Tutorial on using Jackson to transform Java objects to and from JSON

Introduction to a,
There is an ObjectMapper class in Jackson that is useful for interchangeability between Java objects and JSON.
1.JAVA object to JSON[JSON serialization]

import java.io.IOException; 
import java.text.ParseException; 
import java.text.SimpleDateFormat; 
 
import com.fasterxml.jackson.databind.ObjectMapper; 
 
public class JacksonDemo { 
  public static void main(String[] args) throws ParseException, IOException { 
    User user = new User(); 
    user.setName(" wang ");  
    user.setEmail("xiaomin@sina.com"); 
    user.setAge(20); 
     
    SimpleDateFormat dateformat = new SimpleDateFormat("yyyy-MM-dd"); 
    user.setBirthday(dateformat.parse("1996-10-01"));     
     
     
    ObjectMapper mapper = new ObjectMapper(); 
     
    //The User class turn JSON
    //Output result: {"name":" xiaomin ","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}
    String json = mapper.writeValueAsString(user); 
    System.out.println(json); 
     
    //Java collection to JSON
    //Output: [{" name ":" wang ", "age" : 20, "birthday" : 844099200000, "email" : "xiaomin@sina.com"}]
    List<User> users = new ArrayList<User>(); 
    users.add(user); 
    String jsonlist = mapper.writeValueAsString(users); 
    System.out.println(jsonlist); 
  } 
} 

2.JSON to Java class [JSON deserialization]

import java.io.IOException; 
import java.text.ParseException; 
import com.fasterxml.jackson.databind.ObjectMapper; 
 
public class JacksonDemo { 
  public static void main(String[] args) throws ParseException, IOException { 
    String json = "{"name":" wang ","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}"; 
     
     
    ObjectMapper mapper = new ObjectMapper(); 
    User user = mapper.readValue(json, User.class); 
    System.out.println(user); 
  } 
} 

 
Ii. Jackson supports three modes of use:
1. Data Binding: most convenient to use.
(1) Full Data Binding:

private static final String MODEL_BINDING = "{"name":"name1","type":1}"; 
  public void fullDataBinding() throws Exception{ 
    ObjectMapper mapper = new ObjectMapper(); 
    Model user = mapper.readValue(MODEL_BINDING, Model.class);//ReadValue into an entity class.
    System.out.println(user.getName()); 
    System.out.println(user.getType()); 
  } 

Model class:

private static class Model{ 
    private String name; 
    private int type; 
     
    public String getName() { 
      return name; 
    } 
    public void setName(String name) { 
      this.name = name; 
    } 
    public int getType() { 
      return type; 
    } 
    public void setType(int type) { 
      this.type = type; 
    } 
  } 

(2) the Raw Data Binding:

 
  public void rawDataBinding() throws Exception{ 
    ObjectMapper mapper = new ObjectMapper(); 
    HashMap map = mapper.readValue(MODEL_BINDING,HashMap.class);//ReadValue to a raw data type.
    System.out.println(map.get("name")); 
    System.out.println(map.get("type")); 
  } 

  (3) the generic Data Binding:

private static final String GENERIC_BINDING = "{"key1":{"name":"name2","type":2},"key2":{"name":"name3","type":3}}"; 
  public void genericDataBinding() throws Exception{ 
    ObjectMapper mapper = new ObjectMapper(); 
    HashMap<String,Model> modelMap = mapper.readValue(GENERIC_BINDING,new TypeReference<HashMap<String,Model>>(){});//ReadValue to a schema data.
    Model model = modelMap.get("key2"); 
    System.out.println(model.getName()); 
    System.out.println(model.getType()); 
  } 

