17、Java——JSON介绍与解析

9、JSON

JSON(JavaScript Object Notation) 是一种轻量级的数据交换格式。 JSON 官方:http://www.json.org
JSON 数据格式的特点
JSON 建构于两种结构：
1、 “名称/值”对的集合
2.、 值的有序列表（数组）

JSON 表示名称 / 值对的方式 ：{ “firstName”: “vince”,“lastName”:“ma”,“email”:“finally_m@foxmail.com” }
表示数组
{ “user”: [{ “firstName”: “vince”,“lastName”:“ma”,“email”: “finally_m@foxmail.com” },
{ “firstName”: “lin”,“lastName”:“jacks”,“email”: “jacks@qq.com”}]
}

10、GSON组件的使用

GSON是Google开发的Java API，用于转换Java对象和Json对象。
下载地址：http://www.mvnrepository.com/artifact/com.google.code.gson/gson
解析JSON：JsonReader reader = new JsonReader(new StringReader(jsonData));
生成JSON：
private String createJSON(ArrayList users) {
JSONObject jsonObject = new JSONObject();JSONArray array = new JSONArray();
int size = users.size();
try {for (int i = 0; i < size; i++) {
User user = users.get(i); JSONObject object = new JSONObject();
object.put(“name”, user.name);object.put(“age”, user.age);array.put(object);}
jsonObject.put(“users”,array);
return jsonObject.toString();
} catch (JSONException e) {e.printStackTrace();}
return null;
}
static class User {String name; int age;//…}

[
  {
    "id": 912345678901,
    "text": "How do I read a JSON stream in Java?",
    "geo": null,
    "user": {
      "name": "json_newb",
      "followers_count": 41
    }
  },
  {
    "id": 912345678902,
    "text": "@json_newb just use JsonReader!",
    "geo": [50.454722, -104.606667],
    "user": {
      "name": "jesse",
      "followers_count": 2
    }
  }
]

public class User {
    private String name;
    private int followers_count;

    public User() {
    }

    public User(String name, int followers_count) {
        this.name = name;
        this.followers_count = followers_count;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getFollowers_count() {
        return followers_count;
    }

    public void setFollowers_count(int followers_count) {
        this.followers_count = followers_count;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", followers_count=" + followers_count +
                '}';
    }
}

public class Message {
    private long id;
    private String text;
    private ArrayList<Double> geo;
    private User user;

    public Message() {
    }

    public Message(int id, String text, ArrayList<Double> geo, User user) {
        this.id = id;
        this.text = text;
        this.geo = geo;
        this.user = user;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public ArrayList<Double> getGeo() {
        return geo;
    }

    public void setGeo(ArrayList<Double> geo) {
        this.geo = geo;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    @Override
    public String toString() {
        return "Message{" +
                "id=" + id +
                ", text='" + text + '\'' +
                ", geo=" + geo +
                ", user=" + user +
                '}';
    }
}

public class JsonDemo2 {

    /**
     * 使用JsonReader解析复杂的JSON数据
     */
    @Test
    public void parseJSONMessages(){

        InputStream is = Thread.currentThread().getContextClassLoader()
                .getResourceAsStream("com/vince/json/message.json");
        InputStreamReader in = new InputStreamReader(is);
        JsonReader jsonReader = new JsonReader(in);
        ArrayList<Message> list = readMessageArray(jsonReader);
        for (Message m: list){
            System.out.println(m);
        }

    }

    //读取Message数组
    private ArrayList<Message> readMessageArray(JsonReader jsonReader) {
        ArrayList<Message> list = new ArrayList<>();
        try {
            jsonReader.beginArray();
            while(jsonReader.hasNext()){
                list.add(readMessage(jsonReader));
            }
            jsonReader.endArray();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return list;
    }

