Useful example of a shutdown hook in Java?

Solution 1:

You could do the following:

• Let the shutdown hook set some AtomicBoolean (or volatile boolean) "keepRunning" to false
• (Optionally, .interrupt the working threads if they wait for data in some blocking call)
• Wait for the working threads (executing writeBatch in your case) to finish, by calling the Thread.join() method on the working threads.
• Terminate the program

Some sketchy code:

• Add a static volatile boolean keepRunning = true;

• In run() you change to

  for (int i = 0; i < N && keepRunning; ++i)
      writeBatch(pw, i);
  

• In main() you add:

  final Thread mainThread = Thread.currentThread();
  Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {
          keepRunning = false;
          mainThread.join();
      }
  });
  

That's roughly how I do a graceful "reject all clients upon hitting Control-C" in terminal.

From the docs:

When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt.

That is, a shutdown hook keeps the JVM running until the hook has terminated (returned from the run()-method.