原文链接

前言

Android中提供了SQLite数据库进行数据的持久化 ，并提供了对应API访问数据库，而Room框架提供了SQLite数据访问抽象层，为高效的数据库访问层带来便捷

APP可以缓存用户数据，当APP离线时便从SQLite读取数据，当重新连线时即可完成和服务器数据的同步

谷歌官方强烈推荐使用Room框架操作SQLite数据库

room_architecture.png

Hello World

首先在build.gradle中添加必要依赖

dependencies {
    def room_version = "1.1.1"

    implementation "android.arch.persistence.room:runtime:$room_version"
    annotationProcessor "android.arch.persistence.room:compiler:$room_version" // use kapt for Kotlin

    // optional - RxJava support for Room
    implementation "android.arch.persistence.room:rxjava2:$room_version"

    // optional - Guava support for Room, including Optional and ListenableFuture
    implementation "android.arch.persistence.room:guava:$room_version"

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

创建实体类User,@Entity表示该类对应数据库中的表，@ColumnInfo后面的name属性对应数据库中的字段名，并实现该实体类的Getter，Setter方法

@Entity
public class User {
    @PrimaryKey
    private int uid;

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

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

    public int getUid() {
        return uid;
    }

    public void setUid(int uid) {
        this.uid = uid;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "User{" +
                "uid=" + uid +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                '}';
    }
}

创建实体类对应的dao层UserDao，完成User的增删改查(CRUD)接口定义，@Dao注解定义一个dao层,参数赋值(传递)使用:clumn_name进行赋值

@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);
}

创建AppDatabase，@Database注解表示这是一个数据库操作类，entities对应Entity实体类，version用于数据库版本升级，并在该抽象类中定义一个返回dao层的抽象方法

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

初始化用于操作数据库的实例对象AppDatabase，需要注意的是不能在主线程中初始化，必须新开启一个线程进行初始化，否则会报错，或者无法创建数据库

new Thread(new Runnable() {
    @Override
    public void run() {
        AppDatabase db = Room.databaseBuilder(getApplicationContext(),
                AppDatabase.class, "database-name").build();
    }
}).start();

测试

增加，也可以传一个User数组

for (int i = 0; i < 10; i++) {
    User user = new User();
    user.setUid(i);
    user.setFirstName("Shell" + i);
    user.setLastName("Hub" + i);
    insertAll(db, user);
}

如果报以下错误，修改dao层的注解为@Insert(onConflict = OnConflictStrategy.REPLACE)

android.database.sqlite.SQLiteConstraintException: UNIQUE constraint failed: User.uid (code 1555 SQLITE_CONSTRAINT_PRIMARYKEY)

查询所有数据

for (User user : db.userDao().getAll()) {
    System.out.println(user);
}

other...

Room单例模式

最好使用设计模式中的单例模式获取数据库实例，因为每次获取数据库实例都很耗时并且耗内存，我们可以自定义一个类继承Application并定义一个public static方法获取数据库实例

public class App extends Application {
    private static Context context;
    private static final String DATABASE_NAME = "SHELLHUB";
    private static AppDatabase DB_INSTANCE;
    @Override
    public void onCreate() {
        super.onCreate();
        context = getApplicationContext();

        new Thread(new Runnable() {
            @Override
            public void run() {
                DB_INSTANCE = Room.databaseBuilder(getApplicationContext(),
                        AppDatabase.class, DATABASE_NAME).build();

            }
        }).start();
    }

    public static AppDatabase getDB() {
        return DB_INSTANCE;
    }
}