数据持久化

1.简介

数据持久化就是指将内存中的瞬时数据保存到存储设备中。
在Android系统中主要提供了三种方式用于简单的实现数据持久化功能。

• 文件存储
• SharedPreferences存储
• SQLite数据库存储

  2.文件存储

1. 写文件

openFileOutput

@Override
public  void onDestroy() {
    super.onDestroy();
    String InputText = edit.getText().toString();
    this.save(InputText);
}
public void save(String inputText) {
    FileOutputStream out = null;
    BufferedWriter writer = null;
    try {
        out = openFileOutput("data", Context.MODE_PRIVATE);
        writer = new BufferedWriter(new OutputStreamWriter(out));
        Log.i("ok", "111");
        writer.write(inputText);
    }catch(IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if(writer != null) {
                writer.close();
            }
        }catch(IOException e) {
            e.printStackTrace();
        }
    }
}

2. 读文件

openFileOutput

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();
            }
        }
    }
    Log.i("info", content.toString());
    return content.toString();
}

文件存储的方式不适合保存一些较为复杂的文本数据。

学习Android持久化技术遇到的问题

3. SharedPreferences存储

将数据以键值对形式存在文件中(XML文件)。

1. 存数据

saveData.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        SharedPreferences.Editor editor = getSharedPreferences("data", MODE_PRIVATE).edit();
        editor.putString("name", "tom");
        editor.putInt("age", 28);
        editor.putBoolean("married", false);
        editor.apply();
    }
});

2. 取数据

restoreData.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        SharedPreferences perf = getSharedPreferences("data", MODE_PRIVATE);
        String name = perf.getString("name", "");
        int age = perf.getInt("age", 0);
        boolean married = perf.getBoolean("married", false);
        Log.d("MainActivity", "name is " + name);
        Log.d("MainActivity", "age is " + age);
        Log.d("MainActivity", "married is " + married);
    }
});

SharedPreferences存储

4. SQLite数据库存储

SQLiteOpenHelper抽象类
继承类中重写onCreate和onUpgrade方法
执行getReadableDataBase或getWritableDataBase创建数据库，此时重写的onCreate方法会执行
数据库文件位于/data/data//databases/目录下

MyDatabaseHelper.java

package com.example.databasetest;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.Toast;
public class MyDatabaseHelper extends SQLiteOpenHelper {
    public  static final String CREARE_BOOK = "create table Book ("
            + "id integer primary key autoincrement, "
            + "author text, "
            + "price real, "
            + "pages integer, "
            + "name text)";
    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) {
        db.execSQL(CREARE_BOOK);
        Toast.makeText(mContext, "Created successed", Toast.LENGTH_SHORT).show();
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVsersion) {
    }
}

MainActivity.java

package com.example.databasetest;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
    private  MyDatabaseHelper dbHelper;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dbHelper = new MyDatabaseHelper(this, "BookStore.db", null, 1);
        Button button = findViewById(R.id.create_database);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // 创建数据库执行onCreate方法
                dbHelper.getWritableDatabase();   
            }
        });
    }
}

增: db.put
删: db.delete
改：db.update
查: db.query

SQLite数据库存储

5. 使用LitePal操作数据库

todo