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 hoojo_@126.com * @version 1.0 */ @SuppressWarnings("unchecked") public class JacksonTest { private JsonGenerator jsonGenerator = null; private ObjectMapper bjectMapper = 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"); bjectMapper = 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; bjectMapper = 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":"hoojo_@126.com"}

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

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("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(); } }

转换后结果如下：
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、 将List集合转换成json

[java] view plaincopyprint?

1. /**
2. * function:将list集合转换成json字符串
3. * @author hoojo
4. * @createDate 2010-11-23 下午06:05:59
5. */
6. @Test
7. public void writeListJSON() {
8. try {
9. List list = new ArrayList();
10. list.add(bean);
11. bean = new AccountBean();
12. bean.setId(2);
13. bean.setAddress("address2");
14. bean.setEmail("email2");
15. bean.setName("haha2");
16. list.add(bean);
17. System.out.println("jsonGenerator");
18. //list转换成JSON字符串
19. jsonGenerator.writeObject(list);
20. System.out.println();
21. System.out.println("ObjectMapper");
22. //用objectMapper直接返回list转换成的JSON字符串
23. System.out.println("1###" + objectMapper.writeValueAsString(list));
24. System.out.print("2###");
25. //objectMapper list转换成JSON字符串
26. objectMapper.writeValue(System.out, list);
27. } catch (IOException e) {
28. e.printStackTrace();
29. }
30. }

/** * 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":"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。

[java] view plaincopyprint?

1. @Test
2. public void writeOthersJSON() {
3. try {
4. String[] arr = { "a", "b", "c" };
5. System.out.println("jsonGenerator");
6. String str = "hello world jackson!";
7. //byte
8. jsonGenerator.writeBinary(str.getBytes());
9. //boolean
10. jsonGenerator.writeBoolean(true);
11. //null
12. jsonGenerator.writeNull();
13. //float
14. jsonGenerator.writeNumber(2.2f);
15. //char
16. jsonGenerator.writeRaw("c");
17. //String
18. jsonGenerator.writeRaw(str, 5, 10);
19. //String
20. jsonGenerator.writeRawValue(str, 5, 5);
21. //String
22. jsonGenerator.writeString(str);
23. jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));
24. System.out.println();
25. //Object
26. jsonGenerator.writeStartObject();//{
27. jsonGenerator.writeObjectFieldStart("user");//user:{
28. jsonGenerator.writeStringField("name", "jackson");//name:jackson
29. jsonGenerator.writeBooleanField("sex", true);//sex:true
30. jsonGenerator.writeNumberField("age", 22);//age:22
31. jsonGenerator.writeEndObject();//}
32. jsonGenerator.writeArrayFieldStart("infos");//infos:[
33. jsonGenerator.writeNumber(22);//22
34. jsonGenerator.writeString("this is array");//this is array
35. jsonGenerator.writeEndArray();//]
36. jsonGenerator.writeEndObject();//}
37. AccountBean bean = new AccountBean();
38. bean.setAddress("address");
39. bean.setEmail("email");
40. bean.setId(1);
41. bean.setName("haha");
42. //complex Object
43. jsonGenerator.writeStartObject();//{
44. jsonGenerator.writeObjectField("user", bean);//user:{bean}
45. jsonGenerator.writeObjectField("infos", arr);//infos:[array]
46. jsonGenerator.writeEndObject();//}
47. } catch (Exception e) {
48. e.printStackTrace();
49. }
50. }

@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

[java] view plaincopyprint?

1. @Test
2. public void readJson2Entity() {
3. String json = "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}";
4. try {
5. AccountBean acc = objectMapper.readValue(json, AccountBean.class);
6. System.out.println(acc.getName());
7. System.out.println(acc);
8. } catch (JsonParseException e) {
9. e.printStackTrace();
10. } catch (JsonMappingException e) {
11. e.printStackTrace();
12. } catch (IOException e) {
13. e.printStackTrace();
14. }
15. }

@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

[java] view plaincopyprint?

1. /**
2. * function:json字符串转换成list
3. * @author hoojo
4. * @createDate 2010-11-23 下午06:12:01
5. */
6. @Test
7. public void readJson2List() {
8. String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
9. "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
10. try {
11. List> list = objectMapper.readValue(json, List.class);
12. System.out.println(list.size());
13. for (int i = 0; i < list.size(); i++) {
14. Map map = list.get(i);
15. Set set = map.keySet();
16. for (Iterator it = set.iterator();it.hasNext();) {
17. String key = it.next();
18. System.out.println(key + ":" + map.get(key));
19. }
20. }
21. } catch (JsonParseException e) {
22. e.printStackTrace();
23. } catch (JsonMappingException e) {
24. e.printStackTrace();
25. } catch (IOException e) {
26. e.printStackTrace();
27. }
28. }

/** * function:json字符串转换成list