数据存储

数据存储

Android数据存储的几种形式

1. Internal Storage
Store private data on the device memory.
   通过mContext.getFilesDir()来得到data/data/包名/File目录

2. External Storage
Store public data on the shared external storage.

   TextView tv = (TextView) findViewById(R.id.tv_sdsize);
   File path = Environment.getExternalStorageDirectory();
   StatFs stat = new StatFs(path.getPath());
   long blockSize = stat.getBlockSize();
   long availableBlocks = stat.getAvailableBlocks();
   long sizeAvailSize = blockSize * availableBlocks;
   String str = Formatter.formatFileSize(this, sizeAvailSize);
   tv.setText(str);

3. SharedPreferences
Store private primitive data in key-value pairs.
   会在data/data/包名/shared_prefes里面去创建相应的xml文件，根节点是Map，其实内部就是将数据保存到Map集合中，
   然后将该集合中的数据写到xml文件中进行保存。

   <map>
       <string name="password">123</string>
       <boolean name="isLogin" value="false"/>
   </map>

   //获取系统的一个sharedpreference文件  名字叫sp
   SharedPreferences sp = context.getSharedPreferences("sp", Context.MODE_WORLD_READABLE+Context.MODE_WORLD_WRITEABLE);
   //创建一个编辑器 可以编辑 sp
   Editor editor = sp.edit();
   editor.putString("name", name);
   editor.putString("password", password);
   editor.putBoolean("boolean", false);
   editor.putInt("int", 8888);
   editor.putFloat("float",3.14159f);
   //注意:调用 commit 提交 数据到文件.
   editor.commit();
   //editor.clear();

4. SQLiteDatabase
Store structured data in a private database.
Android平台中嵌入了一个关系型数据库SQLite，和其他数据库不同的是SQLite存储数据时不区分类型，例如一个字段声明为Integer类型，
   我们也可以将一个字符串存入，
   一个字段声明为布尔型，我们也可以存入浮点数。除非是主键被定义为Integer，这时只能存储64位整数创建数据库的表时可以不指定数据类型，例如：

   CREATE TABLE person(id INTEGER PRIMARY KEY AUTOINCREMENT, name VARCHAR(20))  
   CREATE TABLE person(id INTEGER PRIMARY KEY AUTOINCREMENT, name)

   SQLite支持大部分标准SQL语句，增删改查语句都是通用的，分页查询语句和MySQL相同

   SELECT * FROM person LIMIT 20 OFFSET 10       
   SELECT * FROM person LIMIT 10,20    
   SELECT * FROM reading_history ORDER BY _id DESC LIMIT 3, 4
   DELETE FROM test WHERE _id IN (SELECT _id FROM test ORDER BY _id DESC LIMIT 3, 20)

   SQLite数据库的使用

   • 继承SQLiteOpenHelper

     public class NoteSQLiteOpenHelper extends SQLiteOpenHelper {
     
         /**
          * name 数据库的名称 cursorfactory 游标工厂 一般设置null 默认游标工厂 version 数据库的版本
          * 版本号从1开始的
          * 
          * @param context
          */
         public NoteSQLiteOpenHelper(Context context) {
             super(context, "note.db", null, 1);
         }
     
         /**
          * oncreate 方法 会在数据库第一创建的时候的是被调用 适合做数据库表结构的初始化
          */
         @Override
         public void onCreate(SQLiteDatabase db) {
             db.execSQL("create table account (id integer primary key autoincrement , name  varchar(20), money varchar(20) )");
         }
     
         @Override
         public void onUpgrade(SQLiteDatabase db, int arg1, int arg2) {
             db.execSQL("DROP TABLE IF EXISTS " + account);
             this.onCreate(db);
         }
     }

   • 获取

     public class NoteDao {
         private NoteSQLiteOpenHelper helper;
     
         public NoteDao(Context context) {
             helper = new NoteSQLiteOpenHelper(context);
         }
     
         /**
          * 添加一条账目信息 到数据库
          * 
          * @param name
          *            花销的名称
          * @param money
          *            金额
          */
         public void add(String name, float money) {
             SQLiteDatabase db = helper.getWritableDatabase();
             db.execSQL("insert into account (name,money) values (?,?)",
                     new Object[] { name, money });
             // 记住 关闭.
             db.close();
         }
     
         public void delete(int id) {
             SQLiteDatabase db = helper.getWritableDatabase();
             db.execSQL("delete from account where id=?", new Object[] { id });
             db.close();
         }
     
         public void update(int id, float newmoney) {
             SQLiteDatabase db = helper.getWritableDatabase();
             db.execSQL("update account set money =? where id=?", new Object[] {
                     newmoney, id });
             db.close();
         }
     