2. Tree Model: the most flexible.

private static final String TREE_MODEL_BINDING = "{"treekey1":"treevalue1","treekey2":"treevalue2","children":[{"childkey1":"childkey1"}]}"; 
  public void treeModelBinding() throws Exception{ 
    ObjectMapper mapper = new ObjectMapper(); 
    JsonNode rootNode = mapper.readTree(TREE_MODEL_BINDING); 
    //Path does the same thing as get, but returns missing node instead of Null when the node is not found.
    String treekey2value = rootNode.path("treekey2").getTextValue();// 
    System.out.println("treekey2value:" + treekey2value); 
    JsonNode childrenNode = rootNode.path("children"); 
    String childkey1Value = childrenNode.get(0).path("childkey1").getTextValue(); 
    System.out.println("childkey1Value:"+childkey1Value); 
     
    //Create a root node
    ObjectNode root = mapper.createObjectNode(); 
    //Create child node 1
    ObjectNode node1 = mapper.createObjectNode(); 
    node1.put("nodekey1",1); 
    node1.put("nodekey2",2); 
    //Bind child node 1
    root.put("child",node1); 
    //An array of node
    ArrayNode arrayNode = mapper.createArrayNode(); 
    arrayNode.add(node1); 
    arrayNode.add(1); 
    // The binding An array of node
    root.put("arraynode", arrayNode); 
    //JSON reads to the tree node
    JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING); 
    //Bind JSON node
    root.put("valuetotreenode",valueToTreeNode); 
    //JSON is bound to the JSON node object
    JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//Bind JSON to the JSON node object.
    //Bind JSON node
    root.put("bindJsonNode",bindJsonNode); 
    System.out.println(mapper.writeValueAsString(root)); 
  } 

3. Streaming API: best performance.
 
For high performance programs, it is recommended to use the stream API, otherwise use other methods
JsonFactory is used to create either a JsonGenerator or a JsonParser.

package com.jingshou.jackson; 
 
import java.io.File; 
import java.io.IOException; 
 
import com.fasterxml.jackson.core.JsonEncoding; 
import com.fasterxml.jackson.core.JsonFactory; 
import com.fasterxml.jackson.core.JsonGenerator; 
import com.fasterxml.jackson.core.JsonParser; 
import com.fasterxml.jackson.core.JsonToken; 
 
public class JacksonTest6 { 
 
  public static void main(String[] args) throws IOException { 
    JsonFactory jfactory = new JsonFactory(); 
      
     
    JsonGenerator jGenerator = jfactory.createGenerator(new File( 
        "c:\user.json"), JsonEncoding.UTF8); 
    jGenerator.writeStartObject(); // { 
    
    jGenerator.writeStringField("name", "mkyong"); // "name" : "mkyong" 
    jGenerator.writeNumberField("age", 29); // "age" : 29 
    
    jGenerator.writeFieldName("messages"); // "messages" : 
    jGenerator.writeStartArray(); // [ 
    
    jGenerator.writeString("msg 1"); // "msg 1" 
    jGenerator.writeString("msg 2"); // "msg 2" 
    jGenerator.writeString("msg 3"); // "msg 3" 
    
    jGenerator.writeEndArray(); // ] 
    
    jGenerator.writeEndObject(); // } 
    jGenerator.close(); 
     
     
    JsonParser jParser = jfactory.createParser(new File("c:\user.json")); 
    // loop until token equal to "}" 
    while (jParser.nextToken() != JsonToken.END_OBJECT) { 
    
      String fieldname = jParser.getCurrentName(); 
      if ("name".equals(fieldname)) { 
    
       // current token is "name", 
       // move to next, which is "name"'s value 
       jParser.nextToken(); 
       System.out.println(jParser.getText()); // display mkyong 
    
      } 
    
      if ("age".equals(fieldname)) { 
    
       // current token is "age",  
       // move to next, which is "name"'s value 
       jParser.nextToken(); 
       System.out.println(jParser.getIntValue()); // display 29 
    
      } 
    
      if ("messages".equals(fieldname)) { 
    
       jParser.nextToken(); // current token is "[", move next 
    
       // messages is array, loop until token equal to "]" 
       while (jParser.nextToken() != JsonToken.END_ARRAY) { 
    
             // display msg1, msg2, msg3 
         System.out.println(jParser.getText());  
    
       } 
    
      } 
    
     } 
     jParser.close(); 
 
  } 
 
}