Jetpack——Room

概述

Room是谷歌公司推出的数据库处理框架，该框架同样基于SQLite，但它通过注解技术极大简化了数据库操作，减少了原来相当一部分编码工作量。
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，标识导入指定版本的Room库：

implementation ‘androidx.room:room-runtime:2.2.5’
annotationProcessor ‘androidx.room:room-compiler:2.2.5’

annotationProcessor：注解处理器

使用步骤

以录入书籍信息为例，使用Room框架的编码过程分为下列五步：

1、编写书籍信息表对应的实体类，该类添加“@Entity”注解

package com.example.study_android.entity;import androidx.room.Entity;
import androidx.room.PrimaryKey;@Entity
public class BookInfo {@PrimaryKey(autoGenerate = true)private int id;public String name;public String author;public double price;@Overridepublic String toString() {return "BookInfo{" +"id=" + id +", name='" + name + '\'' +", author='" + author + '\'' +", price=" + price +'}';}
}

2、编写书籍信息表对应的持久化类，该类添加“@Dao”注解

@Dao
public interface BookDao {@Insertvoid insert(BookInfo... book);@Deletevoid delete(BookInfo... book);@Query("DELETE FROM BookInfo")void deleteAll();@Updateint 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、编写书籍信息表对应的数据库类，该类从RoomDatabase派生而来，并添加“@Database”注解

package com.example.study_android.database;import androidx.room.Database;
import androidx.room.RoomDatabase;import com.example.study_android.dao.BookDao;
import com.example.study_android.entity.BookInfo;@Database(entities = {BookInfo.class}, version = 1, exportSchema = true)
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();
}

entities：表示该数据库有哪些表
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

在build.gradle中添加导出路径

        javaCompileOptions {annotationProcessorOptions {//指定数据库schema导出的位置arguments += mapOf("room.schemaLocation" to "$projectDir/schemas","room.incremental" to "true")}}

4、在自定义的Application类中声明书籍数据库的唯一实例

public class MyApplication extends Application {private static MyApplication mApp;// 声明一个书籍数据库对象private BookDatabase bookDatabase;public static MyApplication getInstance() {return mApp;}// App启动时调用@Overridepublic void onCreate() {super.onCreate();mApp = this;// 构建书籍数据库的实例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();}public BookDatabase getBookDB() {return bookDatabase;}
}

5、在操作书籍信息表的地方获取数据表的持久化对象

bookDao = MyApplication.getInstance().getBookDB().bookDao();@Overridepublic void onClick(View view) {String name = et_book_name.getText().toString();String author = et_author.getText().toString();String price = et_price.getText().toString();switch (view.getId()) {case R.id.btn_save:BookInfo book = new BookInfo();book.name = name;book.author = author;book.price = Double.parseDouble(price);bookDao.insert(book);ToastUtil.show(this, "保存成功");break;case R.id.btn_del:BookInfo b2 = new BookInfo();b2.id = 1;bookDao.delete(b2);break;case R.id.btn_edit:BookInfo b3 = new BookInfo();BookInfo b4=bookDao.queryByName(name);b3.id = b4.id;b3.name = name;bookDao.update(b3);break;case R.id.btn_query:List<BookInfo> list = bookDao.queryAll();StringBuilder sb = new StringBuilder();for (BookInfo b : list) {sb.append(b.toString());}tv_result.setText(sb);break;}}