         /**
          * 返回数据库所有的条目
          * 
          * @return
          */
         public List<NoteBean> findAll() {
             // 得到可读的数据库
             SQLiteDatabase db = helper.getReadableDatabase();
             List<NoteBean> noteBeans = new ArrayList<NoteBean>();
             // 获取到数据库查询的结果游标
             Cursor cursor = db.rawQuery("select id,money,name from account ", null);
             while (cursor.moveToNext()) {
                 int id = cursor.getInt(cursor.getColumnIndex("id"));
                 String name = cursor.getString(cursor.getColumnIndex("name"));
                 float money = cursor.getFloat(cursor.getColumnIndex("money"));
                 NoteBean bean = new NoteBean(id, money, name);
                 noteBeans.add(bean);
                 bean = null;
             }
     
             db.close();
             return noteBeans;
         }
     
         /**
          * 模拟一个转账的操作. 使用数据库的事务
          * 
          * @throws Exception
          */
         public void testTransaction() throws Exception {
             // 得到可写的数据库
             SQLiteDatabase db = helper.getWritableDatabase();
             db.beginTransaction(); // 开始事务
             try {
                 db.execSQL("update account set money = money - 5 where id=? ",
                         new String[] { "2" });
                 db.execSQL("update account set money = money + 5 where id=? ",
                         new String[] { "3" });
                 db.setTransactionSuccessful();
             } catch (Exception e) {
                 // TODO: handle exception
             } finally {
                 db.endTransaction();//关闭事务.
                 db.close();
             }
         }
     }

     public class NoteDao2 {
         private NoteSQLiteOpenHelper helper;
     
         public NoteDao2(Context context) {
             helper = new NoteSQLiteOpenHelper(context);
         }
     
         /**
          * 添加一条账目信息 到数据库
          * 
          * @param name
          *            花销的名称
          * @param money
          *            金额
          * 
          * @return true 插入成功 false 失败
          */
         public boolean add(String name, float money) {
             SQLiteDatabase db = helper.getWritableDatabase();
             ContentValues values = new ContentValues();
             values.put("name", name);
             values.put("money", money);
             long rawid = db.insert("account", null, values);
             db.close();
             if (rawid > 0) {
                 return true;
             } else {
                 return false;
             }
         }
     
         public boolean delete(int id) {
             SQLiteDatabase db = helper.getWritableDatabase();
             int result = db.delete("account", "id=?", new String[] { id + "" });
             db.close();
             if (result > 0) {
                 return true;
             } else {
                 return false;
             }
         }
     
         public boolean update(int id, float newmoney) {
             SQLiteDatabase db = helper.getWritableDatabase();
             ContentValues values = new ContentValues();
             values.put("id", id);
             values.put("money", newmoney);
             int result = db.update("account", values, "id=?", new String[] { id
                     + "" });
             db.close();
             if (result > 0) {
                 return true;
             } else {
                 return false;
             }
         }
     
         /**
          * 返回数据库所有的条目
          * 
          * @return
          */
         public List<NoteBean> findAll() {
             // 得到可读的数据库
             SQLiteDatabase db = helper.getReadableDatabase();
             List<NoteBean> noteBeans = new ArrayList<NoteBean>();
             Cursor cursor = db.query("account", new String[] { "id", "name",
                     "money" }, null, null, null, null, null);
             while (cursor.moveToNext()) {
                 int id = cursor.getInt(cursor.getColumnIndex("id"));
                 String name = cursor.getString(cursor.getColumnIndex("name"));
                 float money = cursor.getFloat(cursor.getColumnIndex("money"));
                 NoteBean bean = new NoteBean(id, money, name);
                 noteBeans.add(bean);
                 bean = null;
             }
             db.close();
             return noteBeans;
         }
     }

5. Network
Store data on the web with your own network server.

清除缓存&清除数据

清除数据会清除`/data/data/`包名中的所有文件     
清楚缓存会清楚`getCacheDir()`目录下的内容，也就是`/data/data/<当前应用程序包名>/cache/`

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• Good Luck!