基础复习——利用Room简化数据库操作——重点

妖狐艹你老母 2023-09-28 21:24 10阅读 0赞

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使用数据库帮助器编码的时候，开发者每次都得手工实现以下代码逻辑：

(1)重写数据库帮助器的onCreate方法，添加该表的建表语句；(2)在插入记录之时，必须将数据实例的属性值逐一赋给该表的各字段；(3)在查询记录之时，必须遍历结果集游标，把各字段值逐一赋给数据实例；(4)每次读写操作之前，都要先开启数据库连接；读写操作之后，又要关闭数据库连接；

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实体类 BookInfo：

package com.example.myapplication.entity;
    
    import androidx.annotation.NonNull;
    import androidx.room.Entity;
    import androidx.room.PrimaryKey;
    
    //书籍信息
    @Entity
    public class BookInfo
    {
        @PrimaryKey // 该字段是主键，不能重复
        @NonNull // 主键必须是非空字段
        private String name; // 书籍名称
        
        private String author; // 作者
        private String press; // 出版社
        private double price; // 价格
    
        public void setName(String name) {
            this.name = name;
        }
    
        public String getName() {
            return this.name;
        }
    
        public void setAuthor(String author) {
            this.author = author;
        }
    
        public String getAuthor() {
            return this.author;
        }
    
        public void setPress(String press) {
            this.press = press;
        }
    
        public String getPress() {
            return this.press;
        }
    
        public void setPrice(double price) {
            this.price = price;
        }
    
        public double getPrice() {
            return this.price;
        }
    
    }

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持久化类BookDao：

package com.example.myapplication.dao;
    
    import androidx.room.Dao;
    import androidx.room.Delete;
    import androidx.room.Insert;
    import androidx.room.OnConflictStrategy;
    import androidx.room.Query;
    import androidx.room.Update;
    import com.example.myapplication.entity.BookInfo;
    import java.util.List;
    
    @Dao
    public interface BookDao
    {
        @Query("SELECT * FROM BookInfo") // 设置查询语句
        List<BookInfo> queryAllBook(); // 加载所有书籍信息
    
        @Query("SELECT * FROM BookInfo WHERE name = :name") // 设置带条件的查询语句
        BookInfo queryBookByName(String name); // 根据名字加载书籍
    
        @Insert(onConflict = OnConflictStrategy.REPLACE) // 记录重复时替换原记录
        void insertOneBook(BookInfo book); // 插入一条书籍信息
    
        @Insert
        void insertBookList(List<BookInfo> bookList); // 插入多条书籍信息
    
        @Update(onConflict = OnConflictStrategy.REPLACE)// 出现重复记录时替换原记录
        int updateBook(BookInfo book); // 更新书籍信息
    
        @Delete
        void deleteBook(BookInfo book); // 删除书籍信息
    
        @Query("DELETE FROM BookInfo WHERE 1=1") // 设置删除语句
        void deleteAllBook(); // 删除所有书籍信息
    }

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数据库类 bookDatabase：

package com.example.myapplication.database;
    
    import androidx.room.Database;
    import androidx.room.RoomDatabase;
    import com.example.myapplication.dao.BookDao;
    import com.example.myapplication.entity.BookInfo;
    
    //entities表示该数据库有哪些表，version表示数据库的版本号
    //exportSchema表示是否导出数据库信息的json串，建议设为false，若设为true还需指定json文件的保存路径
    @Database(entities = {BookInfo.class},version = 1, exportSchema = false)
    public abstract class BookDatabase extends RoomDatabase
    {
        // 获取该数据库中某张表的持久化对象
        public abstract BookDao bookDao();
    }

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在自定义的Application类中声明书籍数据库的唯一实例:

package com.example.myapplication;
    
    import android.app.Application;
    import android.util.Log;
    import androidx.room.Room;
    import com.example.myapplication.database.BookDatabase;
    import java.util.HashMap;
    
    public class MainApplication extends Application
    {
        private final static String TAG = "MainApplication";
    
        private static MainApplication mApp; // 声明一个当前应用的静态实例
    
        // 声明一个公共的信息映射对象，可当作全局变量使用
        public HashMap<String, String> infoMap = new HashMap<String, String>();
        public static int goodsCount = 0;
    
        private BookDatabase bookDatabase; // 声明一个书籍数据库对象
    
        // 利用单例模式获取当前应用的唯一实例
        public static MainApplication getInstance()
        {
            return mApp;
        }
    
        @Override
        public void onCreate()
        {
            super.onCreate();
            Log.d(TAG, "onCreate");
    
            mApp = this; // 在打开应用时对静态的应用实例赋值
    
            // 构建书籍数据库的实例
            bookDatabase = Room.databaseBuilder(mApp, BookDatabase.class,"BookInfo")
                    .addMigrations() // 允许迁移数据库(发生数据库变更时，Room默认删除原数据库再创建新数据库。如此一来原来的记录会丢失，故而要改为迁移方式以便保存原有记录)
                    .allowMainThreadQueries() // 允许在主线程中操作数据库(Room默认不能在主线程中操作数据库)
                    .build();
        }
    
