javap for Ammonite REPL

predef.scala

// ~/.ammonite/predef.sc
// Works on Scala 3, easy to adapt to Scala2
// Make sure:
// 1. You run Ammonite for Scala 3: 3.1-2.5.2+ (see https://github.com/com-lihaoyi/Ammonite/releases )
// 2. You provide required JVM exports:
// JAVA_OPTS="--add-exports=jdk.jdeps/com.sun.tools.javap=ALL-UNNAMED" ~/Downloads/3.1-2.5.2
import java.io.{File, FileOutputStream}
import scala.util.Using
import scala.reflect.ClassTag

// Works only in Scala 3+

def javapWithOptions[T <: AnyRef](options: Array[String])(using ct: ClassTag[T]): Unit = {
  val targetFile = File.createTempFile("javap", "repl.class")
  Using(ct.runtimeClass.getClassLoader.getResourceAsStream(ct.runtimeClass.getName.replace('.', '/') + ".class")) { classStream =>
    Using(new FileOutputStream(targetFile)) { outStream =>
      classStream.transferTo(outStream)
      outStream.flush()
    }
  }

  val task = new com.sun.tools.javap.JavapTask
  // Doesn't work without this line in Ammonite, but works in scala REPL ¯\_(ツ)_/¯
  scala.util.Try(task.handleOptions(Array("-cp", "Dog")))
  task.handleOptions(Array(targetFile.getAbsolutePath))
  task.handleOptions(Array("-c", "-p"))
  task.run()
}

def javap[T <: AnyRef: ClassTag]: Unit = javapWithOptions[T](Array.empty)