use flatbuffers java from sbt

build.sbt

import scala.sys.process.Process

scalaVersion := "2.12.6"

libraryDependencies ++= Seq(
  "com.google.flatbuffers" % "flatbuffers-java" % "1.9.0"
)

sourceGenerators in Compile += Def.task {
  val files = (file("src/main/fbs") ** "*.fbs").get.map(_.getAbsolutePath).toList
  val out = (sourceManaged in Compile).value
  val args = "flatc" :: "--java" :: "-o" :: out.getAbsolutePath :: files
  Process(args).!
  (out ** "*.java").get
}