How to gracefully shutdown a Java application that

Question

There is a an answered question on Best Way to Gracefully Shutdown a Java Command Line Program. A shutdown hook does the job in case when a program was terminated by Ctrl+C.

My question is how to gracefully exit if the command line itself is closed during the execution of a Java program? I tested with shutdown hook but it didn't work in this case. I cannot find out what happens to the virtual machine in this scenario. Is the process and all its threads killed instantly? What kind of signal does closing command line produce?

So, how this particular problem can be solved?

EDIT: The problem concerns Windows environment.

Answer 1

In Java, there is a special Runtime method for that : addShutdownHook.

This allows you to initialize a thread that the JVM will try to run just before stopping. It is the place to put any cleanup you want to execute even in case of Ctrl-C of closing of parent window. Extract from javadoc : A shutdown hook is simply an initialized but unstarted thread. 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.

The shutdown hook is call even if the program ends normally. In that case, it is cleaner to remove the registered hook before exiting with removeShutdownHook (still a method from Runtime)

EDIT :

In the case of Windows environment, there are no real signals, but special callbacks when system is shutting down. AFAIK, the system hook is correctly called in that case, but I admit I never really tested that. In Windows, processes can be asked to terminate with 2 ways :

• PostQuitMessage function posts a WM_QUIT message in process event loop - normally the process should exit, but it can do its cleanup (equivallent of Unix SIG_TERM)
• TerminateProcess immediately stops the process and all its threads (equivallent of Unix SIG_KILL)

Console processes can use a ConsoleControlHandler that can intercept Ctrl-C, Ctrl-Break or Ctrl-Close events. First two are generated through keyboard, last is generated when the user closes the console. But normally, the Oracle JVM should use the system hook mechanisme when getting the Ctrl-Close event that is processed the same as a SIGTERM.

Answer 2

Logically, SIGHUP (terminal hangup) should be raised.

Actually, I've just checked my guess with a simple shell script. Yes, when a user closes a terminal emulator in which an application was started (and from which it wasn't detached), then the application receives SIGHUP. So set up a SIGHUP handler and react accordingly. A usual behaviour is to terminate an application, but your intents may be different.

Also if your Java application performs any STDIN/STDOUT operations, it should be closed or at least re-cofigured when HUP is received, because an attempt to read/write from non existing terminal would lead to SIGPIPE and/or program block.

For Windows take a look at Catch Windows terminal closing on running process

Answer 3

Edit for windows environment:

I don't have much experience on windows environment but if you want your application to keep running, it's generally deployed as Windows service (it's similar to daemon on Linux). You would typically start/stop/restart service through a utility that lists all services (I think you get to it via control panel -> Administrative Tools -> Services. I would guess that issuing a "stop" via this tool would signal a graceful shutdown. And if you kill the service via the task manager, then it won't be a graceful shutdown.

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Is this a Linux based or Windows based environment? In Linux, if you ran the program in background (and exit the shell with 'exit' command), it'll continue running. You can put your application in the background by adding an & at the end. Also, a lot of applications/services run in the background. If you execute a Tomcat startup script with the startup.sh command, it'll continue running in the background even when you quit the terminal you launched it from. On windows too, the concept should be similar.

In terms of closing application, you use kill command on Linux systems. kill command on a process sends a SIGTERM signal to it. Applications can implement code to intercept SIGTERM and shutdown gracefully on detecting a SIGTERM. If the application doesn't handle SIGTERM gracefully, then it won't respond to a SIGTERM / kill. In that case, you need to explicitly give it a SIGKILL (kill -9) to kill it forcefully. In that case, graceful shutdown is not possible.