Jackson 框架

使用說明

Jackson可以輕松的將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。

前面有介紹過json-lib這個框架，在線博文：http://www.cnblogs.com/duanxz/archive/2013/05/03/3056174.html
相比json-lib框架，Jackson所依賴的jar包較少，簡單易用并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。

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一、準備工作

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

然后在工程中導入這個jar包即可開始工作

官方示例：http://wiki.fasterxml.com/JacksonInFiveMinutes

因為下面的程序是用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，那么還需要stax2-api.jar

2、 測試類基本代碼如下(下面的代碼還不能直接運行）

package com.dxz.json;import java.io.IOException; import org.codehaus.jackson.JsonEncoding; import org.codehaus.jackson.JsonGenerator; import org.codehaus.jackson.map.ObjectMapper; import org.junit.After; import org.junit.Before; import org.junit.Test; /*** <b>function:</b>Jackson 將java對象轉換成JSON字符串，也可以將JSON字符串轉換成java對象* jar-lib-version: jackson-all-1.6.2* @createDate 2010-11-23 下午04:54:53* @file JacksonTest.java*/ @SuppressWarnings("unchecked") public class JacksonTest{private JsonGenerator jsonGenerator = null;private ObjectMapper objectMapper = null;private AccountBean bean = null;@Beforepublic void init() {bean = new AccountBean();bean.setAddress("china-Guangzhou");bean.setEmail("abc@gmail.com");bean.setId(1);bean.setName("duanxz");objectMapper = new ObjectMapper();try {jsonGenerator = objectMapper.getJsonFactory().createJsonGenerator(System.out, JsonEncoding.UTF8);} catch (IOException e) {e.printStackTrace();}}@Afterpublic 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

AccountBean.java

/*** AccountBean*/ package com.dxz.json;public class AccountBean { private int id; private String name;private String email;private String address;private Birthday birthday;@Overridepublic String toString() {return this.name + "#" + this.id + "#" + this.address + "#" + this.birthday + "#" + this.email;}//getter、setterpublic int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public Birthday getBirthday() {return birthday;}public void setBirthday(Birthday birthday) {this.birthday = birthday;} }

Birthday.java

package com.dxz.json;public class Birthday {private String birthday;public Birthday(String birthday) {super();this.birthday = birthday;}//getter、setterpublic Birthday() {}@Overridepublic String toString() {return this.birthday;} }

二、Java對象轉換成JSON

1、 JavaBean(Entity/Model)轉換成JSON（JacksonTest中的方法）

/*** <b>function:</b>將java對象轉換成json字符串* @author hoojo* @createDate 2010-11-23 下午06:01:10*/@Testpublic void writeEntityJSON() {try {System.out.println("jsonGenerator");//writeObject可以轉換java對象，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();}}

該方法執行結果：

jsonGenerator {"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1} ObjectMapper {"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","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的創建依賴于ObjectMapper對象。也就是說如果你要使用JsonGenerator來轉換JSON，那么你必須創建一個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，轉換后可以通過這個流來輸出轉換后的內容。或是提供一個File，將轉換后的內容寫入到File中。當然，這個參數也可以接收一個JSONGenerator，然后通過JSONGenerator來輸出轉換后的信息。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，那么是一個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。

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2、 將Map集合轉換成Json字符串

/*** <b>function:</b>將map轉換成json字符串* @createDate 2010-11-23 下午06:05:26*/@Testpublic 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("hoojo@qq.com");map.put("account2", bean);System.out.println("writeMapJSON*****jsonGenerator");jsonGenerator.writeObject(map);System.out.println("");System.out.println("writeMapJSON*****objectMapper");objectMapper.writeValue(System.out, map);} catch (IOException e) {e.printStackTrace();}}

轉換后結果如下：

writeMapJSON*****jsonGenerator {"account2":{"email":"hoojo@qq.com","birthday":null,"address":"china-Beijin","name":null,"id":0},"name":"duanxz","account":{"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}} writeMapJSON*****objectMapper {"account2":{"email":"hoojo@qq.com","birthday":null,"address":"china-Beijin","name":null,"id":0},"name":"duanxz","account":{"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1}}

3、 將List集合轉換成json

/*** <b>function:</b>將list集合轉換成json字符串*/@Testpublic 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轉換成JSON字符串 jsonGenerator.writeObject(list);System.out.println();System.out.println("ObjectMapper");// 用objectMapper直接返回list轉換成的JSON字符串System.out.println("1###" + objectMapper.writeValueAsString(list));System.out.print("2###");// objectMapper list轉換成JSON字符串 objectMapper.writeValue(System.out, list);} catch (IOException e) {e.printStackTrace();}}

結果如下：

jsonGenerator [{"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","id":2}] ObjectMapper 1###[{"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","id":2}] 2###[{"email":"abc@gmail.com","birthday":null,"address":"china-Guangzhou","name":"duanxz","id":1},{"email":"email2","birthday":null,"address":"address2","name":"haha2","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，這里就不再轉換了。~.~

