Jackson 使用简介

博客分类： J2EE
jsonXML
http://jackson.codehaus.org/

lib:
jackson-core-asl-1.6.0.jar
jackson-mapper-asl-1.6.0.jar

心得：最方便的是Full Data Binding 方式，不用额外处理，但是POJO的属性名需要与JSON的变量名一一对应。对于需要做JSON变量名和POJO属性名转换的话，使用Tree Model。

Jackson提供了三种方式处理JSON
Streaming API， 基于事件处理JSON，类似StAX API。性能最好，其它两种方法都是建立在它之上
Tree Model 提供了内建文档树方式来展现JSON，类似XML DOM。最灵活。
Data Binding 使用注释或者属性访问器JSON与POJOs之间转换。最方便。

以下是官方网站的例子
1．Full Data Bindaing(POJO) Example

{ 
  "name" : { "first" : "Joe", "last" : "Sixpack" }, 
  "gender" : "MALE", 
  "verified" : false, 
  "userImage" : "Rm9vYmFyIQ==" 
} 

Java代码

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;  
    }  

}  

Java代码

ObjectMapper mapper = new ObjectMapper(); // can reuse, share globally  
User user = mapper.readValue(new File("user.json"), User.class);  
mapper.writeValue(new File("user-modified.json"), user);  

2．Simple Data Binding Example

Map<String, Object> userData = mapper.readValue(new File("user.json"), Map.class); 
userData = new HashMap<String, Object>(); 
Map<String, String> nameStruct = new HashMap<String, String>(); 
nameStruct.put("first", "Joe"); 
nameStruct.put("last", "Sixpack"); 
userData.put("name", nameStruct); 
userData.put("gender", "MALE"); 
userData.put("verified", Boolean.FALSE); 
userData.put("userImage", "Rm9vYmFyIQ=="); 
mapper.writeValue(new File("user-modified.json"), userData); 
Concrete Java types that Jackson will use for simple data binding are: 
JSON Type	 Java Type
object	 LinkedHashMap<String,Object>
array	 ArrayList<Object>
string	 String
number (no fraction)	 Integer, Long or BigInteger (smallest applicable)
number (fraction)	 Double (configurable to use BigDecimal)
true false	 Boolean
null	 null

3．The Model Example
Java代码

ObjectMapper m = new ObjectMapper();  
// can either use mapper.readTree(JsonParser), or bind to JsonNode  
JsonNode rootNode = m.readValue(new File("user.json"), JsonNode.class);  
// ensure that "last name" isn't "Xmler"; if is, change to "Jsoner"  
JsonNode nameNode = rootNode.path("name");  
String lastName = nameNode.path("last").getTextValue();  
if ("xmler".equalsIgnoreCase(lastName)) {  
    ((ObjectNode)nameNode).put("last", "Jsoner");  
}  
// and write it out:  
m.writeValue(new File("user-modified.json"), rootNode);