Thursday, September 12, 2013

Ctrl-C shutdown hook in Scala

If you need to intercept Ctrl-C, SIGINT, SIGHUP, etc. in Scala, and handle it on the main thread, it is necessary to join() back to the main thread from within the shutdown hook as below because, unlike C signal handlers, the process terminates at the end of the shutdown handler, even though there is no explicit call to System.exit().

object ctrlctest {
  @volatile var keepRunning = true

  def main(args: Array[String]) {
    val mainThread = Thread.currentThread();
    Runtime.getRuntime.addShutdownHook(new Thread() {override def run = {
      println("inside addShutDownHook handler")
      keepRunning = false
    while (keepRunning) {println("in while loop")}
    // Graceful shutdown code goes here that needs to be on the main thread
    println("Exiting main")
And the resulting output is:
in while loop
in while loop
in while loop
inside addShutDownHook handler
in while loop
Exiting main