Gson fromJson() 用法

1.純數組JSON 字符串轉換	[ {"name": "zhangsan","age": "10","phone": "11111","email": "[e-mail]"},  {"name": "lisi","age": "20","phone": "22222","email": "[e-mail]"},   ...] 解析代碼如下： public class UserBean {     //變量名跟JSON數據字段名必須一致     private String name ;     private String age;     private String phone;     private String email;     ... }    //拿到本地JSON 並轉成String  String strByJson = ConstructTestJsonStr();//此處函數內容省略    //Json的解析類對象  JsonParser parser = new JsonParser();  //將JSON的String 轉成一個JsonArray對象  JsonArray jsonArray = parser.parse(strByJson).getAsJsonArray();    Gson gson = new Gson();  ArrayList<UserBean> userBeanList = new ArrayList<>();    //加強for循環遍歷JsonArray  for (JsonElement user : jsonArray) {         //使用GSON，直接轉成Bean對象         UserBean userBean = gson.fromJson(user, UserBean.class);         userBeanList.add(userBean);  }
2.有數據頭的純數組 數據轉換	{"muser": [{"name": "zhangsan","age": "10","phone": "11111","email": "[e-mail]"}, {"name": "lisi","age": "20","phone": "22222","email": "[e-mail]"}, ...] } //解析有數據頭的純數組 //拿到本地JSON 並轉成String String strByJson = ConstructTestJsonStr();//此處函數內容省略；   //先轉JsonObject JsonObject jsonObject = new JsonParser().parse(strByJson).getAsJsonObject(); //再轉JsonArray 加上數據頭 JsonArray jsonArray = jsonObject.getAsJsonArray("muser");   Gson gson = new Gson(); ArrayList<UserBean> userBeanList = new ArrayList<>();   //循環遍歷 for (JsonElement user : jsonArray) {         //通過反射 得到UserBean.class         UserBean userBean = gson.fromJson(user, new TypeToken<UserBean>() {}.getType());         userBeanList.add(userBean); }
3.有數據頭的復雜 數據轉換	{ "code": 200,"msg": "OK", "muser": [{"name": "zhangsan","age": "10","phone": "11111","email":"[e-mail]"},       {"name": "lisi","age": "20","phone": "22222","email": "[e-mail]"},     ...] }  被解析對象不是純數組數據，還有其它字段+數組（含頭），方法如下：  根據 JSON 建立Bean，注意這里的 Bean 是返回所有字段，因為 GSON 能直接解析成 List ，所以 Bean 是下面這樣的，同樣把占地方的get/set省略： public class ResultBean {     //注意變量名與字段名必須一致     private int code;     private String msg;     private List<UserBean> muser;     public class UserBean{         private String name ;         private String age;         private String phone;         private String email;         ...     }     ... } //有消息頭 復雜數據 常規方式 方法如下 //拿到Json字符串 String strByJson =  ConstructTestJsonStr();//此處函數內容省略； //GSON直接解析成對象 ResultBean resultBean = new Gson().fromJson(strByJson,ResultBean.class); //對象中拿到集合 List<ResultBean.UserBean> userBeanList = resultBean.getMuser(); 三句話搞定，由此看出GSON的強大
4.只解析復雜JSON中的 數組或數組中的某部分內容	有數據頭 復雜數據 截取方式，如對第3例中JSON數據內容只想取“muser”數組中年齡（age）大於30歲的怎么辦？ 方法1：全部解析，再從 List 中取。（但假如有1萬條數據？全部解析太麻煩），方法2：條件遍歷！（以下代碼示例）： //解析代碼：   //拿到JSON字符串 String strByJson =  ConstructTestJsonStr();//此處函數內容省略； List<UserBean> userBeanList = new ArrayList<>();   //拿到數組 JsonObject jsonObject = new JsonParser().parse(strByJson).getAsJsonObject(); JsonArray jsonArray = jsonObject.getAsJsonArray("muser");   //循環遍歷數組  for (JsonElement user : jsonArray) {         UserBean userBean = new Gson().fromJson(user, new TypeToken<UserBean>() {         }.getType());         //根據條件過濾         if (Integer.parseInt(userBean.getAge()) > 30) {             userBeanList.add(userBean);         }   }
5.嵌套超復雜JSON 數據轉換	三種方式：1，全部解析出來；2，要什么解析什么；3，JsonReader，第1/2招前述已解釋，下面示例JsonReader（類似逐節點解釋XML） //通過JsonReader的方式去解析 private void parseComplexJArrayByReader() throws IOException {     String strByJson = JsonToStringUtil.getStringByJson(this, R.raw.juser_4);     JsonReader reader = new JsonReader(new StringReader(strByJson));     try {         reader.beginObject();         String tagName = reader.nextName();         if (tagName.equals("group")) {             //讀group這個節點             readGroup(reader);         }         reader.endObject();     } finally {         reader.close();     } }   //讀group這個節點 private void readGroup(JsonReader reader) throws