        @Override
        public void onTerminate()
        {
            super.onTerminate();
            Log.d(TAG, "onTerminate");
        }
    
        // 获取书籍数据库的实例
        public BookDatabase getBookDB()
        {
            return bookDatabase;
        }
    }

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写布局：

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写代码：

package com.example.myapplication;
    
    import androidx.appcompat.app.AppCompatActivity;
    import android.os.Bundle;
    import android.text.TextUtils;
    import android.view.View;
    import android.widget.EditText;
    
    import com.example.myapplication.dao.BookDao;
    import com.example.myapplication.entity.BookInfo;
    
    public class RoomWriteActivity extends AppCompatActivity implements View.OnClickListener {
        private EditText et_name;
        private EditText et_author;
        private EditText et_press;
        private EditText et_price;
        private BookDao bookDao; // 声明一个书籍的持久化对象
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_room_write);
            et_name = findViewById(R.id.et_name);
            et_author = findViewById(R.id.et_author);
            et_press = findViewById(R.id.et_press);
            et_price = findViewById(R.id.et_price);
            findViewById(R.id.btn_save).setOnClickListener(this);
            // 从App实例中获取唯一的书籍持久化对象
            bookDao = MainApplication.getInstance().getBookDB().bookDao();
        }
    
        @Override
        public void onClick(View v) {
            if (v.getId() == R.id.btn_save) {
                String name = et_name.getText().toString();
                String author = et_author.getText().toString();
                String press = et_press.getText().toString();
                String price = et_price.getText().toString();
                if (TextUtils.isEmpty(name)) {
                    ToastUtil.show(this, "请先填写书籍名称");
                    return;
                } else if (TextUtils.isEmpty(author)) {
                    ToastUtil.show(this, "请先填写作者姓名");
                    return;
                } else if (TextUtils.isEmpty(press)) {
                    ToastUtil.show(this, "请先填写出版社名称");
                    return;
                } else if (TextUtils.isEmpty(price)) {
                    ToastUtil.show(this, "请先填写价格");
                    return;
                }
                // 以下声明一个书籍信息对象，并填写它的各字段值
                BookInfo info = new BookInfo();
                info.setName(name);
                info.setAuthor(author);
                info.setPress(press);
                info.setPrice(Double.parseDouble(price));
                bookDao.insertOneBook(info); // 往数据库插入一条书籍记录
                ToastUtil.show(this, "数据已写入Room数据库");
            }
        }
    
    }

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读布局：

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读代码：

package com.example.myapplication;
    
    import androidx.appcompat.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.TextView;
    import com.example.myapplication.dao.BookDao;
    import com.example.myapplication.entity.BookInfo;
    import java.util.List;
    
    public class RoomReadActivity extends AppCompatActivity implements View.OnClickListener
    {
        private TextView tv_room;
        private BookDao bookDao; // 声明一个书籍的持久化对象
    
        @Override
        protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_room_read);
    
            tv_room = findViewById(R.id.tv_room);
            findViewById(R.id.btn_delete).setOnClickListener(this);
    
            // 从App实例中获取唯一的书籍持久化对象
            bookDao = MainApplication.getInstance().getBookDB().bookDao();
            readRoom(); // 读取数据库中的所有书籍记录
        }
    
        // 读取数据库中的所有书籍记录
        private void readRoom()
        {
            List<BookInfo> bookList = bookDao.queryAllBook(); // 获取所有书籍记录
            String desc = String.format("数据库查询到%d条记录，详情如下：", bookList.size());
    
            for (int i = 0; i < bookList.size(); i++)
            {
                BookInfo info = bookList.get(i);
                desc = String.format("%s\n第%d条记录信息如下：", desc, i + 1);
                desc = String.format("%s\n　书名为《%s》", desc, info.getName());
                desc = String.format("%s\n　作者为%s", desc, info.getAuthor());
                desc = String.format("%s\n　出版社为%s", desc, info.getPress());
                desc = String.format("%s\n　价格为%f", desc, info.getPrice());
            }
    
            if (bookList.size() <= 0)
            {
                desc = "数据库查询到的记录为空";
            }
    
            tv_room.setText(desc);
        }
    
        @Override
        public void onClick(View v)
        {
            if (v.getId() == R.id.btn_delete)
            {
                bookDao.deleteAllBook(); // 删除所有书籍记录
    
                ToastUtil.show(this, "已删除所有记录");
                readRoom(); // 读取数据库中的所有书籍记录
            }
        }
    
    }

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