Parse and objectize JSON using the @JsonProperty annotation of the Java library Jackson

Overview

--Use Jackson, Java's JSON library --Try parsing a JSON string and making it an object in several ways -Objectize using @ JsonProperty annotation --Try writing without annotations for comparison

JSON to be parsed this time

{
  "user": {
    "name": "Pompom Pudding",
    "score": {
      "kokugo": "good",
      "sansuu": 100
    },
    "favorites": ["soccer", "puzzle"]
  }
}

Parse and objectize JSON in several ways

When directly creating a Map object without preparing a class (no annotation used)

You can get a Map object by passing Map.class as the second argument of ObjectMapper # readValue. Creating an object is simple, but retrieving the desired data from the created object is complicated.

ObjectMapper mapper = new ObjectMapper();
Map obj = mapper.readValue(json, Map.class);
System.out.println(obj);
String name = (String)(((Map)obj.get("user")).get("name"));
System.out.println("name=" + name);

Output example.

{user={name=Pompom Pudding, score={kokugo=good, sansuu=100}, favorites=[soccer, puzzle]}}
name=Pompom Pudding

When preparing a public instance variable in the class (no annotation used)

It can be made into an object by preparing a public instance variable or getter setter. In that case, it is not necessary to write @ JsonProperty annotation.

import java.util.Map;
public class PublicUserObject {
  public Map user;
}

ObjectMapper mapper = new ObjectMapper();
PublicUserObject obj = mapper.readValue(json, PublicUserObject.class);
System.out.println(obj);
String name = (String)obj.user.get("name");
System.out.println("name=" + name);

Output example.

PublicUserObject@e6abefac
name=Pompom Pudding

When preparing a private instance variable in the class (using annotation)

When using a private instance variable, it is necessary to specify the JSON key (name) you want to map with @ JsonProperty.

import java.util.Map;
import com.fasterxml.jackson.annotation.JsonProperty;

public class SimpleUserObject {

  @JsonProperty("user")
  private Map user;

  public Map getUser() {
    return user;
  }

  @Override
  public String toString() {
    return getClass().getName() + ": user:" + user;
  }
}

ObjectMapper mapper = new ObjectMapper();
SimpleUserObject obj = mapper.readValue(json, SimpleUserObject.class);
System.out.println(obj);
String name = (String) obj.getUser().get("name");
System.out.println("name=" + name);

Output example.

SimpleUserObject: user:{name=Pompom Pudding, score={kokugo=good, sansuu=100}, favorites=[soccer, puzzle]}
name=Pompom Pudding

When a private instance variable and access method are prepared in the class (using annotation)

If you create an accessor method a little more firmly, it will be easier to use from an external class.

import java.util.List;
import java.util.Map;
import com.fasterxml.jackson.annotation.JsonProperty;

public class AccessorUserObject {

  @JsonProperty("user")
  private Map user;

  public String getName() {
    return (String) user.get("name");
  }

  private Map getScore() {
    return (Map) user.get("score");
  }

  public String getKokugoScore() {
    return (String) getScore().get("kokugo");
  }

  public int getSansuuScore() {
    return (int) getScore().get("sansuu");
  }

  public List getFavorites() {
    return (List) user.get("favorites");
  }

  @Override
  public String toString() {
    return getClass().getName() +
        ", getName: " + getName() +
        ", getKokugoScore: " + getKokugoScore() +
        ", getSansuuScore: " + getSansuuScore() +
        ", getFavorites: " + getFavorites();
  }
}

ObjectMapper mapper = new ObjectMapper();
AccessorUserObject obj = mapper.readValue(json, AccessorUserObject.class);
System.out.println(obj);
String name = obj.getName();
System.out.println("name=" + name);

Output example.

