So konvertieren Sie ein Java-Objekt in / von JSON (Jackson)
In diesem Tutorial zeigen wir Ihnen, wie Sie die Jackson 1.x-Datenbindung verwenden, um Java-Objekte in / von JSON zu konvertieren.
Note
Jackson 1.x ist ein Wartungsprojekt. Verwenden Sie stattdessenJackson 2.x.
Note
Dieses Tutorial ist veraltet, kein Update mehr. Weitere Informationen finden Sie in den neuestenJackson 2
tutorial – Object to / from JSON.
1. Kurzübersicht
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. Jackson-Abhängigkeit
Für Jackson 1.x enthält es 6 separate Gläser für verschiedene Zwecke. In den meisten Fällen benötigen Sie nurjackson-mapper-asl.
pom.xml
org.codehaus.jackson jackson-mapper-asl 1.9.13
3. POJO (einfaches altes Java-Objekt)
Ein Benutzerobjekt zum Testen.
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-Objekt zu JSON
Konvertieren Sie einuser-Objekt in eine JSON-formatierte Zeichenfolge.
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;
}
}
Ausgabe
//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 nach Java Objekt
Lesen Sie die JSON-Zeichenfolge und konvertieren Sie sie zurück in ein Java-Objekt.
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();
}
}
}
Ausgabe
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 wird in Jackson seit Version 1.4 unterstützt. Damit können Sie steuern, welche Felder angezeigt werden sollen.
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;
}
}
Ausgabe
{"name":"example"}
{"name":"example","age":33}