Transaction
public
abstract
@interface
Transaction
implements
Annotation
androidx.room.Transaction |
Marks a method in a Dao
class as a transaction method.
When used on a non-abstract method of an abstract Dao
class,
the derived implementation of the method will execute the super method in a database transaction.
All the parameters and return types are preserved. The transaction will be marked as successful
unless an exception is thrown in the method body.
Example:
@Dao public abstract class SongDao { @Insert public abstract void insert(Song song); @Delete public abstract void delete(Song song); @Transaction public void insertAndDeleteInTransaction(Song newSong, Song oldSong) { // Anything inside this method runs in a single transaction. insert(newSong); delete(oldSong); } }
When used on a Query
method that has a SELECT
statement, the generated code for
the Query will be run in a transaction. There are 2 main cases where you may want to do that:
- If the result of the query is fairly big, it is better to run it inside a transaction
to receive a consistent result. Otherwise, if the query result does not fit into a single
CursorWindow
, the query result may be corrupted due to changes in the database in between cursor window swaps. - If the result of the query is a POJO with
Relation
fields, these fields are queried separately. To receive consistent results between these queries, you also want to run them in a single transaction.
class AlbumWithSongs extends Album { @Relation(parentColumn = "albumId", entityColumn = "songId") public List<Song> songs; } @Dao public interface AlbumDao { @Transaction @Query("SELECT * FROM album") public List<AlbumWithSongs> loadAll(); }If the query is asynchronous (e.g. returns a
LiveData
or RxJava Flowable
), the transaction is properly handled when the query is run, not when
the method is called.
Putting this annotation on an Insert
, Update
or Delete
method has no
impact because those methods are always run inside a transaction. Similarly, if a method is
annotated with Query
but runs an INSERT, UPDATE or DELETE statement, it is automatically
wrapped in a transaction and this annotation has no effect.
Room will only perform at most one transaction at a time, additional transactions are queued and executed on a first come, first serve order.
Summary
Inherited methods | |
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