[Android] 通过Room操作SQLite数据库

谷歌推荐使用Room操作数据库，Room在 SQLite 上提供了一个抽象层，在充分利用 SQLite强大功能的同时，能够流畅地访问数据库。

Room的三个主要组件：

• 数据库类，用于保存数据库并作为应用持久性数据底层连接的主要访问点。
• 数据实体，@Entity，表示数据库中的表。
• 数据访问对象 (DAO)，@Dao，提供查询、更新、插入和删除数据的方法。

build.gradle添加

dependencies {
    def room_version = "2.4.3"

    implementation "android.arch.persistence.room:runtime:$room_version"
    annotationProcessor "android.arch.persistence.room:compiler:$room_version" 

    implementation "android.arch.persistence.room:rxjava2:$room_version"

    testImplementation "android.arch.persistence.room:testing:$room_version"
}

1. 创建实体类User

@Entity(tableName = "users")
public class User {
    @PrimaryKey
    public int uid;

    @ColumnInfo(name = "first_name")
    public String firstName;

    @ColumnInfo(name = "last_name")
    public String lastName;

	... get ...
	... set ...
}

@Entity实体类，users表的名称，不加默认user
@ColumnInfo列名
@PrimaryKey主键
复合主键
@Entity(primaryKeys = {“first_name”, “last_name”})

主键不设置非空约束，报错

You must annotate primary keys with @NonNull. "xxx" is nullable. SQLite considers this a bug and Room does not allow it. See SQLite docs for details:

@NonNull
    @ColumnInfo(name = "first_name")

2. 创建DAO

@Dao
public interface UserDao {
    @Query("SELECT * FROM user")
    List<User> getAll();

    @Query("SELECT * FROM user WHERE uid IN (:userIds)")
    List<User> loadAllByIds(int[] userIds);

    @Query("SELECT * FROM user WHERE first_name LIKE :first AND " +
            "last_name LIKE :last LIMIT 1")
    User findByName(String first, String last);

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insertAll(User... users);

    @Delete
    void delete(User user);

	@Query("SELECT * FROM user WHERE first_name=:name AND last_name=:names")
    List<DisableInfo> getDisable(String name,String names);
}

增，删，改：将实例与表的主键进行匹配
查询结果将自动映射到对应类型的字段，若未映射将报警告

删除表内所有数据是query ，而不是delete，留个印
@Query(“DELETE FROM appinfo”)
void deleteAll();

3. 数据库

抽象类AppDatabase 定义数据库配置，并作为应用对持久性数据的主要访问点，扩展了RommDataBase

@Database(entities = {User.class}, version = 1, exportSchema = false)
public abstract class AppDataBase extends RoomDatabase {

    public abstract UserDao userDao();
}

4. 使用

AppDataBase db = Room.databaseBuilder(getApplicationContext(),
                        AppDataBase.class, "database-name").build();
                for (int i = 0; i < 10; i++) {
                    User user = new User();
                    user.uid = i;
                    user.firstName = "Shell" + i;
                    user.lastName = "Hub" + i;
                    db.userDao().insertAll( user);
                    List<User> userList = db.userDao().getAll();
                    for(User user1 : userList) {
                        Log.d("mip",""+user1.firstName);
                    }
                }

为了节约获取数据库的时间和资源，采取单例模式
简单实现：

public class Utils {
    private static AppDataBase db = null;
    private static Context context = null;

    public static AppDataBase getDb(){
        if( db == null) {
            db = Room.databaseBuilder(context,
                    AppDataBase.class, "database-name").build();
        }
        return db;
    }

    public static void setContext(Context context){
        Utils.context = context;
    }
}

调用

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Utils.setContext(getApplicationContext());

        new Thread(new Runnable() {
            @Override
            public void run() {
                AppDataBase db = Utils.getDb();
                for (int i = 0; i < 10; i++) {
                    User user = new User();
                    user.uid = i;
                    user.firstName = "Shell" + i;
                    user.lastName = "Hub" + i;
                    db.userDao().insertAll( user);
                    List<User> userList = db.userDao().getAll();
                    for(User user1 : userList) {
                        Log.d("mip",""+user1.firstName);
                    }
                }
            }
        }).start();
    }
}

路过的大佬们有更好的单例实现请告诉我一下，
感谢。