How to gracefully handle the SIGKILL signal in Java

How do you handle clean up when the program receives a kill signal?

For instance, there is an application I connect to that wants any third party app (my app) to send a finish command when logging out. What is the best say to send that finish command when my app has been destroyed with a kill -9?

edit 1: kill -9 cannot be captured. Thank you guys for correcting me.

edit 2: I guess this case would be when the one calls just kill which is the same as ctrl-c

It is impossible for any program, in any language, to handle a SIGKILL. This is so it is always possible to terminate a program, even if the program is buggy or malicious. But SIGKILL is not the only means for terminating a program. The other is to use a SIGTERM. Programs can handle that signal. The program should handle the signal by doing a controlled, but rapid, shutdown. When a computer shuts down, the final stage of the shutdown process sends every remaining process a SIGTERM, gives those processes a few seconds grace, then sends them a SIGKILL.

The way to handle this for anything other than kill -9 would be to register a shutdown hook. If you can use (SIGTERM) kill -15 the shutdown hook will work. (SIGINT) kill -2 DOES cause the program to gracefully exit and run the shutdown hooks.

Registers a new virtual-machine shutdown hook.

The Java virtual machine shuts down in response to two kinds of events:

• The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
• The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.

I tried the following test program on OSX 10.6.3 and on kill -9 it did NOT run the shutdown hook, as expected. On a kill -15 it DOES run the shutdown hook every time.

public class TestShutdownHook
{
    public static void main(String[] args) throws InterruptedException
    {
        Runtime.getRuntime().addShutdownHook(new Thread()
        {
            @Override
            public void run()
            {
                System.out.println("Shutdown hook ran!");
            }
        });

        while (true)
        {
            Thread.sleep(1000);
        }
    }
}

There isn't any way to really gracefully handle a kill -9 in any program.

In rare circumstances the virtual machine may abort, that is, stop running without shutting down cleanly. This occurs when the virtual machine is terminated externally, for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows.

The only real option to handle a kill -9 is to have another watcher program watch for your main program to go away or use a wrapper script. You could do with this with a shell script that polled the ps command looking for your program in the list and act accordingly when it disappeared.

#!/usr/bin/env bash

java TestShutdownHook
wait
# notify your other app that you quit
echo "TestShutdownHook quit"

I would expect that the JVM gracefully interrupts (thread.interrupt()) all the running threads created by the application, at least for signals SIGINT (kill -2) and SIGTERM (kill -15).

This way, the signal will be forwarded to them, allowing a gracefully thread cancellation and resource finalization in the standard ways.

But this is not the case (at least in my JVM implementation: Java(TM) SE Runtime Environment (build 1.8.0_25-b17), Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode).

As other users commented, the usage of shutdown hooks seems mandatory.

So, how do I would handle it?

Well first, I do not care about it in all programs, only in those where I want to keep track of user cancellations and unexpected ends. For example, imagine that your java program is a process managed by other. You may want to differentiate whether it has been terminated gracefully (SIGTERM from the manager process) or a shutdown has occurred (in order to relaunch automatically the job on startup).

As a basis, I always make my long-running threads periodically aware of interrupted status and throw an InterruptedException if they interrupted. This enables execution finalization in way controlled by the developer (also producing the same outcome as standard blocking operations). Then, at the top level of the thread stack, InterruptedException is captured and appropriate clean-up performed. These threads are coded to known how to respond to an interruption request. High cohesion design.

So, in these cases, I add a shutdown hook, that does what I think the JVM should do by default: interrupt all the non-daemon threads created by my application that are still running:

Runtime.getRuntime().addShutdownHook(new Thread() {
    @Override
    public void run() {
        System.out.println("Interrupting threads");
        Set<Thread> runningThreads = Thread.getAllStackTraces().keySet();
        for (Thread th : runningThreads) {
            if (th != Thread.currentThread() 
                && !th.isDaemon() 
                && th.getClass().getName().startsWith("org.brutusin")) {
                System.out.println("Interrupting '" + th.getClass() + "' termination");
                th.interrupt();
            }
        }
        for (Thread th : runningThreads) {
            try {
                if (th != Thread.currentThread() 
                && !th.isDaemon() 
                && th.isInterrupted()) {
                    System.out.println("Waiting '" + th.getName() + "' termination");
                    th.join();
                }
            } catch (InterruptedException ex) {
                System.out.println("Shutdown interrupted");
            }
        }
        System.out.println("Shutdown finished");
    }
});

Complete test application at github: https://github.com/idelvall/kill-test

There are ways to handle your own signals in certain JVMs -- see this article about the HotSpot JVM for example.

By using the Sun internal sun.misc.Signal.handle(Signal, SignalHandler) method call you are also able to register a signal handler, but probably not for signals like INT or TERM as they are used by the JVM.

To be able to handle any signal you would have to jump out of the JVM and into Operating System territory.

What I generally do to (for instance) detect abnormal termination is to launch my JVM inside a Perl script, but have the script wait for the JVM using the waitpid system call.

I am then informed whenever the JVM exits, and why it exited, and can take the necessary action.

You can use Runtime.getRuntime().addShutdownHook(...), but you cannot be guaranteed that it will be called in any case.

There is one way to react to a kill -9: that is to have a separate process that monitors the process being killed and cleans up after it if necessary. This would probably involve IPC and would be quite a bit of work, and you can still override it by killing both processes at the same time. I assume it will not be worth the trouble in most cases.

Whoever kills a process with -9 should theoretically know what he/she is doing and that it may leave things in an inconsistent state.