使用Jackson來實現Java物件與JSON的相互轉換的教程

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一、入門
Jackson中有個ObjectMapper類很是實用,用於Java物件與JSON的互換。
1.JAVA物件轉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字串儲存File、OutputStream等不同的介質中。 
* writeValue(File arg0, Object arg1)把arg1轉成json序列,並儲存到arg0檔案中。 
* writeValue(OutputStream arg0, Object arg1)把arg1轉成json序列,並儲存到arg0輸出流中。 
* writeValueAsBytes(Object arg0)把arg0轉成json序列,並把結果輸出成位元組陣列。 
* writeValueAsString(Object arg0)把arg0轉成json序列,並把結果輸出成字串。 
*/ 
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支援從byte[]、File、InputStream、字串等資料的JSON反序列化。 
*/ 
ObjectMapper mapper = new ObjectMapper(); 
User user = mapper.readValue(json, User.class); 
System.out.println(user); 
} 
} 

 
二、Jackson支援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到一個原始資料型別. 
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到一個範型資料中. 
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); 
//建立根節點 
ObjectNode root = mapper.createObjectNode(); 
//建立子節點1 
ObjectNode node1 = mapper.createObjectNode(); 
node1.put("nodekey1",1); 
node1.put("nodekey2",2); 
//繫結子節點1 
root.put("child",node1); 
//陣列節點 
ArrayNode arrayNode = mapper.createArrayNode(); 
arrayNode.add(node1); 
arrayNode.add(1); 
//繫結陣列節點 
root.put("arraynode", arrayNode); 
//JSON讀到樹節點 
JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING); 
//繫結JSON節點 
root.put("valuetotreenode",valueToTreeNode); 
//JSON繫結到JSON節點物件 
JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//繫結JSON到JSON節點物件. 
//繫結JSON節點 
root.put("bindJsonNode",bindJsonNode); 
System.out.println(mapper.writeValueAsString(root)); 
} 

3、Streaming API:最佳效能。
 
對於效能要求高的程式,推薦使用流API,否則使用其他方法
不管是建立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(); 
} 
} 

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