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}