2.0.0-RC1

What's changed

sbt-assembly 2.0.0 has been refactored to use in-memory processing of library entries, in contrast to its prior versions where library JAR files were unzipped to disk. This has positive performance implications, especially for large projects, machines with slow disks (i.e. spinning hard drives) or systems with slow file systems such as Windows Subsystem for Linux (WSL1) emulated file access.

This overhaul was contributed by Ben Reyes (@fnqista) in #464.

Breaking changes

• MergeStrategy.rename behavior was changed. Directories named LICENSE etc will no longer be renamed anymore. If these directories conflict with files named LICENSE etc the files will be renamed to include the assembly jar name (minus the .jar extension) instead.
• Fails *.class renames via MergeStrategy.rename, where it was a silent no-op previously.
• assemblyCacheUnzip setting was dropped.
• excludedFiles field was dropped from AssemblyOption.

Performance improvements

sbt-assembly 2.0.0-RC1 generally improves the performance compared to the the previous versions, however the effect of speedup may be dependent on the filesystem and hardware configuration. For example, a Play app that used to take 10s to assemble now assembles in 7s. On Windows, we expect more speedup.

In addition, a new setting called ThisBuild / assemblyRepeatableBuild is introduced:

ThisBuild / assemblyRepeatableBuild := false

The setting is set to true (repeatable) by default, but if set to false, sbt-assembly will parallelize JAR creation for an even faster performance. For instance, a Play app that used to take 10s assembled in 5s on Mac. Your mileage may vary.

CustomMergeStrategy.apply

sbt-assembly 2.0.0 introduces CustomMergeStrategy builder to construct custom merge strategies.

// same as discard
CustomMergeStrategy("custom-discard") { conflicts =>
  Right(Vector.empty)
}

Bug fixes

• Fixes the reported number of affected files
• Files that conflict with directories after the merge will now be printed as a clear error message to the user, instead of failing at runtime
• Fixes the caching directory to reflect the crossVersion, where previously, cross-builds (i.e. 2.12.8, 2.13.8) shared the same cache directory.