安卓学习笔记（六）数据存储

数据存储

Android常用的数据持久化方式：文件存储、SharedPreferences存储以及数据库存储。

文件存储

文件保存的数据默认保存在/data/data/packagename/files/中，可通过Android Studio菜单中的View–>Tool Windows–>Device File Explorer中查看数据。

MainActivity：

public class MainActivity extends AppCompatActivity {

    private EditText edit;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        edit = findViewById(R.id.edit);
        String inputText = load();
        if(!TextUtils.isEmpty(inputText)){//判断inputText是否为null或者空字符串
            edit.setText(inputText);
            edit.setSelection(inputText.length());
            Toast.makeText(this,"Restoring succeeded",Toast.LENGTH_SHORT).show();
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        String inputText = edit.getText().toString();
        save(inputText);
    }

    public void save(String inputText){//将数据存储到文件
        FileOutputStream out = null;
        BufferedWriter writer = null;
        try {
            out = openFileOutput("data", Context.MODE_PRIVATE);
            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(){//获取文件中的数据
        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();
    }
}

SharedPreferences存储

SharedPreferences存储的数据默认保存在/data/data/packagename/shared_prefs/中，可以使用File Explorer查看数据。
获取SharedPreferences有三种方法：

（1）Context类中的getSharedPreferences()方法

SharedPreferences pref = getSharedPreferences("data",MODE_PRIVATE);//参数指定存放数据文件的名称和操作模式

（2）Activity类中的getPreferences()方法

SharedPreferences pref = getPreferences(MODE_PRIVATE);//参数指定操作模式

（3）PreferenceManager类中的getDefaultSharedPreferences()方法

SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(this);

MainActivity：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button saveData = findViewById(R.id.save_data);
        saveData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
            	//发送数据到SharedPreferences中
                SharedPreferences.Editor editor = getSharedPreferences("data",MODE_PRIVATE).edit();
                editor.putString("name","youyu4");//第一个参数为键，第二个参数为数据的值
                editor.putInt("age",22);
                editor.putBoolean("married",false);
                editor.apply();
            }
        });
        Button restoreData = findViewById(R.id.restore_data);
        restoreData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
           		//从SharedPreferences中读取数据
                SharedPreferences pref = getSharedPreferences("data",MODE_PRIVATE);
                String name = pref.getString("name","");//第一个参数为键，第二个参数为默认值
                int age = pref.getInt("age",0);
                boolean married = pref.getBoolean("married",false);
                Log.d("MainActivity", "name is " + name);
                Log.d("MainActivity", "age is " + age);
                Log.d("MainActivity", "married is " + married);
            }
        });
    }
}

使用SharedPreferences实现记住账号密码功能

打开之前使用Broadcast实现的登录界面项目，在activity_login中添加一个CheckBox复选框控件，用户可以通过点击的方式来选择是否要记住账号密码。

LoginActivity：

public class LoginActivity extends BaseActivity {

    private EditText accountEdit;
    private EditText passwordEdit;
    private Button login;
    private SharedPreferences.Editor editor;
    private SharedPreferences pref;
    private CheckBox rememberPass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        accountEdit = findViewById(R.id.account);
        passwordEdit = findViewById(R.id.password);
        rememberPass = findViewById(R.id.remember_pass);
        pref = getSharedPreferences("data",MODE_PRIVATE);
        editor = pref.edit();
        Boolean isRemember = pref.getBoolean("isRemember",false);
        if(isRemember){
        	//将账号和密码都设置到文本框中
            accountEdit.setText(pref.getString("account",""));
            passwordEdit.setText(pref.getString("password",""));
            rememberPass.setChecked(true);
        }
        login = findViewById(R.id.login);
        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String account = accountEdit.getText().toString();
                String password = passwordEdit.getText().toString();
                if(account.equals("admin")&&password.equals("123456")){
                    if (rememberPass.isChecked()){//检查复选框是否被选中
                        editor.putString("account",account);
                        editor.putString("password",password);
                        editor.putBoolean("isRemember",true);
                    }else{
                        editor.clear();
                    }
                    editor.apply();
                    Intent intent = new Intent(LoginActivity.this,MainActivity.class);
                    startActivity(intent);
                    finish();
                }else{
                    Toast.makeText(LoginActivity.this,"account or password is invalid",Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

SQLite数据库存储

adb shell

存储的文件默认保存在/data/data/packagename/databases/中，无法使用File Explorer查看数据库中的数据，因此使用adb shell来查看数据。首先将platform-tools目录配置进系统环境变量中，然后打开命令行界面输入"adb shell"，若符号为"$“则证明为普通管理员，无法查看数据库，输入命令"su"后符号变为”#“并成为超级管理员（注意安卓虚拟机7.0级7.0以上版本无法设置超级管理员权限，因此使用Android7.0以下的虚拟器运行项目）然后输入"cd"命令进入/data/data/packagename/databases/数据库目录下输入"ls"可以看到BookStore.db，然后输入"sqlite3 BookStore.db"进入数据库，输入”.table"命令可以看数据库有哪些表，输入".schema"命令可以查看建表语句，输入"select * from Book;"命令可以查询Book表的数据。

使用SQLiteDatabase或SQL操作数据库

新建MyDatabaseHelper类继承自SQLiteOpenHelper：

public class MyDatabaseHelper extends SQLiteOpenHelper {

    public static final String CREATE_BOOK = "create table Book ("
            +"id integer primary key autoincrement,"
            +"author text,"
            +"price real,"
            +"pages integer,"
            +"name text)";

    public static final String CREATE_CATEGORY = "create table Category ("
            +"id integer primary key autoincrement,"
            +"category_name text,"
            +"category_code integer)";

    private Context mContext;

    public MyDatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
        mContext = context;
    }
	
