Jackson Tutorial - 2

1. Examples
   • Full Data Binding (POJO) Example
   • Simple Data Binding Example
   • Data Binding with Generics
   • Three Model Example
   • Streaming API Example

    

2. Full Data Binding(POJO) Example
   • Jackson's org.codehaus.jackson.map.ObjectMapper "just works" for mapping JSON data into plain old Java objects ("POJOs").
   • Json data

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

   • User Instance로의 변환

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

   • 결과물(User.java)

   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; } }

   • Json data로의 변환

   mapper.writeValue(new File("user-modified.json"), user);

    

3. Simple Data Binding Example

   Map<String,Object> 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==");

   • Json data로의 변환

   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

    

4. Data Binding with Generics
   • 제네릭을 사용하기 위해서는 TypeReference를 이용한다.

   Map<String,User> result = mapper.readValue(src, new TypeReference<Map<String,User>>() { });

   • 만약 Map.class만을 사용한다면 형이 지정되지 않는 Map<?, ?>으로 바인딩 된다.
   • 다른 방법으로는, v1.3 이후부터는 TypeFactory를 이용할 수 있다.

    

5. Tree Model Example
   • ObjectMapper를 이용해 Tree Node를 만들고 Json으로 변환.

   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);

   • Tree Node의 생성

   TreeMapper treeMapper = new TreeMapper(); ObjectNode userOb = treeMapper.objectNode(); Object nameOb = userRoot.putObject("name"); nameOb.put("first", "Joe"); nameOb.put("last", "Sixpack"); userOb.put("gender", User.Gender.MALE.toString()); userOb.put("verified", false); byte[] imageData = getImageData(); // or wherever it comes from userOb.put("userImage", imageData);

   • (NOTE: with Jackson 1.2 you can use ObjectMapper directly, using ObjectMapper.createObjectNode() to create userOb

    

Streaming API Example
• WriteJSON.java 작성

JsonFactory f = new JsonFactory(); JsonGenerator g = f.createJsonGenerator(new File("user.json"));   g.writeStartObject(); g.writeObjectFieldStart("name"); g.writeStringField("first", "Joe"); g.writeStringField("last", "Sixpack"); g.writeEndObject(); // for field 'name' g.writeStringField("gender", Gender.MALE); g.writeBooleanField("verified", false); g.writeFieldName("userImage"); // no 'writeBinaryField' (yet?) byte[] binaryData = ...; g.writeBinary(binaryData); g.writeEndObject(); g.close(); // important: will force flushing of output, close underlying output stream

• ObjectMapper를 쓰는 것보다 다소 빠르다. 그러나 손이 많이 간다는 것!
• 이 방법을 쓸 일은 거의 없겠지…..
• Parsing

JsonFactory f = new JsonFactory(); JsonParser jp = f.createJsonParser(new File("user.json")); User user = new User(); jp.nextToken(); // will return JsonToken.START_OBJECT (verify?) while (jp.nextToken() != JsonToken.END_OBJECT) {   String fieldname = jp.getCurrentName();   jp.nextToken(); // move to value, or START_OBJECT/START_ARRAY   if ("name".equals(fieldname)) { // contains an object     Name name = new Name();     while (jp.nextToken() != JsonToken.END_OBJECT) {       String namefield = jp.getCurrentName();       jp.nextToken(); // move to value       if ("first".equals(namefield)) {         name.setFirst(jp.getText());       } else if ("last".equals(namefield)) {         name.setLast(jp.getText());       } else {         throw new IllegalStateException("Unrecognized field '"+fieldname+"'!");       }     }     user.setName(name);   } else if ("gender".equals(fieldname)) {     user.setGender(Gender.valueOf(jp.getText()));   } else if ("verified".equals(fieldname)) {     user.setVerified(jp.getCurrentToken() == JsonToken.VALUE_TRUE);   } else if ("userImage".equals(fieldname)) {     user.setUserImage(jp.getBinaryValue());   } else {     throw new IllegalStateException("Unrecognized field '"+fieldname+"'!");   } } jp.close(); // ensure resources get cleaned up timely and properly

추가적으로, 다음의 메소드로 JsonParser, JsonGenerator에서 데이터 바인딩이나 Tree Model을 사용할 수 있다.
• JsonParser.readValueAs()
• JsonParser.readValueAsTree()
• JsonGenerator.writeObject()
• JsonGenerator.writeTree()  

출처 : http://youreme.blog.me/110082668135