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

1package com.hoo.test;
2
3import java.io.IOException;
4
5import java.io.StringWriter;
6
7import java.util.ArrayList;
8
9import java.util.HashMap;
10
11import java.util.Iterator;
12
13import java.util.LinkedHashMap;
14
15import java.util.List;
16
17import java.util.Map;
18
19import java.util.Set;
20
21import org.codehaus.jackson.JsonEncoding;
22
23import org.codehaus.jackson.JsonGenerationException;
24
25import org.codehaus.jackson.JsonGenerator;
26
27import org.codehaus.jackson.JsonParseException;
28
29import org.codehaus.jackson.map.JsonMappingException;
30
31import org.codehaus.jackson.map.ObjectMapper;
32
33import org.codehaus.jackson.node.JsonNodeFactory;
34
35import org.codehaus.jackson.xml.XmlMapper;
36
37import org.junit.After;
38
39import org.junit.Before;
40
41import org.junit.Test;
42
43import com.hoo.entity.AccountBean;
44
45/**
46
47 * <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
48
49 * jar-lib-version: jackson-all-1.6.2
50
51 * jettison-1.0.1
52
53 * @author hoojo
54
55 * @createDate 2010-11-23 下午04:54:53
56
57 * @file JacksonTest.java
58
59 * @package com.hoo.test
60
61 * @project Spring3
62
63 * @blog http://blog.csdn.net/IBM_hoojo
64
65 * @email hoojo_@126.com
66
67 * @version 1.0
68
69 */
70
71@SuppressWarnings("unchecked")
72
73public class JacksonTest {
74
75private JsonGenerator jsonGenerator = null;
76
77private ObjectMapper objectMapper = null;
78
79private AccountBean bean = null;
80
81@Before
82
83public void init() {
84
85bean = new AccountBean();
86
87bean.setAddress("china-Guangzhou");
88
89bean.setEmail("hoojo_@126.com");
90
91bean.setId(1);
92
93bean.setName("hoojo");
94
95objectMapper = new ObjectMapper();
96
97try {
98
99jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
100
101} catch (IOException e) {
102
103e.printStackTrace();
104
105}
106
107}
108
109@After
110
111public void destory() {
112
113try {
114
115if (jsonGenerator != null) {
116
117jsonGenerator.flush();
118
119}
120
121if (!jsonGenerator.isClosed()) {
122
123jsonGenerator.close();
124
125}
126
127jsonGenerator = null;
128
129objectMapper = null;
130
131bean = null;
132
133System.gc();
134
135} catch (IOException e) {
136
137e.printStackTrace();
138
139}
140
141}
142
143}

3、 所需要的JavaEntity

1package com.hoo.entity;
2
3public class AccountBean {
4
5private int id;
6
7private String name;
8
9private String email;
10
11private String address;
12
13private Birthday birthday;
14
15//getter、setter
16
17@Override
18
19public String toString() {
20
21return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;
22
23}
24
25}

Birthday

1package com.hoo.entity;
2
3public class Birthday {
4
5private String birthday;
6
7public Birthday(String birthday) {
8
9super();
10
11this.birthday = birthday;
12
13}
14
15//getter、setter
16
17public Birthday() {}
18
19@Override
20
21public String toString() {
22
23return this.birthday;
24
25}
26
27}

二、Java对象转换成****JSON

1、 JavaBean(Entity/Model)转换成JSON

1/**
2
3 * <b>function:</b>将java对象转换成json字符串
4
5 * @author hoojo
6
7 * @createDate 2010-11-23 下午06:01:10
8
9 */
10
11@Test
12
13public void writeEntityJSON() {
14
15try {
16
17System.out.println("jsonGenerator");
18
19//writeObject可以转换java对象，eg:JavaBean/Map/List/Array等
20
21jsonGenerator.writeObject(bean);
22
23System.out.println();
24
25System.out.println("ObjectMapper");
26
27//writeValue具有和writeObject相同的功能
28
29objectMapper.writeValue(System.out, bean);
30
31} catch (IOException e) {
32
33e.printStackTrace();
34
35}
36
37}

运行后结果如下：

1jsonGenerator
2
3{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}
4
5ObjectMapper
6
7{"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

1/**
2
3 * <b>function:</b>将map转换成json字符串
4
5 * @author hoojo
6
7 * @createDate 2010-11-23 下午06:05:26
8
9 */
10
11@Test
12
13public void writeMapJSON() {
14
15try {
16
17Map<String, Object> map = new HashMap<String, Object>();
18
19map.put("name", bean.getName());
20
21map.put("account", bean);
22
23bean = new AccountBean();
24
25bean.setAddress("china-Beijin");
26
27bean.setEmail("hoojo@qq.com");
28
29map.put("account2", bean);
30
31System.out.println("jsonGenerator");
32
33jsonGenerator.writeObject(map);
34
35System.out.println("");
36
37System.out.println("objectMapper");
38
39objectMapper.writeValue(System.out, map);
40
41} catch (IOException e) {
42
43e.printStackTrace();
44
45}
46
47}

