Jackson使用說明

一、入門

Jackson中有個ObjectMapper類很是實用，用於Java對象與JSON的互換。需要使用到的jar包基本包含 jackson-databind、jackson-core、 jackson-annotations三個包。
1.JAVA對象轉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("[email protected]"); 
    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中實現。 
     * ObjectMapper有多個JSON序列化的方法，可以把JSON字符串保存File、OutputStream等不同的介質中。 
     * writeValue(File arg0, Object arg1)把arg1轉成json序列，並保存到arg0文件中。 
     * writeValue(OutputStream arg0, Object arg1)把arg1轉成json序列，並保存到arg0輸出流中。 
     * writeValueAsBytes(Object arg0)把arg0轉成json序列，並把結果輸出成字節數組。 
     * writeValueAsString(Object arg0)把arg0轉成json序列，並把結果輸出成字符串。 
     */ 
    ObjectMapper mapper = new ObjectMapper(); 
     
    //User類轉JSON 
    //輸出結果：{"name":"小民","age":20,"birthday":844099200000,"email":"[email protected]"} 
    String json = mapper.writeValueAsString(user); 
    System.out.println(json); 
     
    //Java集合轉JSON 
    //輸出結果：[{"name":"小民","age":20,"birthday":844099200000,"email":"[email protected]"}] 
    List<User> users = new ArrayList<User>(); 
    users.add(user); 
    String jsonlist = mapper.writeValueAsString(users); 
    System.out.println(jsonlist); 
  } 
} 

2.JSON轉Java類[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\":\"[email protected]\"}"; 
     
    /** 
     * ObjectMapper支持從byte[]、File、InputStream、字符串等數據的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種使用方式:
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到一個實體類中. 
    System.out.println(user.getName()); 
    System.out.println(user.getType()); 
  } 

Model類：

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到一個原始數據類型. 
    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到一個範型數據中. 
    Model model = modelMap.get("key2"); 
    System.out.println(model.getName()); 
    System.out.println(model.getType()); 
  } 

2、Tree Model：最靈活。

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與get作用相同,但是當找不到該節點的時候,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); 
     
    //創建根節點 
    ObjectNode root = mapper.createObjectNode(); 
    //創建子節點1 
    ObjectNode node1 = mapper.createObjectNode(); 
    node1.put("nodekey1",1); 
    node1.put("nodekey2",2); 
    //綁定子節點1 
    root.put("child",node1); 
    //數組節點 
    ArrayNode arrayNode = mapper.createArrayNode(); 
    arrayNode.add(node1); 
    arrayNode.add(1); 
    //綁定數組節點 
    root.put("arraynode", arrayNode); 
    //JSON讀到樹節點 
    JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING); 
    //綁定JSON節點 
    root.put("valuetotreenode",valueToTreeNode); 
    //JSON綁定到JSON節點對象 
    JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//綁定JSON到JSON節點對象. 
    //綁定JSON節點 
    root.put("bindJsonNode",bindJsonNode); 
    System.out.println(mapper.writeValueAsString(root)); 
  } 

3、Streaming API：最佳性能。
 
對於性能要求高的程序,推薦使用流API,否則使用其他方法
不管是創建JsonGenerator還是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(); 
 
  } 
 
}