使用Jackson来实现Java对象与JSON的相互转换的教程

一、入门
Jackson中有个ObjectMapper类很是实用，用于Java对象与JSON的互换。
1.JAVA对象转JSON[JSON序列化]

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("小民");  
    user.setEmail("xiaomin@sina.com"); 
    user.setAge(20); 
      
    SimpleDateFormat dateformat = new SimpleDateFormat("yyyy-MM-dd"); 
    user.setBirthday(dateformat.parse("1996-10-01"));     
      
    /** 
     * ObjectMapper是JSON操作的核心，Jackson的所有JSON操作都是在ObjectMapper中实现。 
     * ObjectMapper有多个JSON序列化的方法，可以把JSON字符串保存File、OutputStream等不同的介质中。 
     * writeValue(File arg0, Object arg1)把arg1转成json序列，并保存到arg0文件中。 
     * writeValue(OutputStream arg0, Object arg1)把arg1转成json序列，并保存到arg0输出流中。 
     * writeValueAsBytes(Object arg0)把arg0转成json序列，并把结果输出成字节数组。 
     * writeValueAsString(Object arg0)把arg0转成json序列，并把结果输出成字符串。 
     */
    ObjectMapper mapper = new ObjectMapper(); 
      
    //User类转JSON 
    //输出结果：{"name":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"} 
    String json = mapper.writeValueAsString(user); 
    System.out.println(json); 
      
    //Java集合转JSON 
    //输出结果：[{"name":"小民","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转Java类[JSON反序列化]

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":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}"; 
      
    /** 
     * ObjectMapper支持从byte[]、File、InputStream、字符串等数据的JSON反序列化。 
     */
    ObjectMapper mapper = new ObjectMapper(); 
    User user = mapper.readValue(json, User.class); 
    System.out.println(user); 
  } 
} 

二、Jackson支持3种使用方式:
1、Data Binding：最方便使用.
(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到一个实体类中. 
    System.out.println(user.getName()); 
    System.out.println(user.getType()); 
  } 

Model类：

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)Raw Data Binding：

/** 
  Concrete Java types that Jackson will use for simple data binding are: 
  JSON Type    Java Type 
  object     LinkedHashMap<String,Object> 
  array      ArrayList<Object> 
  string     String 
  number(no fraction) Integer, Long or BigInteger (smallest applicable) 
  number(fraction)  Double(configurable to use BigDecimal) 
  true|false   Boolean 
  null      null 
  */
  public void rawDataBinding() throws Exception{ 
    ObjectMapper mapper = new ObjectMapper(); 
    HashMap map = mapper.readValue(MODEL_BINDING,HashMap.class);//readValue到一个原始数据类型. 
    System.out.println(map.get("name")); 
    System.out.println(map.get("type")); 
  } 

(3)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到一个范型数据中. 
    Model model = modelMap.get("key2"); 
    System.out.println(model.getName()); 
    System.out.println(model.getType()); 
  } 

2、Tree Model：最灵活。

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与get作用相同,但是当找不到该节点的时候,返回missing node而不是Null. 
    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); 
      
    //创建根节点 
    ObjectNode root = mapper.createObjectNode(); 
    //创建子节点1 
    ObjectNode node1 = mapper.createObjectNode(); 
    node1.put("nodekey1",1); 
    node1.put("nodekey2",2); 
    //绑定子节点1 
    root.put("child",node1); 
    //数组节点 
    ArrayNode arrayNode = mapper.createArrayNode(); 
    arrayNode.add(node1); 
    arrayNode.add(1); 
    //绑定数组节点 
    root.put("arraynode", arrayNode); 
    //JSON读到树节点 
    JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING); 
    //绑定JSON节点 
    root.put("valuetotreenode",valueToTreeNode); 
    //JSON绑定到JSON节点对象 
    JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//绑定JSON到JSON节点对象. 
    //绑定JSON节点 
    root.put("bindJsonNode",bindJsonNode); 
    System.out.println(mapper.writeValueAsString(root)); 
  } 

3、Streaming API：最佳性能。

对于性能要求高的程序,推荐使用流API,否则使用其他方法
不管是创建JsonGenerator还是JsonParser,都是使用JsonFactory。

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(); 
       
    /*** write to file ***/
    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(); 
      
    /*** read from file ***/
    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(); 
  
  } 
  
}