數(shù)據(jù)持久化就是指將那些內存中的瞬時數(shù)據(jù)保存到存儲設備中按脚，保證即使在手機或電腦關機的情況下，這些數(shù)據(jù)也不會丟失敦冬。保存在內存中的數(shù)據(jù)是處于瞬時狀態(tài)的辅搬，而保存在存儲設備中的數(shù)據(jù)是處于持久狀態(tài)的，持久化技術則提供了一種機制可以讓數(shù)據(jù)在瞬時狀態(tài)和持久狀態(tài)之間進行轉換

文件存儲

文件存儲流程:

• 寫：使用openFileOutput(""脖旱，Context.MODE_PRIVATE)方法返回一個FileOutputStream堪遂，得到此對象就可以使用Java流的方式將數(shù)據(jù)寫入到文件中了。
• 讀：使用openFileInput("")方法方輝一個FileInputStream對象萌庆，得到這個通過Java流的方式就可以將數(shù)據(jù)讀出來了溶褪。

public class MainActivity extends AppCompatActivity {
//布局文件很簡單，只需要一個EditText組件
    private EditText edit;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        edit = (EditText) findViewById(R.id.edit);
//活動創(chuàng)建時從文件中讀取數(shù)據(jù)
        String inputText = load();
        if (!TextUtils.isEmpty(inputText)){//檢測傳入的字符串是否為null或為空字符串
            edit.setSelection(inputText.length());//設置光標位于末尾
            edit.setText(inputText);
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        String inputText = edit.getText().toString();
//活動銷毀時寫入文件
        save(inputText);
    }

    public void save(String inputText){
        FileOutputStream out = null;
        BufferedWriter writer = null;
        try {
//openFileOutput("data", Context.MODE_PRIVATE)方法第一個參數(shù)為文件名践险，第二個參數(shù)為文件操作模式：MODE_PRIVATE指同名文件覆蓋猿妈，MODE_APPEND指同名文件追加。
//所有文件默認存儲在這條路徑下/data/data/<package name>/files/data
            out = openFileOutput("data", Context.MODE_PRIVATE);
//java流操作
            writer = new BufferedWriter(new OutputStreamWriter(out));
            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{
//openFileInput("data"只有一個參數(shù)"data"巍虫，即文件名
            in = openFileInput("data");
            reader = new BufferedReader(new InputStreamReader(in));
            String line = "";
//reader.readLine()讀取一個文本行彭则，此方法是按行讀即一行一行的讀
            while ((line = reader.readLine())!= null){
                content.append(line);
            }
        }catch (IOException e){
            e.printStackTrace();
        }
        return content.toString();
    }
}

SharedPreferences

SharedPreferences存儲流程:

• 獲取SharedPrecences對象：
  方法一：Context類的getSharedPreferences()
  方法二：Activity類的getPreference()
  方法三：PreferenceManager類的getDefultSharedPreferences()

• 調用SharedPreferences對象的edit()方法來獲取一個SharedPrefences.Editor對象

• 使用putXX()向Editor對象添加數(shù)據(jù)，或用getXX()方法得到數(shù)據(jù)

• 僅在添加數(shù)據(jù)時占遥，調用apply()方法將添加的數(shù)據(jù)提交俯抖，從而完成數(shù)據(jù)存儲操作

//布局也很簡單，加兩個按鈕就行了
public class MainActivity extends AppCompatActivity implements View.OnClickListener{
    private String TAG = "MainActivity";

    private Button save_btn;
    private Button restore_btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        save_btn = (Button) findViewById(R.id.save_btn);
        restore_btn = (Button) findViewById(R.id.restore_data);
        save_btn.setOnClickListener(this);
        restore_btn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.save_btn:
//getSharedPreferences("data",MODE_PRIVATE)方法參數(shù)：
//"data"為文件名瓦胎，MODE_PRIVATE為唯一參數(shù)表示只有當前應用才可以對這個SharedPrefences文件進行讀寫芬萍。
                SharedPreferences.Editor editor = getSharedPreferences("data",MODE_PRIVATE).edit();
                editor.putString("name","Tom");
                editor.putInt("age",23);
                editor.putBoolean("married",false);
                editor.apply();
                break;
            case R.id.restore_data:
                SharedPreferences preferences = getSharedPreferences("data",MODE_PRIVATE);
                String name = preferences.getString("name","data is bull");
                int age = preferences.getInt("age",-1);
                boolean married = preferences.getBoolean("married",false);
                Log.d(TAG,"name is " + name);
                Log.d(TAG, "age is " + age);
                Log.d(TAG, "married is " + married);
                break;
            default:
                break;
        }
    }
}

SQLite數(shù)據(jù)庫存儲

SQLite是Android內置的數(shù)據(jù)庫

• 定義一個SQLiteOpenHelper幫助類(此類的實例有兩個方法getReadableDatabase()、getWritableDatabase()這兩個方法都可以打開數(shù)據(jù)庫凛捏，不同的是當數(shù)據(jù)庫不可寫入時担忧，getReadableDatabase()方法返回的對象將以只讀的方式去打開數(shù)據(jù)庫芹缔，而getWritableDatabase()方法則會出現(xiàn)異常)
• 在Activity中實例化SQLiteOpenHelper對象
• 調用getReadableDatabase()或getWritableDatabase()方法