4、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，那么你即使沒有JavaBean(Entity)也可以完成復雜的Java類型的JSON轉換。下面用到這些類型構建一個復雜的Java對象，并完成JSON轉換。

@Testpublic void writeOthersJSON() {try {String[] arr = { "a", "b", "c" };System.out.println("jsonGenerator");String str = "hello world jackson!";// byte jsonGenerator.writeBinary(str.getBytes());// booleanjsonGenerator.writeBoolean(true);// null jsonGenerator.writeNull();// floatjsonGenerator.writeNumber(2.2f);// charjsonGenerator.writeRaw("c");// StringjsonGenerator.writeRaw(str, 5, 10);// StringjsonGenerator.writeRawValue(str, 5, 5);// String jsonGenerator.writeString(str);jsonGenerator.writeTree(JsonNodeFactory.instance.POJONode(str));System.out.println();// ObjectjsonGenerator.writeStartObject();// {jsonGenerator.writeObjectFieldStart("user");// user:{jsonGenerator.writeStringField("name", "jackson");// name:jacksonjsonGenerator.writeBooleanField("sex", true);// sex:truejsonGenerator.writeNumberField("age", 22);// age:22jsonGenerator.writeEndObject();// }jsonGenerator.writeArrayFieldStart("infos");// infos:[jsonGenerator.writeNumber(22);// 22jsonGenerator.writeString("this is array");// this is arrayjsonGenerator.writeEndArray();// ]jsonGenerator.writeEndObject();// }AccountBean bean = new AccountBean();bean.setAddress("address");bean.setEmail("email");bean.setId(1);bean.setName("haha");// complex ObjectjsonGenerator.writeStartObject();// {jsonGenerator.writeObjectField("user", bean);// user:{bean}jsonGenerator.writeObjectField("infos", arr);// infos:[array]jsonGenerator.writeEndObject();// }} catch (Exception e) {e.printStackTrace();}}

運行后，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"]}

怎么樣？構造的json字符串和輸出的結果是一致的吧。關鍵看懂用JSONGenerator提供的方法，完成一個Object的構建。

三、JSON轉換成Java對象

1、 將json字符串轉換成JavaBean對象

@Testpublic 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解析吃什么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。運行后，結果如下：

haha haha#1#address#null#email

2、 將json字符串轉換成List<Map>集合

/*** <b>function:</b>json字符串轉換成list<map>*/@Testpublic 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();}}

嘗試過將上面的JSON轉換成List，然后List中存放AccountBean，但結果失敗了。但是支持Map集合。因為你轉成List.class，但是不知道List存放何種類型。只好默然Map類型。因為所有的對象都可以轉換成Map結合，運行后結果如下：

2 address:address2 name:haha2 id:2 email:email2 address:address name:haha id:1 email:email

3、 Json字符串轉換成Array數組，由于上面的泛型轉換不能識別到集合中的對象類型。所有這里用對象數組，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。

/*** <b>function:</b>json字符串轉換成Array*/@Testpublic 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();}}

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運行后的結果：

2 haha2#2#address2#null#email2 haha#1#address#null#email

4、 Json字符串轉換成Map集合

/*** <b>function:</b>json字符串轉換Map集合*/@Testpublic 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();}}

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運行后結果如下：

3 success:true A:{address=address2, name=haha2, id=2, email=email2} B:{address=address, name=haha, id=1, email=email}

四、Jackson對XML的支持

Jackson也可以完成java對象到xml的轉換，轉換后的結果要比json-lib更直觀，不過它依賴于stax2-api.jar這個jar包。

/** ?* <b>function:</b>java對象轉換成xml文檔 ?* 需要額外的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轉換成xml ??????????? //xml.writeValue(System.out, bean); ??????????? StringWriter sw = new StringWriter(); ??????????? xml.writeValue(sw, bean); ??????????? System.out.println(sw.toString()); ??????????? //List轉換成xml ??????????? List<AccountBean> list = new ArrayList<AccountBean>(); ??????????? list.add(bean); ??????????? list.add(bean); ??????????? System.out.println(xml.writeValueAsString(list)); ??????????? ??????????? //Map轉換xml文檔 ??????????? 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(); ????? } }

運行上面的方法，結果如下：

XmlMapper <unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown> <unknown><unknown><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></unknown><email><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></email></unknown> <unknown><A><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></A><B><address>china-Guangzhou</address><name>hoojo</name><id>1</id><birthday/><email>hoojo_@126.com</email></B></unknown>

看結果，根節點都是unknown 這個問題還沒有解決，由于根節點沒有轉換出來，所有導致解析xml到Java對象，也無法完成。

轉載于:https://www.cnblogs.com/duanxz/archive/2013/05/07/3064160.html

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