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：http://www.cnblogs.com/hoojo/archive/2011/04/21/2023805.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

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
* @email [e-mail]
* @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("[e-mail]");
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();
}
}
}

3、 所需要的JavaEntity

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;
}
}

Birthday

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;
}
}

二、Java对象转换成JSON

1、 JavaBean(Entity/Model)转换成JSON

/**
* 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();
}
}

运行后结果如下：

jsonGenerator
{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}
ObjectMapper
{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}

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

/**
* 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("[e-mail]");
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();
}
}

转换后结果如下：

jsonGenerator
{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"[e-mail]"},"name":"hoojo",
"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}}
objectMapper
{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"[e-mail]"},"name":"hoojo",
"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"}}

3、 将List集合转换成json

/**
* 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();
}
}

结果如下：

jsonGenerator
[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},
{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
ObjectMapper
1###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},
{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
2###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"[e-mail]"},
{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]

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。

@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();
}
}

运行后，结果如下：

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"]}

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

@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();
}
}

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：

haha
haha#1#address#null#email

2、 将json字符串转换成List

/**
* 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();
}
}

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：

2
address:address2
name:haha2
id:2
email:email2
address:address
name:haha
id:1
email:email

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。

/**
* 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();
}
}

运行后的结果：

2
haha2#2#address2#null#email2
haha#1#address#null#email

4、 Json字符串转换成Map集合

/**
* 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();
}
}

运行后结果如下：

3
success:true
A:{address=address2, name=haha2, id=2, email=email2}
B:{address=address, name=haha, id=1, email=email}

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。

/**
* 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();
}
}

运行上面的方法，结果如下：

XmlMapper
<unknown><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>[e-mail]>unknown>
<unknown><unknown><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>[e-mail]>unknown>
<email><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>[e-mail]>email>unknown>
<unknown><A><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>[e-mail]>A>
<B><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>[e-mail]>B>unknown>

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。