【安卓开发】数据库Room框架的学习和使用

dependencies {
    def room_version="2.3.0"
    implementation "androidx.room:room-runtime:$room_version"
    annotationProcessor "androidx.room:room-compiler:$room_version"
}

@Entity //实体类注解
public class Data{
    @PrimaryKey(autoGenerate = true)//主键自动递增
    private int id;
    @ColumnInfo(name="title")//指定列名，不指定的话列名等于变量名
    private String title;
    @ColumnInfo(name="tt")
    private String tt;
}

@Dao
public interface DataDao {
    @Insert
    void insertData(Data data);
    @Query("delete from data")
    void deleteAll();
    @Query("select * from data")
    List<Data> findAll();
    @Query("delete from data where id =:pos")
    void deleteById(int pos);
    @Query("select * from data where tt=:t1 and title=:t2")
    List<Data> find_by(String t1,String t2);
}

@Database(entities = {Data.class,Person.class},version = 1,exportSchema = false)
public abstract class NDatabase extends RoomDatabase {
    private static NDatabase INSTANCE;
    static public NDatabase getDatabase(Context context){
        if(INSTANCE==null) {
            INSTANCE= Room.databaseBuilder(context,NDatabase.class,"n_database")
                    .allowMainThreadQueries().build();
            //数据库名 n_database
            //表名 Data和Person
        }
        return INSTANCE;
    }
    public abstract DataDao getDataDao();
    public abstract PersonDao getPersonDao();
}

private NDatabase nDatabase;
private DataDao dataDao;

nDatabase=NDatabase.getDatabase(this);
dataDao=nDatabase.getDataDao();

List<Data> t=dataDao.findAll();