JavaオブジェクトをJSONに変換する/JSONから変換する方法(Jackson)

JavaオブジェクトとJSONの変換方法(Jackson)

このチュートリアルでは、Jackson 1.xデータバインディングを使用してJavaオブジェクトとJSONを相互に変換する方法を示します。

Note
Jackson 1.xはメンテナンスプロジェクトです。代わりにJackson 2.xを使用してください。

Note
このチュートリアルは廃止されました。更新はありません。最新のJackson 2 tutorial – Object to / from JSONを参照してください。

1. クイックリファレンス

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' : 'example'}";

//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. ジャクソン依存

Jackson 1.xの場合、目的の異なる6つの個別のjarが含まれています。ほとんどの場合、必要なのはjackson-mapper-aslだけです。

pom.xml

    
        org.codehaus.jackson
        jackson-mapper-asl
        1.9.13
    

3. POJO(Plain Old Java Object)

テスト用のUserオブジェクト。

User.java

package com.example.json;

import java.util.List;

public class User {

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

    //getters and setters
}

4. JavaオブジェクトからJSON

userオブジェクトをJSON形式の文字列に変換します。

JacksonExample.java

package com.example.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("example");
        user.setAge(33);

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

        user.setMessages(msg);

        return user;

    }
}

出力

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

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

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

5. JSONからJavaオブジェクト

JSON文字列を読み取り、それをJavaオブジェクトに変換します。

JacksonExample.java

package com.example.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\":\"example\"}";
            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();
        }

    }

}

出力

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

User [name=example, age=33, messages=[msg 1, msg 2]]

6. @JsonView

@JsonViewはバージョン1.4以降Jacksonでサポートされており、表示するフィールドを制御できます。

6.1 A simple class.

Views.java

package com.example.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.example.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 messages;

    //getter and setters
}

6.3 Enable the @JsonView via writerWithView().

JacksonExample.java

package com.example.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("example");
        user.setAge(33);

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

        user.setMessages(msg);

        return user;

    }
}

出力

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