A minimal sbt project to reproduce the issue of code being recompiled on each docker step even though the sources, artifacts, jdk and any residual caches are the same.
This reproduce the issue by running sbt compile and sbt run in two consecutive (but different)
steps in the docker build.
Issue
To reproduce the issue:
docker build .The build output will show Compiling 1 Scala source to /root/target/scala-2.12/classes
in both sbt compile and sbt run stages, even though it gets previously compiled
in the first stage.
Uncomment the last two lines in build.sbt to enable debug output of the sbt process.
It shows the sources getting invalidated and running the recompilation.
[debug] Initial source changes:
[debug] removed:Set()
[debug] added: Set()
[debug] modified: Set()
[debug] Invalidated products: Set(/root/target/scala-2.12/classes/Main$.class, /root/target/scala-2.12/classes/Main.class)
[debug] External API changes: API Changes: Set()
[debug] Modified binary dependencies: Set()
[debug] Initial directly invalidated classes: Set()
[debug]
[debug] Sources indirectly invalidated by:
[debug] product: Set(/root/src/main/scala/Main.scala)
[debug] binary dep: Set()
[debug] external source: Set()
[debug] All sources are invalidated.
[debug] Recompiling all 1 sources: invalidated sources (1) exceeded 50.0% of all sources
Solution
A workaround is proposed in sbt/sbt#4168 (comment).
-
Java 8: Setting
-Dsbt.io.jdktimestamps=truewhen running SBT as recommended in sbt/sbt#4168 (comment) to workaround this issue. -
Newer: Follow recommendation in sbt/sbt#4168 (comment)