AccessorUserObject, getName: Pompom Pudding, getKokugoScore: good, getSansuuScore: 100, getFavorites: [soccer, puzzle]
name=Pompom Pudding

When a class for mapping is prepared (using annotation)

Prepare a class and an inner class for mapping JSON data.

import java.util.List;
import com.fasterxml.jackson.annotation.JsonProperty;

public class StructedUserObject {

  @JsonProperty("user")
  public User user;

  public static class User {

    public String name;
    public Score score;
    public List favorites;

    @Override
    public String toString() {
      return getClass().getSimpleName()
          + ": name: " + name
          + ", score: " + score
          + ", favorites: " + favorites;
    }
  }

  public static class Score {

    public String kokugo;
    public int sansuu;

    @Override
    public String toString() {
      return getClass().getSimpleName()
          + ": kokugo: " + kokugo
          + ", sansuu: " + sansuu;
    }
  }

  @Override
  public String toString() {
    return getClass().getName() + ": user:" + user;
  }
}

ObjectMapper mapper = new ObjectMapper();
StructedUserObject obj = mapper.readValue(json, StructedUserObject.class);
System.out.println(obj);
String name = obj.user.name;
System.out.println("name=" + name);

Output example.

StructedUserObject: user:User: name: Pompom Pudding, score: Score: kokugo: good, sansuu: 100, favorites: [soccer, puzzle]
name=Pompom Pudding

The file and execution environment used for this operation check

Java and Maven

$ mvn --version
Apache Maven 3.6.1 (d66c9c0b3152b2e69ee9bac180bb8fcc8e6af555; 2019-04-05T04:00:29+09:00)
Maven home: /usr/local/Cellar/maven/3.6.1/libexec
Java version: 11.0.2, vendor: Oracle Corporation, runtime: /Library/Java/JavaVirtualMachines/openjdk-11.0.2.jdk/Contents/Home
Default locale: ja_JP, platform encoding: UTF-8
OS name: "mac os x", version: "10.14.5", arch: "x86_64", family: "mac"

File list

├── pom.xml
├── src
│   └── main
│       └── java
│           └── info
│               └── maigo
│                   └── lab
│                       └── myjacksonsamples
│                           ├── AccessorUserObject.java
│                           ├── Main.java
│                           ├── PublicUserObject.java
│                           ├── SimpleUserObject.java
│                           └── StructedUserObject.java
└── user.json

pom.xml

Prepare a pom.xml file so that it can be built and executed by Maven. Add Jackson libraries jackson-core, jackson-databind, jackson-annotations to dependencies. Add exec-maven-plugin to plugins so that you can run Java programs from Maven.

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>info.maigo.lab</groupId>
  <artifactId>my-jackson-samples</artifactId>
  <version>1.0-SNAPSHOT</version>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>11</maven.compiler.source>
    <maven.compiler.target>11</maven.compiler.target>
  </properties>

  <!-- $ mvn exec:java -->
  <!-- https://www.mojohaus.org/exec-maven-plugin/ -->
  <build>
    <plugins>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>exec-maven-plugin</artifactId>
        <version>1.6.0</version>
        <configuration>
          <mainClass>info.maigo.lab.myjacksonsamples.Main</mainClass>
        </configuration>
      </plugin>
    </plugins>
  </build>

  <dependencies>

    <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-core</artifactId>
      <version>2.9.9</version>
    </dependency>

    <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.9.9</version>
    </dependency>

    <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-annotations -->
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-annotations</artifactId>
      <version>2.9.9</version>
    </dependency>

  </dependencies>

</project>

A command to generate a JAR file.

$ mvn package

A command to execute a sample program.

