Pre-test SBT task: Unable to instantiate JDBC driver - postgresql

Question

I'm having trouble getting an SBT task to run migrations with Flyway; I get an exception when I run the task. Any ideas how I could fix it?
org.flywaydb.core.api.FlywayException: Unable to instantiate JDBC driver: org.postgresql.Driver => Check whether the jar file is present
The following code works, when I run it in BeforeAll, in my tests (ScalaTest), but does not work when I move it into an SBT task.
val flyway = Flyway
.configure()
.locations("filesystem:./**/resources/db/migrations/")
.dataSource("jdbc:postgresql://localhost:5432/my_database", "my_user", "secret")
.load()
flyway.clean()
flyway.migrate()
My /build.sbt file looks like this:
import org.flywaydb.core.Flyway
lazy val migrate = taskKey[Unit]("Migrate database")
lazy val migrateTask = Def.task {
println("Migrate")
val flyway = Flyway
.configure()
.locations("filesystem:./**/resources/db/migrations/")
.dataSource("jdbc:postgresql://localhost:5432/my_database", "my_user", "secret")
.load()
flyway.clean()
flyway.migrate()
}
val IntegrationTest = config("integration") extend Test
lazy val integrationTestSettings = inConfig(IntegrationTest)(Defaults.testSettings) ++ List(
IntegrationTest / fork := false,
IntegrationTest / parallelExecution := false,
IntegrationTest / sourceDirectory := baseDirectory.value / "src/test/integration",
IntegrationTest / test := {
(IntegrationTest / test) dependsOn migrateTask
}.value
)
lazy val root = Project(id = "hello", base = file("."))
.configs(Configs.all: _*)
.settings(
integrationTestSettings,
libraryDependencies += "org.scalatest" %% "scalatest" % "3.1.4",
)
And my /project/build.sbt looks like this:
libraryDependencies ++= List(
"org.flywaydb" % "flyway-core" % "7.6.0",
"org.postgresql" % "postgresql" % "42.2.19",
)
The versions I'm using are:
SBT: 1.4.5
Scala: 2.13.4
Flyway: 7.6.0
Does anyone have any ideas why I'm getting that error, and how I can fix it?
Any help would be greatly appreciated. Thanks :)

Answer 1

Searching on the Flyway repo, the error message is coming from here - https://github.com/flyway/flyway/blob/9033185ab8bfa56b0dae9136c04763cdccc50081/flyway-core/src/main/java/org/flywaydb/core/internal/jdbc/DriverDataSource.java#L165-L182 where it's trying load the database driver from the classloader. These ClassLoader techniques sometimes clash with the sbt set up layered ClassLoader to run sbt itself. That's my speculation on what's happening.
How do we work around this?
You said that running it as part the test worked, so maybe you could create a subproject for this purpose?
ThisBuild / scalaVersion := "2.13.4"
lazy val migrate = taskKey[Unit]("Migrate database")
lazy val root = (project in file("."))
.settings(
name := "hello",
migrate := (migrateProj / run).toTask("").value
)
// utility project to run database migration
lazy val migrateProj = (project in file("migrate"))
.settings(
libraryDependencies ++= List(
"org.flywaydb" % "flyway-core" % "7.6.0",
"org.postgresql" % "postgresql" % "42.2.19",
),
Compile / run / fork := true,
publish / skip := true,
)
migrate/Migrate.scala
object Migrate extends App {
println("migrate")
// rest of the code here...
}
Now you can run
sbt:flyway> migrate
[info] running (fork) Migrate
[info] migrate
[success] Total time: 4 s, completed Mar 6, 2021 9:03:07 PM
Details about layered ClassLoader
ClassLoader techniques sometimes clash with the sbt set up layered ClassLoader to run sbt itself. sbt-the-Bash-script allows users to choose the sbt version using project/build.properties, and Scala version using build.sbt. Both of these make sbt build declarative and repeatable, and generally a good thing. But how can sbt launcher written using Scala 2.10 launch sbt 1.4.x written using Scala 2.12, which then launch your Scala 2.13 application? Each of these boundary cross is done by creating a layered ClassLoader, like the movie Inception.