Jackson 框架，轻易转换JSON

Jackson可以轻松的将Java对象转换成json对象和xml文档，同样也可以将json、xml转换成Java对象。

前面有介绍过json-lib这个框架，在线博文：/article/4791846.html

相比json-lib框架，Jackson所依赖的jar包较少，简单易用并且性能也要相对高些。而且Jackson社区相对比较活跃，更新速度也比较快。

一、准备工作

1、 下载依赖库jar包

Jackson的jar all下载地址：http://jackson.codehaus.org/1.7.6/jackson-all-1.7.6.jar

然后在工程中导入这个jar包即可开始工作

官方示例：http://wiki.fasterxml.com/JacksonInFiveMinutes

因为下面的程序是用junit测试用例运行的，所以还得添加junit的jar包。版本是junit-4.2.8

如果你需要转换xml，那么还需要stax2-api.jar

2、 测试类基本代码如下

[code] [code]package com.hoo.test;

import java.io.IOException;

import java.io.StringWriter;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.Iterator;

import java.util.LinkedHashMap;

import java.util.List;

import java.util.Map;

import java.util.Set;

import org.codehaus.jackson.JsonEncoding;

import org.codehaus.jackson.JsonGenerationException;

import org.codehaus.jackson.JsonGenerator;

import org.codehaus.jackson.JsonParseException;

import org.codehaus.jackson.map.JsonMappingException;

import org.codehaus.jackson.map.ObjectMapper;

import org.codehaus.jackson.node.JsonNodeFactory;

import org.codehaus.jackson.xml.XmlMapper;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

import com.hoo.entity.AccountBean;

/**

* function:Jackson 将java对象转换成JSON字符串，也可以将JSON字符串转换成java对象

* jar-lib-version: jackson-all-1.6.2

* jettison-1.0.1

* @author hoojo

* @createDate 2010-11-23 下午04:54:53

* @file JacksonTest.java

* @package com.hoo.test

* @project Spring3

* @blog http://blog.csdn.net/IBM_hoojo[/code] 
* @email hoojo_@126.com


* @version 1.0


*/


@SuppressWarnings("unchecked")


public class JacksonTest {


private JsonGenerator jsonGenerator = null;


private ObjectMapper objectMapper = null;


private AccountBean bean = null;




@Before


public void init() {


bean = new AccountBean();


bean.setAddress("china-Guangzhou");


bean.setEmail("hoojo_@126.com");


bean.setId(1);


bean.setName("hoojo");




objectMapper = new ObjectMapper();


try {


jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);


} catch (IOException e) {


e.printStackTrace();


}


}




@After


public void destory() {


try {


if (jsonGenerator != null) {


jsonGenerator.flush();


}


if (!jsonGenerator.isClosed()) {


jsonGenerator.close();


}


jsonGenerator = null;


objectMapper = null;


bean = null;


System.gc();


} catch (IOException e) {


e.printStackTrace();


}


}


}

[/code]
[/code]

3、 所需要的JavaEntity

[code]
[code]package com.hoo.entity;


 


public class AccountBean {


private int id;


private String name;


private String email;


private String address;


private Birthday birthday;




//getter、setter




@Override


public String toString() {


return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;


}


}

[/code]
[/code]

Birthday

[code]
[code]package com.hoo.entity;


 


public class Birthday {


private String birthday;




public Birthday(String birthday) {


super();


this.birthday = birthday;


}


 


//getter、setter


 


public Birthday() {}




@Override


public String toString() {


return this.birthday;


}


}

[/code]
[/code]

二、Java对象转换成JSON

1、 JavaBean(Entity/Model)转换成JSON

[code]
[code]/**


* function:将java对象转换成json字符串


* @author hoojo


* @createDate 2010-11-23 下午06:01:10


*/


@Test


public void writeEntityJSON() {




try {


System.out.println("jsonGenerator");


//writeObject可以转换java对象，eg:JavaBean/Map/List/Array等


jsonGenerator.writeObject(bean);


System.out.println();




System.out.println("ObjectMapper");