$ mvn exec:java

Java source code

Main.java

package info.maigo.lab.myjacksonsamples;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Map;

import com.fasterxml.jackson.databind.ObjectMapper;

public class Main {

    public static void main(String[] args) throws IOException {
        String json = Files.readString(Path.of("user.json"), StandardCharsets.UTF_8);
        createMappedUserObject(json);
        createPublicUserObject(json);
        createSimpleUserObject(json);
        createAccessorUserObject(json);
        createStructedUserObject(json);
    }

    private static void createMappedUserObject(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        Map obj = mapper.readValue(json, Map.class);
        System.out.println(obj);
        String name = (String) (((Map) obj.get("user")).get("name"));
        System.out.println("name=" + name);
    }

    private static void createPublicUserObject(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        PublicUserObject obj = mapper.readValue(json, PublicUserObject.class);
        System.out.println(obj);
        String name = (String) obj.user.get("name");
        System.out.println("name=" + name);
    }

    private static void createSimpleUserObject(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        SimpleUserObject obj = mapper.readValue(json, SimpleUserObject.class);
        System.out.println(obj);
        String name = (String) obj.getUser().get("name");
        System.out.println("name=" + name);
    }

    private static void createAccessorUserObject(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        AccessorUserObject obj = mapper.readValue(json, AccessorUserObject.class);
        System.out.println(obj);
        String name = obj.getName();
        System.out.println("name=" + name);
    }

    private static void createStructedUserObject(String json) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        StructedUserObject obj = mapper.readValue(json, StructedUserObject.class);
        System.out.println(obj);
        String name = obj.user.name;
        System.out.println("name=" + name);
    }
}

PublicUserObject.java

package info.maigo.lab.myjacksonsamples;

import java.util.Map;

public class PublicUserObject {

    public Map user;

}

SimpleUserObject.java

package info.maigo.lab.myjacksonsamples;

import com.fasterxml.jackson.annotation.JsonProperty;

import java.util.Map;

public class SimpleUserObject {

    @JsonProperty("user")
    private Map user;


    public Map getUser() {
        return user;
    }

    @Override
    public String toString() {
        return getClass().getName() + ": user:" + user;
    }
}

AccessorUserObject.java

package info.maigo.lab.myjacksonsamples;

import java.util.List;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonProperty;

public class AccessorUserObject {

    @JsonProperty("user")
    private Map user;

    public String getName() {
        return (String) user.get("name");
    }

    private Map getScore() {
        return (Map) user.get("score");
    }

    public String getKokugoScore() {
        return (String) getScore().get("kokugo");
    }

    public int getSansuuScore() {
        return (int) getScore().get("sansuu");
    }

    public List getFavorites() {
        return (List) user.get("favorites");
    }

    @Override
    public String toString() {
        return getClass().getName() +
                ", getName: " + getName() +
                ", getKokugoScore: " + getKokugoScore() +
                ", getSansuuScore: " + getSansuuScore() +
                ", getFavorites: " + getFavorites();
    }
}

StructedUserObject.java

package info.maigo.lab.myjacksonsamples;

import java.util.List;

import com.fasterxml.jackson.annotation.JsonProperty;

public class StructedUserObject {

    @JsonProperty("user")
    public User user;

    public static class User {

        public String name;
        public Score score;
        public List favorites;

        @Override
        public String toString() {
            return getClass().getSimpleName()
                    + ": name: " + name
                    + ", score: " + score
                    + ", favorites: " + favorites;
        }
    }

    public static class Score {

        public String kokugo;
        public int sansuu;

        @Override
        public String toString() {
            return getClass().getSimpleName()
                    + ": kokugo: " + kokugo
                    + ", sansuu: " + sansuu;
        }
    }

    @Override
    public String toString() {
        return getClass().getName() + ": user:" + user;
    }
}

user.json

{
  "user": {
    "name": "Pompom Pudding",
    "score": {
      "kokugo": "good",
      "sansuu": 100
    },
    "favorites": ["soccer", "puzzle"]
  }
}

Reference material

• FasterXML/jackson: Main Portal page for the Jackson project
• Jackson Annotations · FasterXML/jackson-annotations Wiki -Jackson usage memo -Qiita
• niwasawa/java-jackson-samples: Sample codes of Jackson, JSON library for Java.