Jar Jar Abrams

in

Jar Jar Abrams is an experimental Scala extension of Jar Jar Links, a utility to shade Java libraries.

For library authors, the idea of other library is a double-edged sword. On one hand, using other libraries avoids unnecessary duplication of work, not using other libraries is almost hypocritical. On the other hand, each library you add would add a transitive dependency to your users, increasing the possibility of conflict. This is partly due to the fact that within a single running program you can one have one version of a library.

This type of conflict happens often in a setup, where a program runs on top of a runtime or a framework. sbt plugins are like that. Spark is another example. One way to mitigate this to shade the transitive libraries under your own package. In 2004, herbyderby (Chris Nokleberg) created a tool called Jar Jar Links that can repackage libraries.

In 2015, Wu Xiang added shading support to sbt-assembly using Jar Jar Links. This was a step forward, but the challenges remained. One of the issues was that Scala compiler includes ScalaSignature information into the *.class files, but Jar Jar was not aware of it. Fast forward to 2020, Jeroen ter Voorde at Simacan contributed ScalaSignature conversion in sbt-assembly#393. (Thanks Simacan!) I wanted to split this up into its own library, since this might be useful beyond sbt. This is Jar Jar Abrams.

core API

At the core there's Shader object that implements shadeDirectory function.

package com.eed3si9n.jarjarabrams
 
object Shader {
  def shadeDirectory(
      rules: Seq[ShadeRule],
      dir: Path,
      mappings: Seq[(Path, String)],
      verbose: Boolean
  ): Unit = ...
}

The function expects the dir to be a directory containing unzipped JAR file.

sbt-jarjar-abrams

To demonstrate the usage, I created an sbt plugin that shades one library at a time.

Add the following to project/plugins.sbt:

addSbtPlugin("com.eed3si9n.jarjarabrams" % "sbt-jarjar-abrams" % "0.1.0")

build.sbt would look like this:

ThisBuild / version := "0.1.0-SNAPSHOT"
ThisBuild / organization := "com.example"
ThisBuild / scalaVersion := "2.12.11"
 
lazy val shadedJawn = project
  .enablePlugins(JarjarAbramsPlugin)
  .settings(
    name := "shaded-jawn",
    jarjarLibraryDependency := "org.typelevel" %% "jawn-parser" % "1.0.0",
    jarjarShadeRules += ShadeRuleBuilder.moveUnder("org.typelevel", "shaded")
  )
 
lazy val use = project
  .dependsOn(shadedJawn)

jawn-parser is now shaded under shaded package. We can confirm that using the REPL:

sbt:jarjar> use/console
[info] Starting scala interpreter...
Welcome to Scala 2.12.11 (OpenJDK 64-Bit Server VM, Java 1.8.0_232).
Type in expressions for evaluation. Or try :help.
 
scala> shaded.org.typelevel.jawn.Facade
res0: shaded.org.typelevel.jawn.Facade.type = shaded.org.typelevel.jawn.Facade$@131cedd

We can try stacking multiple-layers of shaded libraries by mimicking the original dependency graph:

ThisBuild / organization := "com.example"
ThisBuild / scalaVersion := "2.12.11"
 
lazy val shadedJawn = project
  .enablePlugins(JarjarAbramsPlugin)
  .settings(
    name := "shaded-jawn",
    jarjarLibraryDependency := "org.typelevel" %% "jawn-parser" % "1.0.0",
    jarjarShadeRules += ShadeRuleBuilder.moveUnder("org.typelevel", "shaded")
  )
 
lazy val shadedJawnAst = project
  .enablePlugins(JarjarAbramsPlugin)
  .dependsOn(shadedJawn)
  .settings(
    name := "shaded-jawn-ast",
    jarjarLibraryDependency := "org.typelevel" %% "jawn-ast" % "1.0.0",
    jarjarShadeRules += ShadeRuleBuilder.moveUnder("org.typelevel", "shaded")
  )
 
lazy val use = project
  .dependsOn(shadedJawnAst)

Here's REPL:

sbt:jarjar> use/console
[info] Starting scala interpreter...
Welcome to Scala 2.12.11 (OpenJDK 64-Bit Server VM, Java 1.8.0_232).
Type in expressions for evaluation. Or try :help.
 
scala> shaded.org.typelevel.jawn.ast.JParser.parseUnsafe("""{ "x": 10 }""")
res0: shaded.org.typelevel.jawn.ast.JValue = {"x":10}

use at your own risk

I want to note again that all this is experimental. Many libraries depend on things like config files and other runtime behaviors that Jar Jar Abrams won't convert.

I'm cautiously optimistic that this may be able to shade some transitive libraries out of sbt, so sbt plugin authors can freely use different versions.