Jackson可以輕松的將Java對象轉換成json對象和xml文檔，同樣也可以將json、xml轉換成Java對象。
前面有介紹過json-lib這個框架，在線博文：[http://www.cnblogs.com/hoojo/archive/2011/04/21/2023805.html](http://www.cnblogs.com/hoojo/archive/2011/04/21/2023805.html)
相比json-lib框架，Jackson所依賴的jar包較少，簡單易用并且性能也要相對高些。而且Jackson社區相對比較活躍，更新速度也比較快。
### 一、準備工作
1、 下載依賴庫jar包
Jackson的jar all下載地址：[http://jackson.codehaus.org/1.7.6/jackson-all-1.7.6.jar](http://jackson.codehaus.org/1.7.6/jackson-all-1.7.6.jar)
然后在工程中導入這個jar包即可開始工作
官方示例：[http://wiki.fasterxml.com/JacksonInFiveMinutes](http://wiki.fasterxml.com/JacksonInFiveMinutes)
因為下面的程序是用junit測試用例運行的，所以還得添加junit的jar包。版本是junit-4.2.8
如果你需要轉換xml，那么還需要stax2-api.jar
2、 測試類基本代碼如下
~~~
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 將java對象轉換成JSON字符串，也可以將JSON字符串轉換成java對象
* 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 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();
}
}
}
~~~
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對象轉換成JSON
1、 JavaBean(Entity/Model)轉換成JSON
~~~
/**
* function:將java對象轉換成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可以轉換java對象，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();
}
}
~~~
運行后結果如下：
~~~
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"}
~~~
上面分別利用JsonGenerator的writeObject方法和ObjectMapper的writeValue方法完成對Java對象的轉換，二者傳遞的參數及構造的方式不同；JsonGenerator的創建依賴于ObjectMapper對象。也就是說如果你要使用JsonGenerator來轉換JSON，那么你必須創建一個ObjectMapper。但是你用ObjectMapper來轉換JSON，則不需要JSONGenerator。
objectMapper的writeValue方法可以將一個Java對象轉換成JSON。這個方法的參數一，需要提供一個輸出流，轉換后可以通過這個流來輸出轉換后的內容。或是提供一個File，將轉換后的內容寫入到File中。當然，這個參數也可以接收一個JSONGenerator，然后通過JSONGenerator來輸出轉換后的信息。第二個參數是將要被轉換的Java對象。如果用三個參數的方法，那么是一個Config。這個config可以提供一些轉換時的規則，過指定的Java對象的某些屬性進行過濾或轉換等。
2、 將Map集合轉換成Json字符串
~~~
/**
* function:將map轉換成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();
}
}
~~~
轉換后結果如下：
~~~
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"}}
~~~
3、 將List集合轉換成json
~~~
/**
* function:將list集合轉換成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轉換成JSON字符串
jsonGenerator.writeObject(list);
System.out.println();
System.out.println("ObjectMapper");
//用objectMapper直接返回list轉換成的JSON字符串
System.out.println("1###" + objectMapper.writeValueAsString(list));
System.out.print("2###");
//objectMapper list轉換成JSON字符串
objectMapper.writeValue(System.out, list);
} catch (IOException e) {
e.printStackTrace();
}
}
~~~
結果如下：
~~~
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"}]
~~~
外面就是多了個[]中括號；同樣Array也可以轉換，轉換的JSON和上面的結果是一樣的，這里就不再轉換了。~.~
4、下面來看看jackson提供的一些類型，用這些類型完成json轉換；如果你使用這些類型轉換JSON的話，那么你即使沒有JavaBean(Entity)也可以完成復雜的Java類型的JSON轉換。下面用到這些類型構建一個復雜的Java對象，并完成JSON轉換。
~~~
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();
}
}
~~~
運行后，結果如下：
~~~
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"]}
~~~
怎么樣？構造的json字符串和輸出的結果是一致的吧。關鍵看懂用JSONGenerator提供的方法，完成一個Object的構建。
### 三、JSON轉換成Java對象
1、 將json字符串轉換成JavaBean對象
~~~
@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();
}
}
~~~
很簡單，用到了ObjectMapper這個對象的readValue這個方法，這個方法需要提供2個參數。第一個參數就是解析的JSON字符串，第二個參數是即將將這個JSON解析吃什么Java對象，Java對象的類型。當然，還有其他相同簽名方法，如果你有興趣可以一一嘗試使用方法，當然使用的方法和當前使用的方法大同小異。運行后，結果如下：
~~~
haha
haha#1#address#null#email
~~~
2、 將json字符串轉換成List集合
~~~
/**
* function:json字符串轉換成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();
}
}
~~~
嘗試過將上面的JSON轉換成List，然后List中存放AccountBean，但結果失敗了。但是支持Map集合。因為你轉成List.class，但是不知道List存放何種類型。只好默然Map類型。因為所有的對象都可以轉換成Map結合，運行后結果如下：
~~~
2
address:address2
name:haha2
id:2
email:email2
address:address
name:haha
id:1
email:email
~~~
3、 Json字符串轉換成Array數組，由于上面的泛型轉換不能識別到集合中的對象類型。所有這里用對象數組，可以解決這個問題。只不過它不再是集合，而是一個數組。當然這個不重要，你可以用Arrays.asList將其轉換成List即可。
~~~
/**
* function:json字符串轉換成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();
}
}
~~~
運行后的結果：
~~~
2
haha2#2#address2#null#email2
haha#1#address#null#email
~~~
4、 Json字符串轉換成Map集合
~~~
/**
* function:json字符串轉換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();
}
}
~~~
運行后結果如下：
~~~
3
success:true
A:{address=address2, name=haha2, id=2, email=email2}
B:{address=address, name=haha, id=1, email=email}
~~~
### 四、Jackson對XML的支持
Jackson也可以完成java對象到xml的轉換，轉換后的結果要比json-lib更直觀，不過它依賴于stax2-api.jar這個jar包。
~~~
/**
* function:java對象轉換成xml文檔
* 需要額外的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轉換成xml
//xml.writeValue(System.out, bean);
StringWriter sw = new StringWriter();
xml.writeValue(sw, bean);
System.out.println(sw.toString());
//List轉換成xml
List list = new ArrayList();
list.add(bean);
list.add(bean);
System.out.println(xml.writeValueAsString(list));
//Map轉換xml文檔
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();
}
}
~~~
運行上面的方法，結果如下：
~~~
XmlMapper
<unknown><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>unknown>
<unknown><unknown><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>unknown>
<email><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>email>unknown>
<unknown><A><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>A>
<B><address>china-Guangzhouaddress><name>hoojoname><id>1id><birthday/><email>hoojo_@126.comemail>B>unknown>
~~~
看結果，根節點都是unknown 這個問題還沒有解決，由于根節點沒有轉換出來，所有導致解析xml到Java對象，也無法完成。
