java转jackson_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 users = new ArrayList();

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

array ArrayList

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 modelMap = mapper.readValue(GENERIC_BINDING,new TypeReference>(){});//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();

}

}