//writeValue具有和writeObject相同的功能


objectMapper.writeValue(System.out, bean);


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

运行后结果如下：

[code]
[code]jsonGenerator


{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}


ObjectMapper


{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}

[/code]
[/code]

上面分别利用JsonGenerator的writeObject方法和ObjectMapper的writeValue方法完成对Java对象的转换，二者传递的参数及构造的方式不同；JsonGenerator的创建依赖于ObjectMapper对象。也就是说如果你要使用JsonGenerator来转换JSON，那么你必须创建一个ObjectMapper。但是你用ObjectMapper来转换JSON，则不需要JSONGenerator。

objectMapper的writeValue方法可以将一个Java对象转换成JSON。这个方法的参数一，需要提供一个输出流，转换后可以通过这个流来输出转换后的内容。或是提供一个File，将转换后的内容写入到File中。当然，这个参数也可以接收一个JSONGenerator，然后通过JSONGenerator来输出转换后的信息。第二个参数是将要被转换的Java对象。如果用三个参数的方法，那么是一个Config。这个config可以提供一些转换时的规则，过指定的Java对象的某些属性进行过滤或转换等。

2、 将Map集合转换成Json字符串

[code]
[code]/**


* function:将map转换成json字符串


* @author hoojo


* @createDate 2010-11-23 下午06:05:26


*/


@Test


public void writeMapJSON() {


try {


Map map = new HashMap();


map.put("name", bean.getName());


map.put("account", bean);


bean = new AccountBean();


bean.setAddress("china-Beijin");


bean.setEmail("hoojo@qq.com");


map.put("account2", bean);




System.out.println("jsonGenerator");


jsonGenerator.writeObject(map);


System.out.println("");




System.out.println("objectMapper");


objectMapper.writeValue(System.out, map);


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

转换后结果如下：

[code]
[code]jsonGenerator


{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",


"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}


objectMapper


{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",


"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}

[/code]
[/code]

3、 将List集合转换成json

[code]
[code]/**


* function:将list集合转换成json字符串


* @author hoojo


* @createDate 2010-11-23 下午06:05:59


*/


@Test


public void writeListJSON() {


try {


List list = new ArrayList();


list.add(bean);




bean = new AccountBean();


bean.setId(2);


bean.setAddress("address2");


bean.setEmail("email2");


bean.setName("haha2");


list.add(bean);




System.out.println("jsonGenerator");


//list转换成JSON字符串


jsonGenerator.writeObject(list);


System.out.println();


System.out.println("ObjectMapper");


//用objectMapper直接返回list转换成的JSON字符串


System.out.println("1###" + objectMapper.writeValueAsString(list));


System.out.print("2###");


//objectMapper list转换成JSON字符串


objectMapper.writeValue(System.out, list);


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

结果如下：

[code]
[code]jsonGenerator


[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},


{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]


ObjectMapper


1###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},


{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]


2###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},


{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

[/code]
[/code]

外面就是多了个[]中括号；同样Array也可以转换，转换的JSON和上面的结果是一样的，这里就不再转换了。~.~

4、下面来看看jackson提供的一些类型，用这些类型完成json转换；如果你使用这些类型转换JSON的话，那么你即使没有JavaBean(Entity)也可以完成复杂的Java类型的JSON转换。下面用到这些类型构建一个复杂的Java对象，并完成JSON转换。

[code]
[code]@Test


public void writeOthersJSON() {


try {


String[] arr = { "a", "b", "c" };


System.out.println("jsonGenerator");


String str = "hello world jackson!";


//byte


jsonGenerator.writeBinary(str.getBytes());


//boolean


jsonGenerator.writeBoolean(true);


//null


jsonGenerator.writeNull();


//float


jsonGenerator.writeNumber(2.2f);


//char


jsonGenerator.writeRaw("c");


//String


jsonGenerator.writeRaw(str, 5, 10);


//String


jsonGenerator.writeRawValue(str, 5, 5);


//String


jsonGenerator.writeString(str);


jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));


System.out.println();