转换后结果如下：

1jsonGenerator
2
3{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",
4
5"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}
6
7objectMapper
8
9{"account2":{"address":"china-Beijin","name":null,"id":0,"birthday":null,"email":"hoojo@qq.com"},"name":"hoojo",
10
11"account":{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"}}

3、 将List集合转换成json

1/**
2
3 * <b>function:</b>将list集合转换成json字符串
4
5 * @author hoojo
6
7 * @createDate 2010-11-23 下午06:05:59
8
9 */
10
11@Test
12
13public void writeListJSON() {
14
15try {
16
17List<AccountBean> list = new ArrayList<AccountBean>();
18
19list.add(bean);
20
21bean = new AccountBean();
22
23bean.setId(2);
24
25bean.setAddress("address2");
26
27bean.setEmail("email2");
28
29bean.setName("haha2");
30
31list.add(bean);
32
33System.out.println("jsonGenerator");
34
35//list转换成JSON字符串
36
37jsonGenerator.writeObject(list);
38
39System.out.println();
40
41System.out.println("ObjectMapper");
42
43//用objectMapper直接返回list转换成的JSON字符串
44
45System.out.println("1###" + objectMapper.writeValueAsString(list));
46
47System.out.print("2###");
48
49//objectMapper list转换成JSON字符串
50
51objectMapper.writeValue(System.out, list);
52
53} catch (IOException e) {
54
55e.printStackTrace();
56
57}
58
59}

结果如下：

1jsonGenerator
2
3[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
4
5{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
6
7ObjectMapper
8
91###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
10
11{"address":"address2","name":"haha2","id":2,"birthday":null,"email":"email2"}]
12
132###[{"address":"china-Guangzhou","name":"hoojo","id":1,"birthday":null,"email":"hoojo_@126.com"},
14
15{"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。

1@Test
2
3public void writeOthersJSON() {
4
5try {
6
7String[] arr = { "a", "b", "c" };
8
9System.out.println("jsonGenerator");
10
11String str = "hello world jackson!";
12
13//byte
14
15jsonGenerator.writeBinary(str.getBytes());
16
17//boolean
18
19jsonGenerator.writeBoolean(true);
20
21//null
22
23jsonGenerator.writeNull();
24
25//float
26
27jsonGenerator.writeNumber(2.2f);
28
29//char
30
31jsonGenerator.writeRaw("c");
32
33//String
34
35jsonGenerator.writeRaw(str, 5, 10);
36
37//String
38
39jsonGenerator.writeRawValue(str, 5, 5);
40
41//String
42
43jsonGenerator.writeString(str);
44
45jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));
46
47System.out.println();
48
49//Object
50
51jsonGenerator.writeStartObject();//{
52
53jsonGenerator.writeObjectFieldStart("user");//user:{
54
55jsonGenerator.writeStringField("name", "jackson");//name:jackson
56
57jsonGenerator.writeBooleanField("sex", true);//sex:true
58
59jsonGenerator.writeNumberField("age", 22);//age:22
60
61jsonGenerator.writeEndObject();//}
62
63jsonGenerator.writeArrayFieldStart("infos");//infos:[
64
65jsonGenerator.writeNumber(22);//22
66
67jsonGenerator.writeString("this is array");//this is array
68
69jsonGenerator.writeEndArray();//]
70
71jsonGenerator.writeEndObject();//}
72
73AccountBean bean = new AccountBean();
74
75bean.setAddress("address");
76
77bean.setEmail("email");
78
79bean.setId(1);
80
81bean.setName("haha");
82
83//complex Object
84
85jsonGenerator.writeStartObject();//{
86
87jsonGenerator.writeObjectField("user", bean);//user:{bean}
88
89jsonGenerator.writeObjectField("infos", arr);//infos:[array]
90
91jsonGenerator.writeEndObject();//}
92
93} catch (Exception e) {
94
95e.printStackTrace();
96
97}
98
99}

运行后，结果如下：

1jsonGenerator
2
3"aGVsbG8gd29ybGQgamFja3NvbiE=" true null 2.2c world jac  worl "hello world jackson!" "hello world jackson!"
4
5{"user":{"name":"jackson","sex":true,"age":22},"infos":[22,"this is array"]}
6
7{"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

1@Test
2
3public void readJson2Entity() {
4
5String json = "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}";
6
7try {
8
9AccountBean acc = objectMapper.readValue(json, AccountBean.class);
10
11System.out.println(acc.getName());
12
13System.out.println(acc);
14
15} catch (JsonParseException e) {
16
17e.printStackTrace();
18
19} catch (JsonMappingException e) {
20
21e.printStackTrace();
22
23} catch (IOException e) {
24
25e.printStackTrace();
26
27}
28
29}

