How to gracefully handle the SIGKILL signal in Java

Question

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 lt Thread gt  runningThreads   Thread getAllStackTraces   keySet            for  Thread th   runningThreads                if  th    Thread currentThread                     amp  amp   th isDaemon                     amp  amp  th getClass   getName   startsWith  org brutusin                      System out println  Interrupting      th getClass        termination                    th interrupt                                    for  Thread th   runningThreads                try                   if  th    Thread currentThread                     amp  amp   th isDaemon                     amp  amp  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

User · Answer

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

[java] How to gracefully handle the SIGKILL signal in Java

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