https://issuetracker.google.com/issues/207477240
When Room opens an existing database, it checks the identity hash stored in the existing database and the identity hash of the schema Room is trying to upgrade the database with. If the hashes are different and the database version wasn't updated, Room considers the situation to be an error since it cannot verify the data integrity.
According to the implementation (RoomOpenHelper.java), there are actually two hashes the existing database's hash is compared with. As it's said in a comment inside the class, this is due to the fixes with the identity hash being inconsisted if entity fields were reordered. This issue existed in Room of version 1 and was fixed shortly after, but in order to sustain backward compatibility, the old hash couldn't be deleted so a new field was introduced.
It is still required to check the schema hash against both new and legacy hashes because there might be a situation when a user has been using an app without cleaning the local storage since the app's version that used Room of version 1. If this is true, then the hash that is transferred from database to database with each database upgrade is equal to the legacy hash provided that the database schema remains unchanged.
Check out the revision e9bb0a40b45 and verify the following conditions:
- Kotlin plugin
1.4.31(org.jetbrains.kotlin:kotlin-gradle-plugin:1.4.31) - Room version
2.2.6 - An Room's entity defined in a particular way:
@Entity(tableName = "feed_entity")
data class FeedEntity(
@ColumnInfo(name = "name")
val name: String
) {
@ColumnInfo(name = "id")
@PrimaryKey(autoGenerate = true)
var id: Int = 0 // Note that the id field is defined inside the body of the entity
}When the app is built, take a look at the generated source file for the Room database (AppDatabase_Impl.java). There, find the place where identityHash and legacyHash are written. In my case, the values were the following:
| Field name | Hash |
|---|---|
identityHash |
9a8792da20d5fc06ca562ccc9b118cc6 |
legacyHash |
8df0cc88464e07e64cf3a80503685ddd |
Checkout the revision 552af4cb58 and verify the following conditions:
- Kotlin plugin of the version
1.5.0(org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.0) - Room's version is
2.3.0
Note that the schema wasn't changed!
Try to build the app now and go to the generated source file of the Room database (AppDatabase_Impl.java). There, once again find the place where the hashes are written. In my case, the hashes were the following:
| Field name | Hash |
|---|---|
identityHash |
9a8792da20d5fc06ca562ccc9b118cc6 |
legacyHash |
4d32e475b12cf85dc092715855f4705a |
Note that while the identityHash wasn't changed (which is logical since the database schema wasn't changed at all), the legacyHash is now different. The comment in the RoomOpenHelper.java class above the legacyHash field says the following:
/** * Room v1 had a bug where the hash was not consistent if fields are reordered. * The new has fixes it but we still need to accept the legacy hash. */
However, as you've already seen, not only haven't we changed the schema of the database, but also we haven't altered the entity class!
Remembering the fact that the legacyHash might still be important under some condition, a simple library update will lead to crashes. If the old hash was 8df0cc88464e07e64cf3a80503685ddd (the value of the old legacyHash), now it is checked against identityHash (9a8792da20d5fc06ca562ccc9b118cc6) and the new legacyHash (4d32e475b12cf85dc092715855f4705a). Obviously, both of these checks return false and given the fact that the version of the database remained the same, IllegalStateException is thrown.
A simple library update under specific circumstances may lead to unexpected exceptions being thrown by Room