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：

1haha
2
3haha#1#address#null#email

2、 将json字符串转换成List<Map>集合

1/**
2
3 * <b>function:</b>json字符串转换成list<map>
4
5 * @author hoojo
6
7 * @createDate 2010-11-23 下午06:12:01
8
9 */
10
11@Test
12
13public void readJson2List() {
14
15String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
16
17"{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
18
19try {
20
21List<LinkedHashMap<String, Object>> list = objectMapper.readValue(json, List.class);
22
23System.out.println(list.size());
24
25for (int i = 0; i < list.size(); i++) {
26
27Map<String, Object> map = list.get(i);
28
29Set<String> set = map.keySet();
30
31for (Iterator<String> it = set.iterator();it.hasNext();) {
32
33String key = it.next();
34
35System.out.println(key + ":" + map.get(key));
36
37}
38
39}
40
41} catch (JsonParseException e) {
42
43e.printStackTrace();
44
45} catch (JsonMappingException e) {
46
47e.printStackTrace();
48
49} catch (IOException e) {
50
51e.printStackTrace();
52
53}
54
55}

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：

12
2
3address:address2
4
5name:haha2
6
7id:2
8
9email:email2
10
11address:address
12
13name:haha
14
15id:1
16
17email: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。

1/**
2
3 * <b>function:</b>json字符串转换成Array
4
5 * @author hoojo
6
7 * @createDate 2010-11-23 下午06:14:01
8
9 */
10
11@Test
12
13public void readJson2Array() {
14
15String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
16
17"{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
18
19try {
20
21AccountBean[] arr = objectMapper.readValue(json, AccountBean[].class);
22
23System.out.println(arr.length);
24
25for (int i = 0; i < arr.length; i++) {
26
27System.out.println(arr[i]);
28
29}
30
31} catch (JsonParseException e) {
32
33e.printStackTrace();
34
35} catch (JsonMappingException e) {
36
37e.printStackTrace();
38
39} catch (IOException e) {
40
41e.printStackTrace();
42
43}
44
45}

运行后的结果：

12
2
3haha2#2#address2#null#email2
4
5haha#1#address#null#email

4、 Json字符串转换成Map集合

1/**
2
3 * <b>function:</b>json字符串转换Map集合
4
5 * @author hoojo
6
7 * @createDate Nov 27, 2010 3:00:06 PM
8
9 */
10
11@Test
12
13public void readJson2Map() {
14
15String json = "{\"success\":true,\"A\":{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"+
16
17"\"B\":{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}}";
18
19try {
20
21Map<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
22
23System.out.println(maps.size());
24
25Set<String> key = maps.keySet();
26
27Iterator<String> iter = key.iterator();
28
29while (iter.hasNext()) {
30
31String field = iter.next();
32
33System.out.println(field + ":" + maps.get(field));
34
35}
36
37} catch (JsonParseException e) {
38
39e.printStackTrace();
40
41} catch (JsonMappingException e) {
42
43e.printStackTrace();
44
45} catch (IOException e) {
46
47e.printStackTrace();
48
49}
50
51}

运行后结果如下：

13
2
3success:true
4
5A:{address=address2, name=haha2, id=2, email=email2}
6
7B:{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。

1/**
2
3 * <b>function:</b>java对象转换成xml文档
4
5 * 需要额外的jar包 stax2-api.jar
6
7 * @author hoojo
8
9 * @createDate 2010-11-23 下午06:11:21
10
11 */
12
13@Test
14
15public void writeObject2Xml() {
16
17//stax2-api-3.0.2.jar
18
19System.out.println("XmlMapper");
20
21XmlMapper xml = new XmlMapper();
22
23try {
24
25//javaBean转换成xml
26
27//xml.writeValue(System.out, bean);
28
29StringWriter sw = new StringWriter();
30
31xml.writeValue(sw, bean);
32
33System.out.println(sw.toString());
34
35//List转换成xml
36
37List<AccountBean> list = new ArrayList<AccountBean>();
38
39list.add(bean);
40
41list.add(bean);
42
43System.out.println(xml.writeValueAsString(list));
44
45//Map转换xml文档
46
47Map<String, AccountBean> map = new HashMap<String, AccountBean>();
48
49map.put("A", bean);
50
51map.put("B", bean);
52
53System.out.println(xml.writeValueAsString(map));
54
55} catch (JsonGenerationException e) {
56
57e.printStackTrace();
58
59} catch (JsonMappingException e) {
60
61e.printStackTrace();
62
63} catch (IOException e) {
64
65e.printStackTrace();
66
67}
68
69}

运行上面的方法，结果如下：

1XmlMapper
2
3<unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
4
5<unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown>
6
7<email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></email></unknown>
8
9<unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></A>
10
11<B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</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。