Android 应用开发 Jetpack Room导入_jetpack 自增长id

Room框架同样基于SQL，但是通过注解技术极大简化了数据库操作，减少了原来的编码工作

在使用Room之前，要先修改模块的build.gradle文件，往

dependencies节点添加下面两行配置，表示导入指定版本的Room库。

implementation 'androidx.room:room-runtime:2.2.5'
annotationProcessor 'android.room:room-compiler:2.2.5'

Room框架编码步骤有以下五步

1.构建一个实体类保存书籍信息，注意打上注解Entity，对于id打上@PrimaryKey(autoGenerate = true)表示自动增长

package com.example.chapter06.enity;
 
import androidx.room.Entity;
import androidx.room.PrimaryKey;
 
@Entity
public class BookInfo {
 
    @PrimaryKey(autoGenerate = true)
    private int id;
    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 setId(int id) {
        this.id = id;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public void setAuthor(String author) {
        this.author = author;
    }
 
    public void setPress(String press) {
        this.press = press;
    }
 
    public void setPrice(double price) {
        this.price = price;
    }
 
    public int getId() {
        return id;
    }
 
    public String getName() {
        return name;
    }
 
    public String getAuthor() {
        return author;
    }
 
    public String getPress() {
        return press;
    }
 
    public double getPrice() {
        return price;
    }
 
    @Override
    public String toString() {
        return "BookInfo{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", author='" + author + '\'' +
                ", press='" + press + '\'' +
                ", price=" + price +
                '}';
    }
}

2.编写书籍信息表对应的持久化类，该类添加@Dao注解（Database access operator）

package com.example.chapter06.dao;
 
import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;
 
import com.example.chapter06.enity.BookInfo;
 
import java.util.List;
 
@Dao
public interface BookDao {
    @Insert
    void insert(BookInfo... book);
 
    @Delete
    void delete(BookInfo... book);
 
    @Query("DELETE FROM BookInfo")
    void deleteAll();
 
    @Update
    int update(BookInfo... book);
 
    @Query("SELECT * FROM BookInfo")
    List<BookInfo> queryAll();
 
    @Query("SELECT * FROM BookInfo WHERE name = :name ORDER BY id DESC limit 1")
    BookInfo queryByName(String name);
}

 3.提供一个database

package com.example.chapter06.database;
 
import androidx.room.Database;
import androidx.room.RoomDatabase;
 
import com.example.chapter06.dao.BookDao;
import com.example.chapter06.enity.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 = true)
public abstract class BookDataBase extends RoomDatabase {
    //
    public abstract BookDao bookDao();
 
}
 
//如果exportSchema 被设置为true了，需要在build.gradle中设置
//        javaCompileOptions {
//            annotationProcessorOptions {
//                arguments = ["room.schemaLocation": "$projectDir/schemas".toString()]//指定数据库schema导出的位置
//            }
//        }

4.在自定义的Application类中声明书籍数据库的唯一实例

//构建数据库的实例

bookDataBase = Room.databaseBuilder(this, BookDataBase.class, "book")
        // 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）
        .addMigrations()
        //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）
        .allowMainThreadQueries().build();

5.获取实例

bookDao = MyApplication.getInstance().getBookDataBase().bookDao();

增删改查

public void onClick(View view) {
        String name = et_name.getText().toString();
        String author = et_author.getText().toString();
        String press = et_press.getText().toString();
        String price = et_price.getText().toString();
 
        switch (view.getId()) {
            case R.id.btn_save:
                //以下声明一个书籍信息对象，并填写它的各个字段值
                BookInfo b1 = new BookInfo();
                b1.setName(name);
                b1.setAuthor(author);
                b1.setPress(press);
                b1.setPrice(Double.parseDouble(price));
                bookDao.insert(b1);
                ToastUtil.show(this, "保存成功");
                break;
            case R.id.btn_query:
                List<BookInfo> list = bookDao.queryAll();
                for (BookInfo b : list) {
                    Log.d("xr", b.toString());
                }
                break;
            case R.id.btn_delete:
                BookInfo b2 = new BookInfo();
                b2.setId(1);
                bookDao.delete(b2);
                break;
            case R.id.btn_update:
                BookInfo b3 = new BookInfo();
                BookInfo b4 = bookDao.queryByName(name);
                b3.setId(b4.getId());
                b3.setName(name);
                b3.setAuthor(author);
                b3.setPress(press);
                b3.setPrice(Double.parseDouble(price));
                bookDao.update(b3);
                break;
        }
    }