//Object


jsonGenerator.writeStartObject();//{


jsonGenerator.writeObjectFieldStart("user");//user:{


jsonGenerator.writeStringField("name", "jackson");//name:jackson


jsonGenerator.writeBooleanField("sex", true);//sex:true


jsonGenerator.writeNumberField("age", 22);//age:22


jsonGenerator.writeEndObject();//}




jsonGenerator.writeArrayFieldStart("infos");//infos:[


jsonGenerator.writeNumber(22);//22


jsonGenerator.writeString("this is array");//this is array


jsonGenerator.writeEndArray();//]




jsonGenerator.writeEndObject();//}






AccountBean bean = new AccountBean();


bean.setAddress("address");


bean.setEmail("email");


bean.setId(1);


bean.setName("haha");


//complex Object


jsonGenerator.writeStartObject();//{


jsonGenerator.writeObjectField("user", bean);//user:{bean}


jsonGenerator.writeObjectField("infos", arr);//infos:[array]


jsonGenerator.writeEndObject();//}




} catch (Exception e) {


e.printStackTrace();


}


}

[/code]
[/code]

运行后，结果如下：

[code]
[code]jsonGenerator


"aGVsbG8gd29ybGQgamFja3NvbiE=" true null 2.2c world jac  worl "hello world jackson!" "hello world jackson!"


{"user":{"name":"jackson","sex":true,"age":22},"infos":[22,"this is array"]}


{"user":{"address":"address","name":"haha","id":1,"birthday":null,"email":"email"},"infos":["a","b","c"]}

[/code]
[/code]

怎么样？构造的json字符串和输出的结果是一致的吧。关键看懂用JSONGenerator提供的方法，完成一个Object的构建。

三、JSON转换成Java对象

1、 将json字符串转换成JavaBean对象

[code]
[code]@Test


public void readJson2Entity() {


String json = "{/"address/":/"address/",/"name/":/"haha/",/"id/":1,/"email/":/"email/"}";


try {


AccountBean acc = objectMapper.readValue(json, AccountBean.class);


System.out.println(acc.getName());


System.out.println(acc);


} catch (JsonParseException e) {


e.printStackTrace();


} catch (JsonMappingException e) {


e.printStackTrace();


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

很简单，用到了ObjectMapper这个对象的readValue这个方法，这个方法需要提供2个参数。第一个参数就是解析的JSON字符串，第二个参数是即将将这个JSON解析吃什么Java对象，Java对象的类型。当然，还有其他相同签名方法，如果你有兴趣可以一一尝试使用方法，当然使用的方法和当前使用的方法大同小异。运行后，结果如下：

[code]
[code]haha


haha#1#address#null#email

[/code]
[/code]

2、 将json字符串转换成List集合

[code]
[code]/**


* function:json字符串转换成list


* @author hoojo


* @createDate 2010-11-23 下午06:12:01


*/


@Test


public void readJson2List() {


String json = "[{/"address/": /"address2/",/"name/":/"haha2/",/"id/":2,/"email/":/"email2/"},"+


"{/"address/":/"address/",/"name/":/"haha/",/"id/":1,/"email/":/"email/"}]";


try {


List> list = objectMapper.readValue(json, List.class);


System.out.println(list.size());


for (int i = 0; i < list.size(); i++) {


Map map = list.get(i);


Set set = map.keySet();


for (Iterator it = set.iterator();it.hasNext();) {


String key = it.next();


System.out.println(key + ":" + map.get(key));


}


}


} catch (JsonParseException e) {


e.printStackTrace();


} catch (JsonMappingException e) {


e.printStackTrace();


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

尝试过将上面的JSON转换成List，然后List中存放AccountBean，但结果失败了。但是支持Map集合。因为你转成List.class，但是不知道List存放何种类型。只好默然Map类型。因为所有的对象都可以转换成Map结合，运行后结果如下：

[code]
[code]2


address:address2


name:haha2


id:2


email:email2


address:address


name:haha


id:1


email:email

[/code]
[/code]

