially or even fully. e.g.
[java] 
public static void main (String[] args) {
...
} 
[/java]