1Jackson 是什麼?
(1)崇尚速度的(傳說比其他Java json parser和data binder執行起來更快)(2)數據流動的(讀取,寫入)
(3)完全獨立的(獨立於JDK之外的其他包)
(4)火力十足的(爲通用JDK類提供完整的數據綁定,就像Java bean class, Collection, Map or Enum),可配置的
(5)開放源代碼的(Apache License – or, until 2.1, alternatively LGPL)
Jackson就是這樣一個具有上述五大特性的JSON處理器。它提供JSON解析與JSON構建作爲基礎構造塊,另有一個功能強大的數據綁定器(支持JSON與POJO相互轉換)和樹型結構作爲可選附加塊。
也就是說,你可以通過調用流式API、POJO數據綁定、樹型結構三種方式來生成和解析JSON。
2JSON三種方式
(1)流式API:讀取和寫入JSON內容視作離散事件。org.codehaus.jackson.JsonParser reads
org.codehaus.jackson.JsonGenerator writes
(2)樹型:提供一個JSON文檔的可變內存樹的表示形式。
org.codehaus.jackson.map.ObjectMapper 生成樹;該樹由JsonNode節點組成
樹型類似於XML DOM
(3)數據綁定:提供基於屬性訪問器規約或註解的JSON和POJO相互轉換。
有兩種形式:簡單數據綁定和完整數據綁定:
簡單數據綁定:從Java Map、List、String、Numbers、Boolean和空值進行轉換
完整數據綁定:從任何Java bean類型(以及“簡單”類型)進行轉換
org.codehaus.jackson.map.ObjectMapper 對這兩種形式進行編組處理(寫入 JSON)和逆向編組(讀取 JSON)
(4)使用特點
性能最好的是流式API,另外兩種方式都是基於它實現的;使用最方便的是數據綁定;最靈活的是樹型。
3示例
3.1工程包目錄
3.2完整數據綁定(POJO)
(1)Json字符串:data.json{
"name" : { "first" : "Joe", "last" : "Sixpack" },
"gender" : "MALE",
"verified" : false,
"userImage" : "Rm9vYmFyIQ=="
}
(2)Json轉換成實體類:json2Entity() /**
* 將Json字符串轉換成實體類
*/
public static void json2Entity() {
try {
User user = mapper.readValue(new File("src/com/roctec/json/data.json"), User.class);
System.out.println(user);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
(3)實體類:User.javapackage com.roctec.entity;
import java.util.Arrays;
public class User {
public enum Gender {
MALE, FEMALE
};
public static class Name {
private String _first, _last;
public String getFirst() {
return _first;
}
public String getLast() {
return _last;
}
public void setFirst(String s) {
_first = s;
}
public void setLast(String s) {
_last = s;
}
}
private Gender _gender;
private Name _name;
private boolean _isVerified;
private byte[] _userImage;
public Name getName() {
return _name;
}
public boolean isVerified() {
return _isVerified;
}
public Gender getGender() {
return _gender;
}
public byte[] getUserImage() {
return _userImage;
}
public void setName(Name n) {
_name = n;
}
public void setVerified(boolean b) {
_isVerified = b;
}
public void setGender(Gender g) {
_gender = g;
}
public void setUserImage(byte[] b) {
_userImage = b;
}
@Override
public String toString() {
return "User [_gender=" + _gender + ", _name=[_first=" + _name._first + ", _last=" + _name._last
+ "], _isVerified=" + _isVerified + ", _userImage="
+ Arrays.toString(_userImage) + "]";
}
}
(4)實體類轉換成Json:entity2Json() /**
* 將實體類轉換成Json字符串
*/
public static void entity2Json() {
User user = new User();
User.Name name = new User.Name();
name.setFirst("Joe");
name.setLast("Sixpack");
user.setName(name);
user.setGender(User.Gender.MALE);
user.setVerified(false);
user.setUserImage("Rm9vYmFyIQ==".getBytes());
try {
mapper.writeValue(new File("src/com/roctec/json/entity.json"), user);
} catch (JsonGenerationException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
3.3簡單數據綁定(Map)
(1)Json轉換成Map:json2Map() /**
* 將Json字符串轉換成map
*/
public static void json2Map() {
try {
@SuppressWarnings("unchecked")
Map<String, Object> map = mapper.readValue(new File("src/com/roctec/json/data.json"), Map.class);
for (Map.Entry<String, Object> entity : map.entrySet()) {
System.out.println(entity.getKey() + "=" + entity.getValue());
}
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
(2)Map轉換成Json:map2Json() /**
* 將map轉換成Json字符串
*/
public static void map2Json() {
Map<String,Object> map = new HashMap<String,Object>();
Map<String,String> name = new HashMap<String,String>();
name.put("first", "Joe");
name.put("last", "Sixpack");
map.put("name", name);
map.put("gender", "MALE");
map.put("verified", Boolean.FALSE);
map.put("userImage", "Rm9vYmFyIQ==");
try {
mapper.writeValue(new File("src/com/roctec/json/map.json"), map);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
3.4樹型綁定
(1)Json轉換成樹形:json2Tree() /**
* 將Json字符串轉換成樹形
*/
public static void json2Tree() {
try {
// 使用mapper.readTree(source),或使用mapper.readValue(source, JsonNode.class);
JsonNode rootNode = mapper.readTree(new File("src/com/roctec/json/data.json"));
