Jackson转换JSON
原文：http://www.cnblogs.com/hoojo/archive/2011/04/22/2024628.html#top
Jackson框架
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。
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。
建议：JSON的转换使用Jackson框架；xml的转换使用xStream框架。
1. 准备工作
1.1. 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
1.2. 所需要的JavaEntity
Birthday：
public class Birthday {
private String birthday;
public Birthday() {
}
public Birthday(String birthday) {
this.birthday = birthday;
}
// getter、setter
@Override
public String toString() {
return this.birthday;
}
}AccountBean：
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;
}
}2. Java对象转换成JSON
2.1. JavaBean(Entity/Model)转换成JSON
import java.io.IOException;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.ObjectMapper;
public class WriteEntityJSON {
public static void main(String[] args) {
AccountBean bean = new AccountBean();
bean.setAddress("china-Guangzhou");
bean.setEmail("[email protected]");
bean.setId(1);
bean.setName("hoojo");
try {
ObjectMapper objectMapper = new ObjectMapper();
JsonGenerator jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
System.out.println("jsonGenerator");
// writeObject可以转换java对象，eg:JavaBean/Map/List/Array等
jsonGenerator.writeObject(bean);
System.out.println();
// writeValue具有和writeObject相同的功能
System.out.println("ObjectMapper");
objectMapper.writeValue(System.out, bean);
} catch (IOException e) {
e.printStackTrace();
}
}
}运行后结果如下：
jsonGenerator
{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"hoojo","id":1}
ObjectMapper
{"email":"[email protected]","birthday":null,"address":"china-Guangzhou","name":"hoojo","id":1}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。
2.2. 将Map集合转换成Json字符串
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.ObjectMapper;
public class WriteMapJSON {
public static void main(String[] args) {
AccountBean bean1 = new AccountBean();
bean1.setAddress("china-Suzhou");
bean1.setEmail("[email protected]");
bean1.setId(1);
bean1.setName("zhangsan");
AccountBean bean2 = new AccountBean();
bean2.setAddress("china-Shanghai");
bean2.setEmail("[email protected]");
bean2.setId(2);
bean2.setName("lisi");
Map map = new HashMap();
map.put("name1", bean1.getName());
map.put("account1", bean1);
map.put("name2", bean2.getName());
map.put("account2", bean2);
try {
ObjectMapper objectMapper = new ObjectMapper();
JsonGenerator jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
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":{"email":"[email protected]","birthday":null,"address":"china-Shanghai","name":"lisi","id":2},"account1":{"email":"[email protected]","birthday":null,"address":"china-Suzhou","name":"zhangsan","id":1},"name1":"zhangsan","name2":"lisi"}
ObjectMapper
{"account2":{"email":"[email protected]","birthday":null,"address":"china-Shanghai","name":"lisi","id":2},"account1":{"email":"[email protected]","birthday":null,"address":"china-Suzhou","name":"zhangsan","id":1},"name1":"zhangsan","name2":"lisi"}2.3. 将List集合转换成json
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.ObjectMapper;
public class WriteListJSON {
public static void main(String[] args) {
AccountBean bean1 = new AccountBean();
bean1.setAddress("china-Suzhou");
bean1.setEmail("[email protected]");
bean1.setId(1);
bean1.setName("zhangsan");
AccountBean bean2 = new AccountBean();
bean2.setAddress("china-Shanghai");
bean2.setEmail("[email protected]");
bean2.setId(2);
bean2.setName("lisi");
List list = new ArrayList();
list.add(bean1);
list.add(bean2);
try {
ObjectMapper objectMapper = new ObjectMapper();
JsonGenerator jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
System.out.println("jsonGenerator");
jsonGenerator.writeObject(list);
System.out.println();
System.out.println("ObjectMapper");
objectMapper.writeValue(System.out, list);
} catch (IOException e) {
e.printStackTrace();
}
}
} 转换后结果如下：
jsonGenerator
[{"email":"[email protected]","birthday":null,"address":"china-Suzhou","name":"zhangsan","id":1},{"email":"[email protected]","birthday":null,"address":"china-Shanghai","name":"lisi","id":2}]
ObjectMapper
[{"email":"[email protected]","birthday":null,"address":"china-Suzhou","name":"zhangsan","id":1},{"email":"[email protected]","birthday":null,"address":"china-Shanghai","name":"lisi","id":2}]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。
2.4. Jackson类型使用
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。
import java.io.IOException;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.node.JsonNodeFactory;
public class WriteListJSON {
public static void main(String[] args) {
try {
ObjectMapper objectMapper = new ObjectMapper();
JsonGenerator jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);
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 (IOException e) {
e.printStackTrace();
}
}
}转换后结果如下：
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":{"email":"email","birthday":null,"address":"address","name":"haha","id":1},"infos":["a","b","c"]3. JSON转换成Java对象
3.1. 将json字符串转换成JavaBean对象
import java.io.IOException;
import org.codehaus.jackson.map.ObjectMapper;
public class ReadJson2Entity {
public static void main(String[] args) {
String json = "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}";
try {
ObjectMapper objectMapper = new ObjectMapper();
AccountBean acc = objectMapper.readValue(json, AccountBean.class);
System.out.println(acc.getName());
System.out.println(acc);
} 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#email3.2. 将json字符串转换成List集合
import java.io.IOException;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import org.codehaus.jackson.map.ObjectMapper;
public class ReadJson2List {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"
+ "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
try {
ObjectMapper objectMapper = new ObjectMapper();
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));
}
System.out.println();
}
} 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。只好默认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：
2
address:address2
name:haha2
id:2
email:email2
address:address
name:haha
id:1
email:email3.3. 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。
import java.io.IOException;
import org.codehaus.jackson.map.ObjectMapper;
public class ReadJson2Array {
public static void main(String[] args) {
String json = "[{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"
+ "{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}]";
try {
ObjectMapper objectMapper = new ObjectMapper();
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 (IOException e) {
e.printStackTrace();
}
}
}运行后的结果：
2
haha2#2#address2#null#email2
haha#1#address#null#email3.4. Json字符串转换成Map集合
import java.io.IOException;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import org.codehaus.jackson.map.ObjectMapper;
public class ReadJson2Map {
@SuppressWarnings("unchecked")
public static void main(String[] args) {
String json = "{\"success\":true,\"A\":{\"address\": \"address2\",\"name\":\"haha2\",\"id\":2,\"email\":\"email2\"},"
+ "\"B\":{\"address\":\"address\",\"name\":\"haha\",\"id\":1,\"email\":\"email\"}}";
try {
ObjectMapper objectMapper = new ObjectMapper();
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 (IOException e) {
e.printStackTrace();
}
}
} 运行后的结果：
3
success:true
A:{address=address2, name=haha2, id=2, email=email2}
B:{address=address, name=haha, id=1, email=email}
Jackson也可以完成java对象到xml的转换，但不建议使用该Jar执行转换。这里就不做介绍了。