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;
/**
* <b>function:</b>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 [email protected]
* @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(
"[email protected]"); 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
/**
* <b>function:</b>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":
"[email protected]"}ObjectMapper{
"address":
"china-Guangzhou",
"name":
"hoojo",
"id":1,
"birthday":null,
"email":
"[email protected]"}
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
/**
* <b>function:</b>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<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(
"[email protected]"); 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":
"[email protected]"},
"name":
"hoojo",
"account":{
"address":
"china-Guangzhou",
"name":
"hoojo",
"id":1,
"birthday":null,
"email":
"[email protected]"}}objectMapper{
"account2":{
"address":
"china-Beijin",
"name":null,
"id":0,
"birthday":null,
"email":
"[email protected]"},
"name":
"hoojo",
"account":{
"address":
"china-Guangzhou",
"name":
"hoojo",
"id":1,
"birthday":null,
"email":
"[email protected]"}}
3、 将List集合转换成json
/**
* <b>function:</b>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<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":
"[email protected]"},{
"address":
"address2",
"name":
"haha2",
"id":2,
"birthday":null,
"email":
"email2"}]ObjectMapper1###[{
"address":
"china-Guangzhou",
"name":
"hoojo",
"id":1,
"birthday":null,
"email":
"[email protected]"},{
"address":
"address2",
"name":
"haha2",
"id":2,
"birthday":null,
"email":
"email2"}]2###[{
"address":
"china-Guangzhou",
"name":
"hoojo",
"id":1,
"birthday":null,
"email":
"[email protected]"},{
"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：
hahahaha#1#address#null#email
2、 将json字符串转换成List<Map>集合
/**
* <b>function:</b>json字符串转换成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(); }}
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：
2address:address2name:haha2id:2email:email2address:addressname:hahaid:1email: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。
/**
* <b>function:</b>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：
2haha2#2#address2#null#email2haha#1#address#null#email
4、 Json字符串转换成Map集合
/**
* <b>function:</b>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<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(); }}
运行后结果如下：
3success:trueA:{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。
/**
* <b>function:</b>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<AccountBean> list =
new ArrayList<AccountBean>(); list.add(bean); list.add(bean); System.out.println(xml.writeValueAsString(list));
//Map转换xml文档 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(); }}
运行上面的方法，结果如下：
XmlMapper
<
unknown
><
address
>china-Guangzhou
</
address
><
name
>hoojo
</
name
><
id
>1
</
id
><
birthday
/><
email
>[email protected]
</
email
></
unknown
>
<
unknown
><
unknown
><
address
>china-Guangzhou
</
address
><
name
>hoojo
</
name
><
id
>1
</
id
><
birthday
/><
email
>[email protected]
</
email
></
unknown
>
<
email
><
address
>china-Guangzhou
</
address
><
name
>hoojo
</
name
><
id
>1
</
id
><
birthday
/><
email
>[email protected]
</
email
></
email
></
unknown
>
<
unknown
><
A
><
address
>china-Guangzhou
</
address
><
name
>hoojo
</
name
><
id
>1
</
id
><
birthday
/><
email
>[email protected]
</
email
></
A
>
<
B
><
address
>china-Guangzhou
</
address
><
name
>hoojo
</
name
><
id
>1
</
id
><
birthday
/><
email
>[email protected]
</
email
></
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。
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