How To Convert JAVA object to / from JSON (Jackson 2.9.4)

By | April 4, 2018

How To Convert JAVA object to / from JSON (Jackson 2.9.4) will be explain in next java tutorial. This is how to use Jackson 1.x data binding to convert Java object to / from JSON.

Note
Jackson 1.x is a maintenance project, Jackson 1.x is obsolete, no more update, please refer to the latest and please use Jackson 2.9.4 instead.

1. Quick Reference

1.1 Convert Java object to JSON, writeValue(...)

ObjectMapper mapper = new ObjectMapper();
User user = new User();

//Object to JSON in file
mapper.writeValue(new File("c:\\user.json"), user);

//Object to JSON in String
String jsonInString = mapper.writeValueAsString(user);

 

1.2 Convert JSON to Java object, readValue(...)

ObjectMapper mapper = new ObjectMapper();
String jsonInString = "{'name' : 'tiket'}";

//JSON from file to Object
User user = mapper.readValue(new File("c:\\user.json"), User.class);

//JSON from String to Object
User user = mapper.readValue(jsonInString, User.class);

 

P.S All examples are tested with Jackson 1.9.13

2. Jackson Dependency

For Jackson 1.x, it contains 6 separate jars for different purpose, in most cases, you just need jackson-mapper-asl.

pom.xml
	<dependency>
		<groupId>org.codehaus.jackson</groupId>
		<artifactId>jackson-mapper-asl</artifactId>
		<version>1.9.13</version>
	</dependency>

3. POJO (Plain Old Java Object)

An User object for testing.

User.java
package com.tiket.json;

import java.util.List;

public class User {

	private String name;
	private int age;
	private List<String> messages;

	//getters and setters
}

4. Java Object to JSON

Convert an user object into a JSON formatted string.

JacksonExample.java
package com.tiket.json;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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();

		//For testing
		User user = createDummyUser();
		
		try {
			//Convert object to JSON string and save into file directly 
			mapper.writeValue(new File("D:\\user.json"), user);
			
			//Convert object to JSON string
			String jsonInString = mapper.writeValueAsString(user);
			System.out.println(jsonInString);
			
			//Convert object to JSON string and pretty print
			jsonInString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(user);
			System.out.println(jsonInString);			
		} catch (JsonGenerationException e) {
			e.printStackTrace();
		} catch (JsonMappingException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}

	}

	private static User createDummyUser(){
		
		User user = new User();
		
		user.setName("tiket");
		user.setAge(33);

		List<String> msg = new ArrayList<>();
		msg.add("hello jackson 1");
		msg.add("hello jackson 2");
		msg.add("hello jackson 3");

		user.setMessages(msg);
		
		return user;		
	}
}

Output

//new json file is created in D:\\user.json"

{"name":"tiket","age":33,"messages":["hello jackson 1","hello jackson 2","hello jackson 3"]} { "name" : "tiket", "age" : 33, "messages" : [ "hello jackson 1", "hello jackson 2", "hello jackson 3" ] } 

5. JSON to Java Object

Read JSON string and convert it back to a Java object.

JacksonExample.java
package com.tiket.json;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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 {

			// Convert JSON string from file to Object
			User user = mapper.readValue(new File("G:\\user.json"), User.class);
			System.out.println(user);

			// Convert JSON string to Object
			String jsonInString = "{\"age\":33,\"messages\":[\"msg 1\",\"msg 2\"],\"name\":\"tiket\"}";
			User user1 = mapper.readValue(jsonInString, User.class);
			System.out.println(user1);

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

Output

User [name=tiket, age=33, messages=[hello jackson 1, hello jackson 2, hello jackson 3]] User [name=tiket, age=33, messages=[msg 1, msg 2]] 

6. @JsonView

@JsonView has been supported in Jackson since version 1.4, it lets you control what fields to display.

6.1 A simple class.

Views.java
package com.tiket.json;

public class Views {

	public static class NameOnly{};
	public static class AgeAndName extends NameOnly{};

}

6.2 Annotate on the fields you want to display.

User.java
package com.tiket.json;

import java.util.List;
import org.codehaus.jackson.map.annotate.JsonView;

public class User {

	@JsonView(Views.NameOnly.class)
	private String name;

	@JsonView(Views.AgeAndName.class)
	private int age;
	
	private List<String> messages;

	//getter and setters
}

6.3 Enable the @JsonView via writerWithView().

JacksonExample.java
package com.tiket.json;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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

public class JacksonExample {
	public static void main(String[] args) {

		ObjectMapper mapper = new ObjectMapper();
		//By default all fields without explicit view definition are included, disable this
		mapper.configure(SerializationConfig.Feature.DEFAULT_VIEW_INCLUSION, false);
		 
		//For testing
		User user = createDummyUser();
		
		try {
			//display name only
			String jsonInString = mapper.writerWithView(Views.NameOnly.class).writeValueAsString(user);
			System.out.println(jsonInString);
			
			//display namd ana age
			jsonInString = mapper.writerWithView(Views.AgeAndName.class).writeValueAsString(user);
			System.out.println(jsonInString);					
		} catch (JsonGenerationException e) {
			e.printStackTrace();
		} catch (JsonMappingException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
	
	}

	private static User createDummyUser(){
		
		User user = new User();
		
		user.setName("tiket");
		user.setAge(33);

		List<String> msg = new ArrayList<>();
		msg.add("hello jackson 1");
		msg.add("hello jackson 2");
		msg.add("hello jackson 3");

		user.setMessages(msg);
		
		return user;
		
	}
}

Output

{"name":"tiket"} {"name":"tiket","age":33}

References

  1. Jackson 2.9.4 Maven / Gradle @github

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