第三章：Room数据库使用

• 导入依赖

    //room数据库
    implementation 'androidx.room:room-runtime:2.2.5'
    annotationProcessor "androidx.room:room-compiler:2.2.5"

• 构建数据库字段

package cn.yumakeji.jetpackroomstudy;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.Ignore;
import androidx.room.PrimaryKey;

/**
 * 表名
 * tableName = "cache"
 * <p></p>
 * 本表中 那些字段 不需要 映射到表中
 * ignoredColumns = {"data"}
 */
@Entity(tableName = "user_info", ignoredColumns = {"num1", "num3"})
public class Student {

    //PrimaryKey 必须要有,且不为空,autoGenerate 主键的值是否由Room自动生成,默认false
    @PrimaryKey(autoGenerate = true)
    public int id;

    //指定该字段在表中的列的名字
    @ColumnInfo(name = "name")
    public String name;

    //@ColumnInfo:表中的字段,默认用下面的字段名age
    @ColumnInfo
    public int age;

    //忽略该字段
    @Ignore
    public String studyNum;
    @ColumnInfo
    public String num1;
    @ColumnInfo
    public String num2;
    @ColumnInfo
    public String num3;


    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", studyNum='" + studyNum + '\'' +
                ", num1='" + num1 + '\'' +
                ", num2='" + num2 + '\'' +
                ", num3='" + num3 + '\'' +
                '}';
    }
}

• 创建增删改查

package cn.yumakeji.jetpackroomstudy;

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 java.util.List;

@Dao
public interface StudentDao {
    /**
     * OnConflictStrategy.REPLACE :直接覆盖
     *
     * @param students
     */
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insert(Student... students);

    //只能传递对象的,删除时根据Student中的主键 来比对的
    @Delete
    void delete(Student... student);

    //只能传递对象的,更新时根据Student中的主键 来比对的
    @Update(onConflict = OnConflictStrategy.REPLACE)
    void update(Student... student);

    //获取表中所有数据
    @Query("select * from user_info")
    List<Student> findAll();

    //根据name来查找数据（这里是一对一，如果是一对多,这里可以写List<Student>）
    @Query("select * from user_info where name = :name")
    Student findByName(String name);

    //根据name来查找数据（这里是一对多，如果是一对一,这里可以写Student）
    @Query("select * from user_info where age = :age")
    List<Student> findByAge(int age);

    /**
     * 注意，冒号后面必须紧跟参数名，中间不能有空格。大于小于号和冒号中间是有空格的。
     * select *from cache where【表中列名】 =:【参数名】------>等于
     * where 【表中列名】 < :【参数名】 小于
     * where 【表中列名】 between :【参数名1】 and :【参数2】------->这个区间
     * where 【表中列名】like :参数名----->模糊查询
     * where 【表中列名】 in (:【参数名集合】)---->查询符合集合内指定字段值的记录
     *
     * @param ids
     * @return
     */
    //查找参数集合
    @Query("select * from user_info where id in (:ids)")
    List<Student> findByIds(int[] ids);

}

• 创建数据库

import cn.yumakeji.jetpackroomstudy.global.AppGlobals;

@Database(entities = {Student.class}, version = 1)
public abstract class RoomDataBase extends RoomDatabase {
    private static final RoomDataBase database;

    static {
        //创建一个内存数据库
        //但是这种数据库的数据只存在于内存中，也就是进程被杀之后，数据随之丢失
        //Room.inMemoryDatabaseBuilder()
        database = Room.databaseBuilder(AppGlobals.getApplication(), RoomDataBase.class, "jetpack_demo")
                //是否允许在主线程进行查询
                .allowMainThreadQueries()
                //数据库创建和打开后的回调
                //.addCallback()
                //设置查询的线程池
                //.setQueryExecutor()
                //.openHelperFactory()
                //room的日志模式
                //.setJournalMode()
                //数据库升级异常之后的回滚
                //.fallbackToDestructiveMigration()
                //数据库升级异常后根据指定版本进行回滚
                //.fallbackToDestructiveMigrationFrom()
                //数据库升级的入口（如果没有配置升级的时候会将数据库内容全部清除）
                // .addMigrations(CacheDatabase.sMigration)
                .build();
    }