	//新建数据库
    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_BOOK);
        db.execSQL(CREATE_CATEGORY);
        //Toast.makeText(mContext,"Create succeeded",Toast.LENGTH_SHORT).show();
    }

	//更新数据库
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("drop table if exists Book");
        db.execSQL("drop table if exists Category");
        onCreate(db);
    }
}

新建布局包含增删改查四个按钮，并修改MainActivity.java：

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private MyDatabaseHelper dbHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dbHelper = new MyDatabaseHelper(this,"BookStore.db",null,7);
        Button createDatabase = findViewById(R.id.create_database);
        Button addData = findViewById(R.id.add_data);
        Button updateData = findViewById(R.id.update_data);
        Button deleteData = findViewById(R.id.delete_data);
        Button queryData = findViewById(R.id.query_data);

        //新建数据库
        createDatabase.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dbHelper.getWritableDatabase();
            }
        });

        //增
        addData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db = dbHelper.getWritableDatabase();
                //SQLiteDatabase操作
//                ContentValues values = new ContentValues();
//                values.put("name","aaa");
//                values.put("author","qmt");
//                values.put("pages",666);
//                values.put("price",233.33);
//                db.insert("Book",null,values);
//                values.clear();
//                values.put("name","bbb");
//                values.put("author","sxm");
//                values.put("pages",777);
//                values.put("price",2333.33);
//                db.insert("Book",null,values);
                //SQL操作
                db.execSQL("insert into Book (name,author,pages,price)values(?,?,?,?)",new String[]{"aaa","qmt","666","233.33"});
                db.execSQL("insert into Book (name,author,pages,price)values(?,?,?,?)",new String[]{"bbb","sxm","777","2333.33"});
            }
        });

        //改
        updateData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db = dbHelper.getWritableDatabase();
                //SQLiteDatabase操作
//                ContentValues values = new ContentValues();
//                values.put("price",10.99);
//                db.update("Book",values,"name = ?",new String[] {"aaa"});
                //SQL操作
                db.execSQL("update Book set price = ? where name = ?",new String[] {"10.99","aaa"});
            }
        });

        //删
        deleteData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db = dbHelper.getWritableDatabase();
                //SQLiteDatabase操作
//                db.delete("Book","name = ?",new String[] {"bbb"});
                //SQL操作
                db.execSQL("delete from Book where name = ?",new String[] {"bbb"});
            }
        });

        //查
        queryData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SQLiteDatabase db = dbHelper.getWritableDatabase();
                //SQLiteDatabase操作
//                Cursor cursor = db.query("Book",null,null,null,null,null,null);
                //SQL操作
                Cursor cursor = db.rawQuery("select * from Book",null);
                if(cursor.moveToFirst()){
                    do {
                        //遍历Cursor对象，取出数据并打印
                        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, "name: " + name + ",author: " + author + ",pages:" + pages + ",price:" + price);
                    }while(cursor.moveToNext());
                }
                cursor.close();
            }
        });
    }
}

使用LitePal操作数据库

首先在build.gradle中添加依赖库：

implementation 'org.litepal.android:java:3.0.0'

然后在app/src/main中新建assets目录并新建litepal.xml文件，dbname是数据库名，version是数据库版本号，list为指定的映射模型,，添加Book类和Category类：

<litepal>
    <dbname value="BookStore" />
    <version value="2" />
    <list>
        <mapping class="com.example.youyu4.litepaltest.Book">mapping>
        <mapping class="com.example.youyu4.litepaltest.Category">mapping>
    list>
litepal>

在AndroidManifest中配置LitePalApplication：

    <application
        android:name="org.litepal.LitePalApplication"
		...
		...>
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            intent-filter>
        activity>
    application>

因为LitePal采取的是面向对象映射（ORM）模式，因此定义Book类：

public class Book extends LitePalSupport {

    private int id;
    private String author;
    private String name;
    private double price;
    private int pages;
    private String press;
    
	//Getter和Setter
	...
}

Category类：

public class Category {
    private int id;
    private String categoryName;
    private int categoryCode;
    public void setId(int id) {
        this.id = id;
    }
    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }
    public void setCategoryCode(int categoryCode) {
        this.categoryCode = categoryCode;
    }
}

MainActivity.java：

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button createDatabase = findViewById(R.id.create_database);
        Button addData = findViewById(R.id.add_data);
        Button updateData = findViewById(R.id.update_data);
        Button deleteData = findViewById(R.id.delete_data);
        Button queryData = findViewById(R.id.query_data);

        //创建数据库
        createDatabase.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                LitePal.getDatabase();
            }
        });

        //使用LitePal进行数据操作
        // 增
        addData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Book book = new Book();
                book.setName("aaa");
                book.setAuthor("qmt");
                book.setPages(666);
                book.setPrice(233.33);
                book.setPress("Unknow");
                book.save();
            }
        });

        //改
        updateData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Book book = new Book();
                book.setPrice(14.95);
                book.setPress("Anchor");
//                book.setToDefault("pages");//更新数据为默认值
                book.updateAll("name = ?","aaa");
            }
        });

        //删
        deleteData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                LitePal.deleteAll(Book.class,"price < ?","15");
            }
        });

        //查
        queryData.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                List<Book> books = LitePal.findAll(Book.class);
                for(Book book: books){
                    Log.d(TAG, "name: " + book.getName() + ",author: " + book.getAuthor() + ",pages:" + book.getPages() + ",price:" + book.getPrice() + ",press:" + book.getPress());
                }
            }
        });
    }
}