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Saving Data Using the Room Persistence Library

Room provides an abstraction layer over SQLite to allow fluent database access while harnessing the full power of SQLite.

Apps that handle non-trivial amounts of structured data can benefit greatly from persisting that data locally. The most common use case is to cache relevant pieces of data. That way, when the device cannot access the network, the user can still browse that content while they are offline. Any user-initiated content changes are then synced to the server after the device is back online.

Because Room takes care of these concerns for you, we highly recommend using Room instead of SQLite. However, if you prefer to use SQLite APIs to work with your app's databases, Android still supports direct database access using SQLite.

There are 3 major components in Room:

These components, along with their relationships with the rest of the app, appear in Figure 1:

The app uses the Room database to get the data access
  objects, or DAOs, associated with that database. The app then uses each DAO to
  get entities from the database and save any changes to those entities back to
  the database. Finally, the app uses an entity to get and set values that
  correspond to table columns within the database.
Figure 1. Room architecture diagram

The following code snippet contains a sample database configuration with 1 entity and 1 DAO:

User.java

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

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

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

    // Getters and setters are ignored for brevity,
    // but they're required for Room to work.
}

UserDao.java

@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
    void insertAll(User... users);

    @Delete
    void delete(User user);
}

AppDatabase.java

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

After creating the files above, you get an instance of the created database using the following code:

AppDatabase db = Room.databaseBuilder(getApplicationContext(),
        AppDatabase.class, "database-name").build();
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