IOException {     reader.beginObject();     while (reader.hasNext()) {         String tagName = reader.nextName();         if (tagName.equals("user")) {             readUser(reader);         } else if (tagName.equals("info")) {             readInfo(reader);         }     }     reader.endObject(); }   //讀用戶基本消息 user節點 private void readUser(JsonReader reader) throws IOException {     reader.beginObject();     while (reader.hasNext()) {         String tag = reader.nextName();         if (tag.equals("name")) {             String name = reader.nextString();             nameText.setText(name);         } else if (tag.equals("age")) {             String age = reader.nextString();             ageText.setText(age);         }         ...         else {             reader.skipValue();//忽略         }     }     reader.endObject(); }   //讀用戶其他消息 info節點 private void readInfo(JsonReader reader) throws IOException {     reader.beginObject();     while (reader.hasNext()) {         String tag = reader.nextName();         if (tag.equals("address")) {             String address = reader.nextString();             addressText.setText(address);         } else if (tag.equals("work")) {             String work = reader.nextString();             workText.setText(work);         }         ...         else {             reader.skipValue();//忽略         }     }     reader.endObject(); }
6.簡單數據+LIST+MAP 轉換	public class Student {                    public int id;                    public String nickName;                    public int age;                    public ArrayList<String> books;                    public HashMap<String, String> booksMap; } Student student = new Student(); student.id = 1; student.nickName = "Jack"; student.age = 22; student.email = "[e-mail]"; ArrayList<String> books = new ArrayList<String>(); books.add("數學"); books.add("語文"); books.add("英語"); books.add("物理"); books.add("化學"); books.add("生物"); student.books = books; HashMap<String, String> booksMap = new HashMap<String, String>(); booksMap.put("1", "數學"); booksMap.put("2", "語文"); booksMap.put("3", "英語"); booksMap.put("4", "物理"); booksMap.put("5", "化學"); booksMap.put("6", "生物"); student.booksMap = booksMap;   Gson gson = new Gson(); String result = gson.toJson(student); Student studentG = gson.fromJson(result, Student.class);   Log.e("MainActivity", "id:" + studentG.id); Log.e("MainActivity", "nickName:" + studentG.nickName); Log.e("MainActivity", "age:" + studentG.age); Log.e("MainActivity", "email:" + studentG.email); Log.e("MainActivity", "books size:" + studentG.books.size()); Log.e("MainActivity", "booksMap size:" + studentG.booksMap.size());   打印輸出 id:1 nickName: Jack age:22 email:[e-mail] books size:6 booksMap size:6
7.把List或者Map等集合 的泛型換成自定義個class	泛型定義 public HashMap<String,Book> booksMap; public class Book{               public int id;               public String name; } HashMap<String, Book> booksMap = gson.fromJson(result, new TypeToken<HashMap<String, Book>>() { }.getType());   Gson gson = new Gson(); HashMap<String, Book> booksMap = new HashMap<>(); booksMap.put("book1", new Book(1, "Java基礎開發")); booksMap.put("book2", new Book(2, "Java設計模式")); booksMap.put("book3", new Book(3, "Java算法")); String dataStr = gson.toJson(booksMap); System.out.println("dataStr:" + dataStr); HashMap<String, Book> booksMap2 = gson.fromJson(dataStr, new TypeToken<HashMap<String, Book>>() {         }.getType());         for (Map.Entry<String, Book> entry : booksMap2.entrySet()) { System.out.println("key:"+entry.getKey() + "    value:" + entry.getValue().toString()); } 輸出結果 dataStr:{"book2":{"id":2,"name":"Java設計模式"},"book1":{"id":1,"name":"Java基礎開發"},"book3":{"id":3,"name":"Java算法"}} key:book2    value:Book{id=2, name='Java設計模式'} key:book1    value:Book{id=1, name='Java基礎開發'} key:book3    value:Book{id=3, name='Java算法'}