{% setvar book_path %}/reference/kotlin/androidx/_book.yaml{% endsetvar %} {% include "_shared/_reference-head-tags.html" %}
Room is a Database Object Mapping library that makes it easy to access database on Android applications.
Rather than hiding the details of SQLite, Room tries to embrace them by providing convenient APIs to query the database and also verify such queries at compile time. This allows you to access the full power of SQLite while having the type safety provided by Java SQL query builders.
There are 3 major components in Room.
The database class defines the list of entities and data access objects in the database. It is also the main access point for the underlying connection.
Using Dao classes for database access rather than query builders or direct queries allows you to keep a separation between different components and easily mock the database access while testing your application.
// File: Song.javaYou can create an instance of{@literal @}
Entity public class User {{@literal @}
PrimaryKey private int id; private String name;{@literal @}
ColumnInfo(name = "release_year") private int releaseYear; // getters and setters are ignored for brevity but they are required for Room to work. } // File: SongDao.java{@literal @}
Dao public interface SongDao {{@literal @}
Query("SELECT * FROM song") List<Song> loadAll();{@literal @}
Query("SELECT * FROM song WHERE id IN (:songIds)") List<Song> loadAllBySongId(int... songIds);{@literal @}
Query("SELECT * FROM song WHERE name LIKE :name AND release_year = :year LIMIT 1") Song loadOneByNameAndReleaseYear(String first, int year);{@literal @}
Insert void insertAll(Song... songs);{@literal @}
Delete void delete(Song song); } // File: MusicDatabase.java{@literal @}
Database(entities = {Song.java}) public abstract class MusicDatabase extends RoomDatabase { public abstract SongDao userDao(); }
{@code MusicDatabase}
as follows:
MusicDatabase db = Room .databaseBuilder(getApplicationContext(), MusicDatabase.class, "database-name") .build();Since Room verifies your queries at compile time, it also detects information about which tables are accessed by the query or what columns are present in the response.
You can observe a particular table for changes using the InvalidationTracker class which you can acquire via RoomDatabase.getInvalidationTracker.
For convenience, Room allows you to return LiveData from Query methods. It will automatically observe the related tables as long as the {@code LiveData}
has active observers.
// This live data will automatically dispatch changes as the database changes.
{@literal @}
Query("SELECT * FROM song ORDER BY name LIMIT 5")
LiveData<Song> loadFirstFiveSongs();
You can also return arbitrary data objects from your query results as long as the fields in the object match the list of columns in the query response. This makes it very easy to write applications that drive the UI from persistent storage.
class IdAndSongHeader { int id;If there is a mismatch between the query result and the POJO, Room will print a warning during compilation.{@literal @}
ColumnInfo(name = "header") String header; } // DAO{@literal @}
Query("SELECT id, name || '-' || release_year AS header FROM user") public IdAndSongHeader[] loadSongHeaders();
Please see the documentation of individual classes for details.
FragmentManager.BackStackEntry |
Representation of an entry on the fragment back stack, as created with |
FragmentManager.OnBackStackChangedListener |
Interface to watch for changes to the back stack. |
FragmentOnAttachListener |
Listener for receiving a callback immediately following |
FragmentResultListener |
Listener for handling fragment results. |
FragmentResultOwner |
A class that manages passing data between fragments. |
DialogFragment |
Static library support version of the framework's android.app.DialogFragment. |
Fragment |
Static library support version of the framework's android.app.Fragment. |
Fragment.SavedState |
State information that has been retrieved from a fragment instance through |
FragmentActivity |
Base class for activities that want to use the support-based |
FragmentContainer |
Callbacks to a |
FragmentContainerView |
FragmentContainerView is a customized Layout designed specifically for Fragments. |
FragmentController |
Provides integration points with a |
FragmentFactory |
Interface used to control the instantiation of |
FragmentHostCallback |
Integration points with the Fragment host. |
FragmentManager |
Static library support version of the framework's android.app.FragmentManager. |
FragmentManager.FragmentLifecycleCallbacks |
Callback interface for listening to fragment state changes that happen within a given FragmentManager. |
FragmentManagerNonConfig |
This class is deprecated. Have your |
FragmentPagerAdapter |
This class is deprecated. Switch to androidx.viewpager2.widget.ViewPager2 and use androidx.viewpager2.adapter.FragmentStateAdapter instead. |
FragmentStatePagerAdapter |
This class is deprecated. Switch to androidx.viewpager2.widget.ViewPager2 and use androidx.viewpager2.adapter.FragmentStateAdapter instead. |
FragmentTabHost |
This class is deprecated. Use TabLayout and ViewPager instead. |
FragmentTransaction |
Static library support version of the framework's android.app.FragmentTransaction. |
ListFragment |
Static library support version of the framework's android.app.ListFragment. |
Fragment.InstantiationException |
Thrown by |