    //解析一个Message对象
    private Message readMessage(JsonReader jsonReader) {
        Message m = new Message();
        try {
            jsonReader.beginObject();
            while (jsonReader.hasNext()){
                String name = jsonReader.nextName();
                if("id".equals(name)){
                    m.setId(jsonReader.nextLong());
                }else if("text".equals(name)){
                    m.setText(jsonReader.nextString());
                }else if("geo".equals(name) && jsonReader.peek()!= JsonToken.NULL){
                    m.setGeo(readGeo(jsonReader));
                }else if("user".equals(name)){
                    m.setUser(readUser(jsonReader));
                }else{
                    jsonReader.skipValue();
                }
            }
            jsonReader.endObject();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return m;
    }

    /**
     * 解析User对象
     * @param jsonReader
     * @return
     */
    private User readUser(JsonReader jsonReader) {
        User user = new User();
        try {
            jsonReader.beginObject();
            while (jsonReader.hasNext()){
                String name = jsonReader.nextName();
                if("name".equals(name)){
                    user.setName(jsonReader.nextString());
                }else if("followers_count".equals(name)){
                    user.setFollowers_count(jsonReader.nextInt());
                }else{
                    jsonReader.skipValue();
                }
            }
            jsonReader.endObject();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return user;
    }

    /**
     * 解析GEO
     * @param jsonReader
     * @return
     */
    private ArrayList<Double> readGeo(JsonReader jsonReader) {

        ArrayList<Double> list = new ArrayList<>();
        try {
            jsonReader.beginArray();
            while (jsonReader.hasNext()){
                list.add(jsonReader.nextDouble());
            }
            jsonReader.endArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return list;
    }
}

使用 Gson 直接把 JSON 数据转换成 Java 对象
public Student parserJSON2(String data){
Gson gson = new Gson();
Student s = gson.fromJson(data, Student.class);
return s;
}
使用 Gson 直接把 JSON 数组转换成 Java 对象
public List parserJSON3(String data){
Type type = new TypeToken<ArrayList>(){}.getType();
Gson gson = new Gson();
List list = gson.fromJson(data, type);
return list;
}

public class JsonDemo3 {

    /**
     * 把一组JSON对象转换成一个JAVA对象集合，或者把一个JAVA对象集合转换成JSON数组
     */
    @Test
    public void gson2(){
        Gson gson = new Gson();
        InputStream is = Thread.currentThread().getContextClassLoader()
                .getResourceAsStream("com/vince/json/names.json");
        InputStreamReader in = new InputStreamReader(is);
        TypeToken<List<Name>> typeToken = new TypeToken<List<Name>>(){};
        List<Name> list = gson.fromJson(in, typeToken.getType());
        System.out.println(list);


        String json = gson.toJson(list, typeToken.getType());
        System.out.println(json);
    }

    class MyTypeToken extends TypeToken<List<Name>>{}

    /**
     * 把一个JSON对象转换成JAVA对象，或把一个JAVA对象转换成JSON对象
     */
    @Test
    public void gson1(){
        Gson gson = new Gson();
        InputStream is = Thread.currentThread().getContextClassLoader()
                .getResourceAsStream("com/vince/json/name.json");
        InputStreamReader in = new InputStreamReader(is);
        Name name = gson.fromJson(in, Name.class);
        System.out.println(name);

        String json = gson.toJson(name);
        System.out.println(json);
    }

    @Test
    public void createJSON(){
        List<Name> list = new ArrayList<>();
        list.add(new Name("vince","ma","1791705739@qq.com"));
        list.add(new Name("jack","wang","jack@qq.com"));
        JsonArray array = new JsonArray();
        for (Name n: list){
            JsonObject obj = new JsonObject();
            obj.addProperty("firstName",n.getFirstName());
            obj.addProperty("lastName",n.getLastName());
            obj.addProperty("email",n.getEmail());
            array.add(obj);
        }
        System.out.println(array.toString());
    }
}

11、XML与JSON的比较

从以下6点比较JSON与XML：
1、JSON和XML的数据可读性基本相同
2、JSON和XML同样拥有丰富的解析手段
3、JSON相对于XML来讲，数据的体积小
4、JSON与JavaScript的交互更加方便
5、JSON对数据的描述性比XML较差
6、JSON的速度要远远快于XML
适合的场景：
（1）数据传输：JSON要比XML更有优势
（2）存储数据：XML描述性更强
（3）XML通常用做配置文件（WEB课程中会有详细介绍）