//定義一個SQLiteOpenHelper
public class MyDatabaseHelper extends SQLiteOpenHelper{
//SQL語句
    public static final String CREATE_BOOK = "create table Book ("
            +"id integer primary key autoincrement, "
            +"author text, "
            +"price real, "
            +"pages integer, "
            +"name text)";
    public String DROP_BOOK = "drop table if exists Book";
    public String CREATE_CATEGORY = "create table Category ("
            +"id integer primary key autoincrement, "
            +"category_name text, "
            +"category_code integer)";
    public String DROP_CATEGORY = "drop table if exists Category";

    private Context mContext;
    public MyDatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
        mContext = context;
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
//執(zhí)行SQL語句坯癣，創(chuàng)建表
        db.execSQL(CREATE_BOOK);
        db.execSQL(CREATE_CATEGORY);
        Toast.makeText(mContext,"Create succeeded",Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {//此方法用于更新數(shù)據(jù)庫
//執(zhí)行SQL語句，刪除表
        db.execSQL(DROP_BOOK);
        db.execSQL(DROP_CATEGORY);
        onCreate(db);
    }
}

//布局就是添加5個相應的button
public class MainActivity extends AppCompatActivity {
//使用ButterKnife
    @BindView(R.id.create_database)
    Button createDatabase;
    @BindView(R.id.add_data)
    Button addData;
    @BindView(R.id.updata_data)
    Button updataData;

    private MyDatabaseHelper databaseHelper;
    private String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
//實例化MyDatabaseHelper的三個參數(shù)：
//context最欠；數(shù)據(jù)庫名示罗；允許我們在查詢數(shù)據(jù)的時候返回一個自定義的Cursor惩猫，一般都是null；當前數(shù)據(jù)庫版本號蚜点，可用于對數(shù)據(jù)庫進行升級
        databaseHelper = new MyDatabaseHelper(this,"BookStore.db",null,3);
        ButterKnife.bind(this);
    }

    @OnClick({R.id.create_database,R.id.add_data,R.id.updata_data,R.id.delete_data,R.id.select_data})
    public void onClick(View v){
        switch (v.getId()){
            case R.id.create_database:
                databaseHelper.getWritableDatabase();
                break;
            case R.id.add_data:
                SQLiteDatabase db = databaseHelper.getWritableDatabase();
//                使用ContentValues傳入相應數(shù)據(jù)
                ContentValues values = new ContentValues();
//                開始組裝第一條數(shù)據(jù)
                values.put("name","The Da Vinci Code");
                values.put("author","Dan Brown");
                values.put("pages",454);
                values.put("price",16.96);
                db.insert("Book",null,values);//插入第一條數(shù)據(jù)
                values.clear();
                //開始組裝第二條數(shù)據(jù)
                values.put("name","The Lost Symbol");
                values.put("author","Dan Brown");
                values.put("pages",510);
                values.put("price",19.95);
                db.insert("Book",null,values);//插入第二條數(shù)據(jù)
                break;
            case R.id.updata_data:
                SQLiteDatabase dbUpdata = databaseHelper.getWritableDatabase();
                ContentValues valuesUpdata = new ContentValues();
                valuesUpdata.put("price",10.99);
                dbUpdata.update("Book",valuesUpdata,"name = ?",new String[]{
                    "The Da Vinci Code"
                 });
                break;
            case R.id.delete_data:
                SQLiteDatabase dbDelete = databaseHelper.getWritableDatabase();
                dbDelete.delete("Book","pages > ?", new String[]{"500"});
                break;
            case R.id.select_data:
                SQLiteDatabase dbSelect = databaseHelper.getWritableDatabase();
                //查詢Book表中所有的數(shù)據(jù)
                Cursor cursor = dbSelect.query("Book",null,null,null,null,null,null);
                if (cursor.moveToFirst()){
                    do {
                        //遍歷Cursor對象,取出數(shù)據(jù)并打印
                        String name = cursor.getString(cursor.getColumnIndex("name"));
                        String author = cursor.getString(cursor.getColumnIndex("author"));
                        int pages = cursor.getInt(cursor.getColumnIndex("pages"));
                        double price = cursor.getDouble(cursor.getColumnIndex("price"));
                        Log.d(TAG, "book name is " + name);
                        Log.d(TAG, "book author is " + author);
                        Log.d(TAG, "book price is " + price);
                    }while (cursor.moveToNext());
                }
                cursor.close();
                break;
        }
    }
}

注：須知數(shù)據(jù)庫創(chuàng)建一次后轧房，MyDatabaseHelper就不會再次調用onCreate()方法了。這是就需要調用upgrade()绍绘，只要版本號比之前的大就可以更新了奶镶。