作者：阿米达

--------------------转载请注明：http://blog.csdn.net/liaoqianchuan00/article/details/8241115

使用Jackson的三种方式

• 数据绑定模式：使用最方便
• 流模式：性能最佳
• 树模式：最灵活

以最常用的数据绑定模式为例

Json数据如下

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

json生成具体的java对象实例

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

下面是User类：

    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; }
   }

从Java对象转换成Json

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

树模式例子

你还有另外一种方式来使用Jackson，就是使用树模式。这个就像XML的DOM树。Jackson用JsonNode来生成树。

如下所示：

 ObjectMapper m = new ObjectMapper();
    // can either use mapper.readTree(source), or mapper.readValue(source, JsonNode.class);
    JsonNode rootNode = m.readTree(new File("user.json"));
    // 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);

你也可以自己构造一个树：

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);

流模式

这是最高效的模式。

生成json文本

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

解析Json

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(User.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

详细内容参见：http://wiki.fasterxml.com/JacksonInFiveMinutes