Android Json的使用（2） 使用Jackson解析和生成json

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 globallyUser 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