    /**
     * 获取Student数据库操作对象
     *
     * @return
     */
    public abstract StudentDao getStudentDao();

    public static RoomDataBase get() {
        return database;
    }

    /**
     * 数据库升级的入口（如果没有配置升级的时候会将数据库内容全部清除）
     */
//    static Migration sMigration = new Migration(1, 3) {
//        @Override
//        public void migrate(@NonNull SupportSQLiteDatabase database) {
//            database.execSQL("alter table teacher rename to student");
//            database.execSQL("alter table teacher add column teacher_age INTEGER NOT NULL default 0");
//        }
//    };
}

• 使用
  

public class MainActivity extends AppCompatActivity {

    private List<Student> mList = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        for (int i = 0; i < 10; i++) {
            Student student = new Student();
            student.name = "huangxiaoguo" + i;
            student.age = 18 + i;
            student.num1 = String.valueOf(Math.round(Math.random() * 10) + 1);
            student.num2 = String.valueOf(Math.round(Math.random() * 10) * 10 + 10);
            student.num3 = String.valueOf(Math.round(Math.random() * 10) * 10 + 20);
            mList.add(student);
        }
    }

    /**
     * 增加数据
     *
     * @param view
     */
    public void onInsertClick(View view) {
        for (Student student : mList) {
            RoomDataBase.get().getStudentDao().insert(student);
        }
        /**
         * 多个插入
         */
        RoomDataBase.get().getStudentDao().insert(mList.get(2),mList.get(3),mList.get(4),mList.get(5));
    }

    /**
     * 单条删除数据
     *
     * @param view
     */
    public void onDeleteOneClick(View view) {
        Student student = RoomDataBase.get().getStudentDao().findByName("huangxiaoguo1");
        if (student==null){
            return;
        }
        RoomDataBase.get().getStudentDao().delete(student);
    }

    /**
     * 删除所有数据
     * @param view
     */
    public void onDeleteAllClick(View view) {
        List<Student> all = RoomDataBase.get().getStudentDao().findAll();
        for (Student student : all) {
            RoomDataBase.get().getStudentDao().delete(student);
        }
    }
    /**
     * 单条更新数据
     *
     * @param view
     */
    public void onUpDataOneClick(View view) {
        Student student = RoomDataBase.get().getStudentDao().findByName("huangxiaoguo5");
        if (student==null){
            return;
        }
        student.name="修改了";
        RoomDataBase.get().getStudentDao().update(student);
    }

    /**
     * 获取所有数据
     *
     * @param view
     */
    public void onGetAllClick(View view) {
        List<Student> all = RoomDataBase.get().getStudentDao().findAll();
        if (all==null||all.size()==0){
            Log.e("TTT","没有查询到数据");
            return;
        }
        for (Student student : all) {
            Log.e("TTT",student.toString());
        }
    }

    /**
     * 一对一查询数据
     *
     * @param view
     */
    public void onFindOneByOneClick(View view) {
        Student student = RoomDataBase.get().getStudentDao().findByName("huangxiaoguo3");
        if (student==null){
            return;
        }
        Log.e("TTT",student.toString());
    }

    /**
     * 一对多查询数据
     *
     * @param view
     */
    public void onFindOneBySomeClick(View view) {
        List<Student> list = RoomDataBase.get().getStudentDao().findByAge(22);
        Log.e("TTT",list.toString());
    }

    /**
     * 查询符合集合内数据（String）
     *
     * @param view
     */
    public void onFindInSomeIntClick(View view) {
        List<Student> list = RoomDataBase.get().getStudentDao().findByIds(new int[]{7, 8, 9});
        Log.e("TTT",list.toString());
    }

}

完成