原文：http://www.cnblogs.com/hoojo/archive/2011/04/22/2024628.html#top
Jackson框架
Jackson可以輕鬆的將Java對象轉換成json對象和xml文檔，同樣也可以將json、xml轉換成Java對象。
相比json-lib框架，Jackson所依賴的jar包較少，簡單易用並且性能也要相對高些。而且Jackson社區相對比較活躍，更新速度也比較快。
建議：JSON的轉換使用Jackson框架；xml的轉換使用xStream框架。
1. 準備工作
1.1. 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
然後在工程中導入這個jar包即可開始工作
官方示例：http://wiki.fasterxml.com/JacksonInFiveMinutes
因爲下面的程序是用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
如果你需要轉換xml，那麼還需要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對象轉換成JSON
2.1. JavaBean(Entity/Model)轉換成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可以轉換java對象，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();
}
}
}運行後結果如下：
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}上面分別利用JsonGenerator的writeObject方法和ObjectMapper的writeValue方法完成對Java對象的轉換，二者傳遞的參數及構造的方式不同；JsonGenerator的創建依賴於ObjectMapper對象。也就是說如果你要使用 JsonGenerator來轉換JSON，那麼你必須創建一個ObjectMapper。但是你用ObjectMapper來轉換JSON，則不需要 JSONGenerator。
objectMapper的writeValue方法可以將一個Java對象轉換成JSON。這個方法的參數一，需要提供一個輸出流，轉換後可以通過這個流來輸出轉換後的內容。或是提供一個File，將轉換後的內容寫入到File中。當然，這個參數也可以接收一個JSONGenerator，然後通過JSONGenerator來輸出轉換後的信息。第二個參數是將要被轉換的Java對象。如果用三個參數的方法，那麼是一個Config。這個config可以提供一些轉換時的規則，過指定的Java對象的某些屬性進行過濾或轉換等。
2.2. 將Map集合轉換成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<String, Object> map = new HashMap<String, Object>();
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();
}
}
}轉換後結果如下：
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. 將List集合轉換成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<AccountBean> list = new ArrayList<AccountBean>();
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();
}
}
}轉換後結果如下：
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}]外面就是多了個[]中括號；同樣Array也可以轉換，轉換的JSON和上面的結果是一樣的，這裏就不再轉換了。
2.4. Jackson類型使用
下面來看看jackson提供的一些類型，用這些類型完成json轉換；如果你使用這些類型轉換JSON的話，那麼你即使沒有JavaBean(Entity)也可以完成複雜的Java類型的JSON轉換。下面用到這些類型構建一個複雜的Java對象，並完成JSON轉換。
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();
}
}
}轉換後結果如下：
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轉換成Java對象
3.1. 將json字符串轉換成JavaBean對象
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。第一個參數就是解析的JSON字符串，第二個參數是即將將這個JSON解析吃什麼Java對象，Java對象的類型。當然，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，當然使用的方法 和當前使用的方法大同小異。運行後，結果如下：
haha
haha#1#address#null#email3.2. 將json字符串轉換成List<Map>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<Map<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));
}
System.out.println();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}嘗試過將上面的JSON轉換成List，然後List中存放AccountBean，但結果失敗了。但是支持Map集合。因爲你轉成List.class，但是不知道List存放何種類型。只好默認Map類型。因爲所有的對象都可以轉換成Map結合，運行後結果如下：
2
address:address2
name:haha2
id:2
email:email2
address:address
name:haha
id:1
email:email3.3. Json字符串轉換成Array數組
由於上面的泛型轉換不能識別到集合中的對象類型。所有這裏用對象數組，可以解決這個問題。只不過它不再是集合，而是一個數組。當然這個不重要，你可以用Arrays.asList將其轉換成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();
}
}
}運行後的結果：
2
haha2#2#address2#null#email2
haha#1#address#null#email3.4. Json字符串轉換成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<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 (IOException e) {
e.printStackTrace();
}
}
}運行後的結果：
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對象到xml的轉換，但不建議使用該Jar執行轉換。這裏就不做介紹了。