Jackson可以輕松的將java對象轉(zhuǎn)換成json對象和xml文檔，同樣也可以將json、xml轉(zhuǎn)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

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(qū)相對比較活躍，更新速度也比較快。

一、準(zhǔn)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

然后在工程中導(dǎo)入這個jar包即可開始工作

官方示例：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(zhuǎn)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;
 
/**
 * <b>function:</b>Jackson 將java對象轉(zhuǎn)換成JSON字符串，也可以將JSON字符串轉(zhuǎn)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(zhuǎn)換成JSON
1、 JavaBean(Entity/Model)轉(zhuǎn)換成JSON
/**
 * <b>function:</b>將java對象轉(zhuǎn)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(zhuǎn)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(jié)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(zhuǎn)換，二者傳遞的參數(shù)及構(gòu)造的方式不同；JsonGenerator的創(chuàng)建依賴于ObjectMapper對象。也就是說如果你要使用JsonGenerator來轉(zhuǎn)換JSON，那么你必須創(chuàng)建一個ObjectMapper。但是你用ObjectMapper來轉(zhuǎn)換JSON，則不需要JSONGenerator。
objectMapper的writeValue方法可以將一個Java對象轉(zhuǎn)換成JSON。這個方法的參數(shù)一，需要提供一個輸出流，轉(zhuǎn)換后可以通過這個流來輸出轉(zhuǎn)換后的內(nèi)容。或是提供一個File，將轉(zhuǎn)換后的內(nèi)容寫入到File中。當(dāng)然，這個參數(shù)也可以接收一個JSONGenerator，然后通過JSONGenerator來輸出轉(zhuǎn)換后的信息。第二個參數(shù)是將要被轉(zhuǎn)換的Java對象。如果用三個參數(shù)的方法，那么是一個Config。這個config可以提供一些轉(zhuǎn)換時的規(guī)則，過指定的Java對象的某些屬性進行過濾或轉(zhuǎn)換等。
2、 將Map集合轉(zhuǎn)換成Json字符串
/**
 * <b>function:</b>將map轉(zhuǎn)換成json字符串
 * @author hoojo
 * @createDate 2010-11-23 下午06:05:26
 */
@Test
public void writeMapJSON() {
    try {
        Map<String, Object> map = new HashMap<String, Object>();
        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(zhuǎn)換后結(jié)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(zhuǎn)換成json
/**
 * <b>function:</b>將list集合轉(zhuǎn)換成json字符串
 * @author hoojo
 * @createDate 2010-11-23 下午06:05:59
 */
@Test
public void writeListJSON() {
    try {
        List<AccountBean> list = new ArrayList<AccountBean>();
        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(zhuǎn)換成JSON字符串
        jsonGenerator.writeObject(list);
        System.out.println();
        System.out.println("ObjectMapper");
        //用objectMapper直接返回list轉(zhuǎn)換成的JSON字符串
        System.out.println("1###" + objectMapper.writeValueAsString(list));
        System.out.print("2###");
        //objectMapper list轉(zhuǎn)換成JSON字符串
        objectMapper.writeValue(System.out, list);
    } catch (IOException e) {
        e.printStackTrace();
    }
}
結(jié)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(zhuǎn)換，轉(zhuǎn)換的JSON和上面的結(jié)果是一樣的，這里就不再轉(zhuǎn)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(zhuǎn)換；如果你使用這些類型轉(zhuǎn)換JSON的話，那么你即使沒有JavaBean(Entity)也可以完成復(fù)雜的Java類型的JSON轉(zhuǎn)換。下面用到這些類型構(gòu)建一個復(fù)雜的Java對象，并完成JSON轉(zhuǎn)換。
@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();
    }
}
運行后，結(jié)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(gòu)造的json字符串和輸出的結(jié)果是一致的吧。關(guān)鍵看懂用JSONGenerator提供的方法，完成一個Object的構(gòu)建。
三、JSON轉(zhuǎn)換成Java對象
1、 將json字符串轉(zhuǎn)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(shù)。第一個參數(shù)就是解析的JSON字符串，第二個參數(shù)是即將將這個JSON解析吃什么Java對象，Java對象的類型。當(dāng)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(dāng)然使用的方法和當(dāng)前使用的方法大同小異。運行后，結(jié)果如下：
haha
haha#1#address#null#email
2、 將json字符串轉(zhuǎn)換成List<Map>集合
/**
 * <b>function:</b>json字符串轉(zhuǎn)換成list<map>
 * @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<LinkedHashMap<String, Object>> list = objectMapper.readValue(json, List.class);
        System.out.println(list.size());
        for (int i = 0; i < list.size(); i++) {
            Map<String, Object> map = list.get(i);
            Set<String> set = map.keySet();
            for (Iterator<String> 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(zhuǎn)換成List，然后List中存放AccountBean，但結(jié)果失敗了。但是支持Map集合。因為你轉(zhuǎn)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(zhuǎn)換成Map結(jié)合，運行后結(jié)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(zhuǎn)換成Array數(shù)組，由于上面的泛型轉(zhuǎn)換不能識別到集合中的對象類型。所有這里用對象數(shù)組，可以解決這個問題。只不過它不再是集合，而是一個數(shù)組。當(dāng)然這個不重要，你可以用Arrays.asList將其轉(zhuǎn)換成List即可。
/**
 * <b>function:</b>json字符串轉(zhuǎn)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(jié)果：
2
haha2#2#address2#null#email2
haha#1#address#null#email
4、 Json字符串轉(zhuǎn)換成Map集合
/**
 * <b>function:</b>json字符串轉(zhuǎn)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<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
        System.out.println(maps.size());
        Set<String> key = maps.keySet();
        Iterator<String> 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(jié)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(zhuǎn)換，轉(zhuǎn)換后的結(jié)果要比json-lib更直觀，不過它依賴于stax2-api.jar這個jar包。
/**
 * <b>function:</b>java對象轉(zhuǎn)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(zhuǎn)換成xml
        //xml.writeValue(System.out, bean);
        StringWriter sw = new StringWriter();
        xml.writeValue(sw, bean);
        System.out.println(sw.toString());
        //List轉(zhuǎn)換成xml
        List<AccountBean> list = new ArrayList<AccountBean>();
        list.add(bean);
        list.add(bean);
        System.out.println(xml.writeValueAsString(list));
        
        //Map轉(zhuǎn)換xml文檔
        Map<String, AccountBean> map = new HashMap<String, AccountBean>();
        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(jié)果如下：
XmlMapper
<unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
<unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
<email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></email></unknown>
<unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></A>
<B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></B></unknown>
看結(jié)果，根節(jié)點都是unknown 這個問題還沒有解決，由于根節(jié)點沒有轉(zhuǎn)換出來，所有導(dǎo)致解析xml到Java對象，也無法完成。