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 Listmessages; //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); Listmsg = 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 Listmessages; //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); Listmsg = 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}