ObjectMapper Java to JSON

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In this article, we use Jackson library class ObjectMapper which provides the functionality for reading and writing JSON, either to or from basic  POJOs.

Before getting started, let's define the required Jackson API dependencies.

POM Dependency

Let’s first add the following dependencies to the pom.xml:

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

This dependency will also transitively add the following libraries to the classpath:

• jackson-annotations-2.9.8.jar
• jackson-core-2.9.8.jar
• jackson-databind-2.9.8.jar

Always use the latest versions on the Maven central repository for Jackson databind.

Converting Java Object to JSON Example

Here is an example of creating JSON from Java object using the ObjectMapper.writeValueXXX() methods.

Post.java

package net.javaguides.jackson.pojotojson;

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

public class Post {
    private Long id;
    private String title;
    private String description;
    private String content;
    private Date postedAt = new Date();
    private Date lastUpdatedAt = new Date();
    private Set < Tag > tags = new HashSet < > ();

    public Post() {

    }

    public Post(String title, String description, String content) {
        this.title = title;
        this.description = description;
        this.content = content;
    }

    public Long getId() {
        return id;
    }

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

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public Date getPostedAt() {
        return postedAt;
    }

    public void setPostedAt(Date postedAt) {
        this.postedAt = postedAt;
    }

    public Date getLastUpdatedAt() {
        return lastUpdatedAt;
    }

    public void setLastUpdatedAt(Date lastUpdatedAt) {
        this.lastUpdatedAt = lastUpdatedAt;
    }

    public Set < Tag > getTags() {
        return tags;
    }

    public void setTags(Set < Tag > tags) {
        this.tags = tags;
    }
}

Tag.java

package net.javaguides.jackson.pojotojson;

public class Tag {
    private Long id;
    private String name;

    public Tag() {

    }

    public Tag(Long id, String name) {
        super();
        this.id = id;
        this.name = name;
    }


    public Tag(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

JacksonPojoToJson.java

package net.javaguides.jackson.pojotojson;

import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

/**
 * Convert Java to Json using Jackson API
 * @author Ramesh Fadatare
 *
 */
public class JacksonPojoToJson {
    public static void main(String[] args) throws IOException {
        // Create ObjectMapper
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);

        // create a post
        Post post = new Post();
        post.setTitle("Jackson JSON API Guide");
        post.setId(100 L);
        post.setDescription("Post about Jackson JSON API");
        post.setContent("HTML content here");
        post.setLastUpdatedAt(new Date());
        post.setPostedAt(new Date());

        // create some predefined tags
        Set < Tag > tags = new HashSet < > ();
        Tag java = new Tag(1 L, "Java");
        Tag jackson = new Tag(2 L, "Jackson");
        Tag json = new Tag(3 L, "JSON");
        tags.add(java);
        tags.add(jackson);
        tags.add(json);

        // set tags to post
        post.setTags(tags);

        // Convert object to JSON string
        String postJson = mapper.writeValueAsString(post);
        System.out.println(postJson);

        // Save JSON string to file
        FileOutputStream fileOutputStream = new FileOutputStream("post.json");
        mapper.writeValue(fileOutputStream, post);
        fileOutputStream.close();
    }
}

Output:

{
  "id" : 100,
  "title" : "Jackson JSON API Guide",
  "description" : "Post about Jackson JSON API",
  "content" : "HTML content here",
  "postedAt" : 1556025668077,
  "lastUpdatedAt" : 1556025668077,
  "tags" : [ {
    "id" : 3,
    "name" : "JSON"
  }, {
    "id" : 1,
    "name" : "Java"
  }, {
    "id" : 2,
    "name" : "Jackson"
  } ]
}

Note that we can also write JSON to external file. In this example, we are writing JSON to "post.json" file.