Jackson How to parse JSON

By：Roy.LiuLast updated：2019-08-11

Jackson provide writeValue() and readValue() methods to convert Java objects to / from JSON.

mapper.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, default compact-print
	String jsonString = mapper.writeValueAsString(new Staff());
	// pretty-print
	String jsonString2 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(new Staff());

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

P.S Tested with Jackson 2.9.8

1. Download Jackson

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>

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

2. POJO

A simple Java object, POJO, for testing later.

Staff.java

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.mkyong;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;
import java.io.IOException;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
public class JacksonExample1 {
    public static void main(String[] args) {
        ObjectMapper mapper = new ObjectMapper();
        Staff staff = createStaff();
        try {
            // Java objects to JSON file
            mapper.writeValue(new File("c:\\test\\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("mkyong");
        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":"mkyong","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}

Terminal

{"name":"mkyong","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}
  "name" : "mkyong",
  "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.mkyong;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;
import java.io.IOException;
public class JacksonExample2 {
    public static void main(String[] args) {
        ObjectMapper mapper = new ObjectMapper();
        try {
            // JSON file to Java object
            Staff staff = mapper.readValue(new File("c:\\test\\staff.json"), Staff.class);
            // JSON string to Java object
            String jsonInString = "{\"name\":\"mkyong\",\"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();

Output

Staff{name='mkyong', age=37, position=null, skills=[java, python], salary=null}
  "name" : "mkyong",
  "age" : 37,
  "position" : null,
  "skills" : [ "java", "python" ],
  "salary" : null

5. @JsonProperty – JSON Field Naming

5.1 Default

public class Staff {
    private String name;
	private int age;

Output

{"name":"abc", "age":38}

5.2 Change the property name with @JsonProperty

public class Staff {
    @JsonProperty("custom_name")
    private String name;
	private int age;

Output

{"custom_name":"abc", "age":38}

6. @JsonInclude – Ignore null fields

By default, Jackson will include null fields.

  "name" : "mkyong",
  "age" : 38,
  "position" : null,
  "skills" : null,
  "salary" : null

6.1 @JsonInclude on class level.

Staff.java

import com.fasterxml.jackson.annotation.JsonInclude;
											//	ignore null fields , class level
@JsonInclude(JsonInclude.Include.NON_NULL) 	//  ignore all null fields
public class Staff {
    private String name;
    private int age;
    private String[] position;              
    private List<String> skills;           
    private Map<String, BigDecimal> salary; 
	//...

6.2 @JsonInclude on fields level.

Staff.java

import com.fasterxml.jackson.annotation.JsonInclude;
public class Staff {
    private String name;
    private int age;
	@JsonInclude(JsonInclude.Include.NON_NULL) //ignore null field on this property only
    private String[] position;              
    private List<String> skills;           
    private Map<String, BigDecimal> salary;

Output

  "name" : "mkyong",
  "age" : 38,
  "skill" : null,
  "salary" : null

6.3 Globally.

	ObjectMapper mapper = new ObjectMapper();
	// ignore all null fields globally
	mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

Note
More examples for How to ignore null fields with Jackson

7. @JsonView

7.1 The @JsonView is used to limit fields display for different users. For example:

CompanyViews.java

package com.mkyong;
public class CompanyViews {
    public static class Normal{};
    public static class Manager extends Normal{};

Normal view only displays name and age, Manager view is able to display all.

Staff.java

import com.fasterxml.jackson.annotation.JsonView;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
public class Staff {
    @JsonView(CompanyViews.Normal.class)
    private String name;
    @JsonView(CompanyViews.Normal.class)
    private int age;
    @JsonView(CompanyViews.Manager.class)
    private String[] position;
    @JsonView(CompanyViews.Manager.class)
    private List<String> skills;
    @JsonView(CompanyViews.Manager.class)
    private Map<String, BigDecimal> salary;

JacksonExample.java

package com.mkyong;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import java.io.IOException;
import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
public class JacksonExample {
    public static void main(String[] args) {
        ObjectMapper mapper = new ObjectMapper();
        Staff staff = createStaff();
        try {
            // to enable pretty print
            mapper.enable(SerializationFeature.INDENT_OUTPUT);
            // normal
            String normalView = mapper
				.writerWithView(CompanyViews.Normal.class)
				.writeValueAsString(staff);
            System.out.format("Normal views\n%s\n", normalView);
            // manager
            String managerView = mapper
				.writerWithView(CompanyViews.Manager.class)
				.writeValueAsString(staff);
            System.out.format("Manager views\n%s\n", managerView);
        } catch (IOException e) {
            e.printStackTrace();
    private static Staff createStaff() {
        Staff staff = new Staff();
        staff.setName("mkyong");
        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

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

Note
Read this Jackson @JsonView example

8. @JsonIgnore and @JsonIgnoreProperties

By default, Jackson includes all the fields, even static or transient fields.

8.1 @JsonIgnore to ignore fields on field level.

import com.fasterxml.jackson.annotation.JsonIgnore;
public class Staff {
    private String name;
    private int age;
    private String[] position;
    @JsonIgnore
    private List<String> skills;
    @JsonIgnore
    private Map<String, BigDecimal> salary;

Output

  "name" : "mkyong",
  "age" : 38,
  "position" : [ "Founder", "CTO", "Writer" ]

8.2 @JsonIgnoreProperties to ignore fields on class level.

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
@JsonIgnoreProperties({"salary", "position"})
public class Staff {
    private String name;
    private int age;
    private String[] position;
    private List<String> skills;
    private Map<String, BigDecimal> salary;

Output

  "name" : "mkyong",
  "age" : 38,
  "skills" : [ "java", "python", "node", "kotlin" ]

9. FAQs

9.1 Convert JSON array string to List

	String json = "[{\"name\":\"mkyong\", \"age\":38}, {\"name\":\"laplap\", \"age\":5}]";
	List<Staff> list = Arrays.asList(mapper.readValue(json, Staff[].class));
	// or like this:
	// List<Staff> list = mapper.readValue(json, new TypeReference<List<Staff>>(){});

9.2 Convert JSON string to Map

	String json = "{\"name\":\"mkyong\", \"age\":\"33\"}";
	Map<String, String> map = mapper.readValue(json, Map.class);
	// or like this:
	//Map<String, String> map = mapper.readValue(json, new TypeReference<Map<String, String>>(){});
	map.forEach((k, v) -> System.out.format("[key]:%s \t[value]:%s\n", k, v));

Output

[key]:name 	[value]:mkyong
[key]:age 	[value]:33

9.3 What if some complex JSON structure doesn’t map easily to a Java class?
Answer: Try Jackson TreeModel to convert JSON data into JsonNode, so that we can add, update or delete JSON nodes easily.

From：一号门

Tags: jackson java json

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COMMENTS

Jackson How to parse JSON

1. Download Jackson

2. POJO

3. Java Objects to JSON

4. JSON to Java Object

5. @JsonProperty – JSON Field Naming

6. @JsonInclude – Ignore null fields

7. @JsonView

8. @JsonIgnore and @JsonIgnoreProperties

9. FAQs

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