// 確保"last name"不是"Xmler";如果是,就轉換爲"Jsoner"
JsonNode nameNode = rootNode.path("name");
String lastName = nameNode.path("last").getTextValue();
if("xmler".equalsIgnoreCase(lastName)) {
((ObjectNode)nameNode).put("last", "Jsoner");
}
mapper.writeValue(new File("src/com/roctec/json/tree.json"), rootNode);
} catch (JsonProcessingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
(2)樹形轉換成Json:json2Tree() /**
* 將樹形轉換成Json字符串
*/
public static void tree2Json() {
ObjectNode rootNode = mapper.createObjectNode();
ObjectNode nameNode = rootNode.putObject("name");
nameNode.put("first", "Joe");
nameNode.put("last", "Sixpack");
rootNode.put("gender", User.Gender.MALE.toString());
rootNode.put("verified", false);
byte[] imageBuf = "Rm9vYmFyIQ==".getBytes();
rootNode.put("userImage", imageBuf);
try {
mapper.writeValue(new File("src/com/roctec/json/tree.json"), rootNode);
} catch (JsonGenerationException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
3.5流式API
(1)生成Json:generateJson() /**
* 構造Json字符串
*/
public static void generateJson() {
try {
JsonFactory factory = mapper.getJsonFactory();
JsonGenerator generator = factory.createJsonGenerator(new File("src/com/roctec/json/write.json"), JsonEncoding.UTF8);
generator.writeStartObject();
generator.writeObjectFieldStart("name");
generator.writeStringField("first", "Joe");
generator.writeStringField("last", "Sixpack");
generator.writeEndObject();
generator.writeStringField("gender", User.Gender.MALE.toString());
generator.writeBooleanField("verified", false);
generator.writeFieldName("userImage");
byte[] imageBuf = "Rm9vYmFyIQ==".getBytes();
generator.writeBinary(imageBuf);
generator.writeEndObject();
generator.close();
} catch (IOException e) {
e.printStackTrace();
}
}
(2)解析Json:parseJson() /**
* 解析Json字符串
*/
public static void parseJson() {
try {
JsonFactory factory = mapper.getJsonFactory();
JsonParser parser = factory.createJsonParser(new File("src/com/roctec/json/data.json"));
User user = new User();
parser.nextToken();
while(parser.nextToken() != JsonToken.END_OBJECT) {
String fieldName = parser.getCurrentName();
parser.nextToken();
if("name".equals(fieldName)) {
User.Name name = new User.Name();
while(parser.nextToken() != JsonToken.END_OBJECT) {
String nameField = parser.getCurrentName();
parser.nextToken();
if("first".equals(nameField)) {
name.setFirst(parser.getText());
} else if("last".equals(nameField)) {
name.setLast(parser.getText());
} else {
throw new IllegalStateException("Unrecognized field '" + fieldName + "'!");
}
}
user.setName(name);
} else if("gender".equals(fieldName)) {
user.setGender(User.Gender.valueOf(parser.getText()));
} else if("verified".equals(fieldName)) {
user.setVerified(parser.getCurrentToken() == JsonToken.VALUE_TRUE);
} else if("userImage".equals(fieldName)) {
user.setUserImage(parser.getBinaryValue());
} else {
throw new IllegalStateException("Unrecognized field '" + fieldName + "'!");
}
}
parser.close();
System.out.println(user);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
(3)解析Json數組:parseJsonArray() /**
* 解析Json數組
*/
public static void parseJsonArray() {
String json = "[{\"foo\": \"bar\"}, {\"foo\": \"biz\"}]";
try {
JsonFactory factory = mapper.getJsonFactory();
JsonParser parser = factory.createJsonParser(json);
parser.nextToken();
while(parser.nextToken() == JsonToken.START_OBJECT) {
Foo foo = mapper.readValue(parser, Foo.class);
System.out.println(foo);
}
parser.close();
} catch (JsonParseException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
(4)實體類:Foo.javapackage com.roctec.entity;
public class Foo {
public String foo;
public String getFoo() {
return foo;
}
public void setFoo(String foo) {
this.foo = foo;
}
@Override
public String toString() {
return "Foo [foo=" + foo + "]";
}
}
最後介紹一個竅門:可以通過JsonParser和JsonGenerator直接實現數據綁定和樹型。請查看如下方法:
JsonParser.readValueAs()
JsonParser.readValueAsTree()
JsonGenerator.writeObject()
JsonGenerator.writeTree()
注意一點,確保使用的org.codehaus.jackson.map.MappingJsonFactory是“適用數據綁定”的解析器和生成器實例(而非基本的org.codehaus.jackson.JsonFactory)。