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(); 

  } 

}