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]

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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]

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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：

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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：

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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：

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/**   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：

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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。

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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。

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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();      }    }