scalac Command: Compile Scala in CI

scalac compiles Scala source files into JVM class files.

scalac is the Scala compiler. Real projects build through sbt or Mill, but scalac shows up in CI scripts and minimal images. Its flags control output, classpath, and how strictly warnings are enforced.

Common flags

-d OUT - output directory or .jar for class files
-classpath / -cp PATH - dependency class path
-deprecation - emit warnings for deprecated API use
-feature - warn on language features requiring an import
-Xfatal-warnings - turn all warnings into errors
-release 17 - target JVM bytecode and API level
-language:postfixOps - enable a specific language feature

Example in CI

Compile a Scala source tree strictly, failing on any warning:

shell

scalac -deprecation -feature -Xfatal-warnings -release 17 -d build src/*.scala

Common errors in CI

error: not found: value X - unresolved identifier or missing import
error: type mismatch; found: A, required: B
error: object X is not a member of package Y - missing dependency on classpath
error: No warnings can be incurred under -Werror (Scala 3) - a warning failed the build

Key takeaways

Use -Xfatal-warnings (Scala 2) or -Werror (Scala 3) to keep CI strict.
-release N pins both bytecode and the JDK API surface.
Most CI failures are "not found" identifiers or type mismatches.