3、 Json字符串转换成Array数组，由于上面的泛型转换不能识别到集合中的对象类型。所有这里用对象数组，可以解决这个问题。只不过它不再是集合，而是一个数组。当然这个不重要，你可以用Arrays.asList将其转换成List即可。

[code]
[code]/**


* function:json字符串转换成Array


* @author hoojo


* @createDate 2010-11-23 下午06:14:01


*/


@Test


public void readJson2Array() {


String json = "[{/"address/": /"address2/",/"name/":/"haha2/",/"id/":2,/"email/":/"email2/"},"+


"{/"address/":/"address/",/"name/":/"haha/",/"id/":1,/"email/":/"email/"}]";


try {


AccountBean[] arr = objectMapper.readValue(json, AccountBean[].class);


System.out.println(arr.length);


for (int i = 0; i < arr.length; i++) {


System.out.println(arr[i]);


}




} catch (JsonParseException e) {


e.printStackTrace();


} catch (JsonMappingException e) {


e.printStackTrace();


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

运行后的结果：

[code]
[code]2


haha2#2#address2#null#email2


haha#1#address#null#email

[/code]
[/code]

4、 Json字符串转换成Map集合

[code]
[code]/**


* function:json字符串转换Map集合


* @author hoojo


* @createDate Nov 27, 2010 3:00:06 PM


*/


@Test


public void readJson2Map() {


String json = "{/"success/":true,/"A/":{/"address/": /"address2/",/"name/":/"haha2/",/"id/":2,/"email/":/"email2/"},"+


"/"B/":{/"address/":/"address/",/"name/":/"haha/",/"id/":1,/"email/":/"email/"}}";


try {


Map> maps = objectMapper.readValue(json, Map.class);


System.out.println(maps.size());


Set key = maps.keySet();


Iterator iter = key.iterator();


while (iter.hasNext()) {


String field = iter.next();


System.out.println(field + ":" + maps.get(field));


}


} catch (JsonParseException e) {


e.printStackTrace();


} catch (JsonMappingException e) {


e.printStackTrace();


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

运行后结果如下：

[code]
[code]3


success:true


A:{address=address2, name=haha2, id=2, email=email2}


B:{address=address, name=haha, id=1, email=email}

[/code]
[/code]

四、Jackson对XML的支持

Jackson也可以完成java对象到xml的转换，转换后的结果要比json-lib更直观，不过它依赖于stax2-api.jar这个jar包。

[code]
[code]/**


* function:java对象转换成xml文档


* 需要额外的jar包 stax2-api.jar


* @author hoojo


* @createDate 2010-11-23 下午06:11:21


*/


@Test


public void writeObject2Xml() {


//stax2-api-3.0.2.jar


System.out.println("XmlMapper");


XmlMapper xml = new XmlMapper();




try {


//javaBean转换成xml


//xml.writeValue(System.out, bean);


StringWriter sw = new StringWriter();


xml.writeValue(sw, bean);


System.out.println(sw.toString());


//List转换成xml


List list = new ArrayList();


list.add(bean);


list.add(bean);


System.out.println(xml.writeValueAsString(list));




//Map转换xml文档


Map map = new HashMap();


map.put("A", bean);


map.put("B", bean);


System.out.println(xml.writeValueAsString(map));


} catch (JsonGenerationException e) {


e.printStackTrace();


} catch (JsonMappingException e) {


e.printStackTrace();


} catch (IOException e) {


e.printStackTrace();


}


}

[/code]
[/code]

运行上面的方法，结果如下：

[code]
[code]XmlMapper


china-Guangzhou
hoojo1hoojo_@126.com


china-Guangzhou
hoojo1hoojo_@126.com


china-Guangzhou
hoojo1hoojo_@126.com


china-Guangzhou
hoojo1hoojo_@126.com


china-Guangzhou
hoojo1hoojo_@126.com

[/code]
[/code]

看结果，根节点都是unknown 这个问题还没有解决，由于根节点没有转换出来，所有导致解析xml到Java对象，也无法完成。