Android 数据持久化的简单使用

Android 数据持久化主要有三种：
文件，SharePreference, 数据库

文件存储

不对存储的文件进行任何格式化的处理，所有数据都是直接存入文件中，因此比较适合保存一些简单的文本数据和二进制数据

• 存入读取：

 FileOutputStream out = null;
        BufferedWriter writer = null;
        try {
            out = openFileOutput("data", Context.MODE_PRIVATE);
            /*主要有两种模式：
            1.MODE_PRIVATE:当指定同名文件时，自动覆盖
            2.MODE_APPEND：已存在就追加
            */
            writer = new BufferedWriter(new OutputStreamWriter(out));//得到一个OutputStreamwriter对象
            //再构建一个bufferedWriter
            writer.write(inputText);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (writer != null) {
                    writer.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

public String load() {
        FileInputStream in = null;
        BufferedReader reader = null;
        StringBuilder content = new StringBuilder();
        try {
            in = openFileInput("data");
            reader = new BufferedReader(new InputStreamReader(in));
            String line = "";
            while ((line = reader.readLine()) != null) {
                content.append(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return content.toString();
    }

• java知识小补点
  String,StringBuffer,StringBuilder区别：
  String 利用字符数组保存数据，源码中有final修饰，因而实际上是不可变的
  当String a = "b"+"c"时，实际上创建了两个对象
  而StringBuffer和StringBuilder都只创建一个实例
  SringBuffer 线程安全，而StringBuider线程不安全
  三者的效率，String < StringBuffer < StringBuilder

SharedPreference 存储

这是一种应用私有的，利用键值对存储数据的方式，支持对不同类型的数据类型进行储存，存为xml格式，方便小巧效率高
存入：

SharedPreferences.Editor editor = getSharedPreferences("data", MODE_PRIVATE).edit();
                editor.putString("name", "Tom");
                editor.putInt("age", 28);
                editor.putBoolean("married", false);
                editor.apply();

取出：

SharedPreferences pref = getSharedPreferences("data", MODE_PRIVATE);
                String name = pref.getString("name", "");
                int age = pref.getInt("age", 0);
                boolean married = pref.getBoolean("married", false);

这是context类中的getSharedPreferences（）方法的模式
2.Activity类中的getPreferences()方法
3.PrefenceMangager类中的getDefaultSharedPreferences
e.g.
读取

 SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(WeatherActivity.this);

存入

  SharedPreferences.Editor editor =PreferenceManager.getDefaultSharedPreferences(WeatherActivity.this).edit();
editor.putString("weather",mresponse);
editor.apply();

• SQLite数据库
  SQLite 是一个 Android 自带的轻量型的数据库。
• 利用数据库的 DBHelper 建库建表
• e.g.

 class AreaDBHelper extends SQLiteOpenHelper {
 //定义 private static 语句 用于直观呈现数据库所需要的参数
    private static final int DB_VERSION = 1;
    private static final String DB_NAME = "area.db";
    private static final String PROVINCE_NAME = "province";
    private static final String CITY_NAME = "city";
    private static final String COUNTY_NAME = "county";

     AreaDBHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    //构造方法需要提供四个参数，context, 库名，cursor，版本号

    }

    //建表建库，sql 语句要特别留意！
    @Override
    public void onCreate(SQLiteDatabase db) {
        String sql1 = "create table if not exists "
                + PROVINCE_NAME
                + " (Id integer primary key autoincrement,"
                + "ProvinceName text,"
                + "ProvinceId integer)";
        String sql2 = "create table if not exists "
                + CITY_NAME
                + " (Id integer primary key autoincrement,"
                + "CityName text,"
                +"CityId integer,"
                +"ProvinceId integer)";
         String sql3 = "create table if not exists "
                 +COUNTY_NAME
                 + " (Id integer primary key autoincrement,"
                 + "ProvinceId integer,"
                 + "CityId integer,"
                 + "CountyName text,"
                 + "WeatherId text)";
        db.execSQL(sql1);
        db.execSQL(sql2);
        db.execSQL(sql3);
    }

 //数据库的升级方法
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        String sql1 = "DROP TABLE IF EXISTS" + PROVINCE_NAME;
        String sql2 = "DROP TABLE IF EXISTS" + CITY_NAME;
        String sql3 = "DROP TABLE IF EXISTS" + COUNTY_NAME;
        db.execSQL(sql1);
        db.execSQL(sql2);
        db.execSQL(sql3);
        onCreate(db);
    }

}

• 利用 DBDao 类对有关数据库进行操作

public class DBDao {
    private Context mcontext;
    private AreaDBHelper areaDBHelper;
    /*
    *自定义构造器的做法，将传入的 context 赋值给mcontext
     */
    public DBDao(Context context){
        this.mcontext = context;
        areaDBHelper = new AreaDBHelper(context);
    }

    /**
     * 将省级列表存入数据库
     * @param provinceList
     */
    public void initProvinceTable(List<Province> provinceList){
        SQLiteDatabase db = areaDBHelper.getWritableDatabase();
        ContentValues values = new ContentValues();
        for(Province province:provinceList){
            values.put("ProvinceName",province.getProvinceName());
            values.put("ProvinceId",province.getProvinceCode());
            db.insert("province",null,values);
            values.clear();
        }
        db.close();
    }
    /**
    *在数据库中查询省级
    */
    
   public List<Province> queryProvince(){
       List<Province> provinceList = new ArrayList<>();
       SQLiteDatabase db = areaDBHelper.getWritableDatabase();
       Cursor cursor = db.query("province",null,null,null,null,null,null);
       if(cursor.moveToFirst()){
           do{
               String provinceName = cursor.getString(cursor.getColumnIndex("ProvinceName"));
               int provinceCode = cursor.getInt(cursor.getColumnIndex("ProvinceId"));
               Province province = new Province();
               province.setProvinceCode(provinceCode);
               province.setProvinceName(provinceName);
               provinceList.add(province);
           }while (cursor.moveToNext());
       }
        cursor.close();
       db.close();
       return provinceList;
   }
  }
}

• 数据库是耗资源的操作，应避免频繁访问，用完记得关闭连接