SIGTERM,SIGINT,SIGKILL and Java's shutdownHook 📎
The SIGTERM signal requests termination, whereby SIGKILL ...is sent to a process to cause it to terminate immediately...
A Java application behaves accordingly. The shutdown hook (shutdownListener):
public class App {
public static void main(String[] args) {
var shutdownListener = new Thread(){
public void run(){
System.out.println("shutdown in 10s ");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {}
}
};
Runtime.getRuntime().addShutdownHook(shutdownListener);
Executors.newSingleThreadScheduledExecutor().
scheduleAtFixedRate(() -> System.out.println("."), 0, 2, TimeUnit.SECONDS);
}
}
...is executed for SIGTERM and SIGINT. The JVM executes the hook and waits, in the case above for 10 seconds, with the shutdown procedure until
the completion of the shutdownHook.
To send a signal, you will need the process id first. The jcmd returns PIDs from running Java processes, e.g: [PID] airhacks.App.
kill -TERM [PID] sends the SIGTERM (and kill -INT [PID] SIGINT)
signal to the JVM and initiates the shutdown:
.
.
shutdown in 10s
.
.
The kill -KILL [PID] command immediately kills the JVM, even with the running hook, and produces the following output:
[1] [PID] killed java -cp target/signals.jar airhacks.App
Also checkout kill -l for a list of all available signals.
kubernetes (and so openshift) is using SIGTERM and after a grace period, SIGKILL signals to kill containers in a POD.