Is there a way to dynamically restart a HelloWorld Java program which loops print outputs?

Question

Consider the following simple code that prints "Hi world" forever:

public class WakeMeUpSomehow {

 public static void main(String[] args) {

    while (true) {
    try {
         System.out.println( " Hi world ");
         Thread.sleep(1000);                 //1000 milliseconds is one second.
    } catch(InterruptedException ex) {
         Thread.currentThread().interrupt();
    }
    }
 }
}

Here is the output:

enter image description here

Is there a way to devise an external third program , which watches this program to notice when we kill it (like with CTRL+C in command line); and then this "parent" program resumes the "Hello World" running ?

I think it might look something like this :

enter image description here

So my question is - how can I simulate code like this, which has this kind of fail-safe ability? Is there an way to do this?

thanks !

EDIT: I found a neat link here which is relevant but addresses something a little different- How can I restart a Java application?

With Linux you'd generally use the init process (e.g. upstart or systemd) to supervise long-running services. Maybe Windows has something similar? — Chris Martin, Apr 22 '15 at 01:08
@ChrisMartin - So for Linux this task is easy ? I can try using Linux with a virtual machine — Caffeinated, Apr 22 '15 at 01:13
"Easy" is a stretch, and that'd be overkill just for this, but maybe a good learning exercise? :) — Chris Martin, Apr 22 '15 at 01:42

M A · Accepted Answer · 2015-04-26T20:03:07.100

9

Shutdown hooks are guaranteed to run on normal shutdown events of a program, but not on all kinds of abnormal termination, such as when the JVM is killed forcibly.

One solution may be to use a batch script:

@echo off
setlocal

start /wait java WakeMeUpSomehow

if errorlevel 1 goto retry
echo Finished successfully
exit

:retry
echo retrying...
start /wait java WakeMeUpSomehow

edited Apr 26 '15 at 20:03

answered Apr 26 '15 at 19:57

M A

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1

This can be referred as a 'watchdog script'. – Mikael Lindlöf May 03 '15 at 16:35

score 4 · Answer 2 · answered Apr 22 '15 at 01:05

4

If it's as simple as this, you can simply use a shutdownhook.

Runtime.getRuntime().addShutdownHook(() -> {
    //do whatever you want to be done when closing the program here
});

Though you might encounter some problems, if you want to restart the program and use the same commandline.

answered Apr 22 '15 at 01:05

Paul

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So I'm guessing the `addShutdownHook()` code can go in the original class, then we do anything we like after that – Caffeinated Apr 22 '15 at 01:07
1

Exactly. Just add it before entering the while-loop and it'll do its job – Paul Apr 22 '15 at 01:10
One comment - I did get this to work but.... There's a thread inside the braces correct? – Caffeinated Apr 22 '15 at 18:46
1

the argument is a `Runnable`. `addShutDownHook` creates a `Thread` in initialized state, which will start when the application quits. Inside the braces is the code that'll run when the ShutDownHook is started. – Paul Apr 22 '15 at 18:57
Can any one of you please post full code here ? I tried this but job did not work :( – Sunil Sharma Apr 30 '15 at 06:05
@SunilSharma - this is basically just the structure of it – Caffeinated May 02 '15 at 19:31
@Paul - Ah yes GWB :P – Caffeinated May 03 '15 at 18:43
@Coffee OK sounds good. Can you please tell me what I have to write inside addShutDownHook function to restart the program. – Sunil Sharma May 04 '15 at 03:43

score 4 · Answer 3 · answered Apr 26 '15 at 17:44

first of all it is a challenging task. just try it and let me know whether it is working or not !!!!!

public class WakeMeUpSomehow {
    static class Message extends Thread {

        public void run() {
            try {

                while(true)
                {
                System.out.println("Hello World from run");

                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        try {
            Runtime.getRuntime().addShutdownHook(new Message());
            while(true)
            {
            System.out.println("Hello World");
            Thread.sleep(100);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

When I tried killing program with `CTRL + C` then it actually ran into an infinite loop (printing "hello world from run" ) and I couldn't `CTRL + C` it — Caffeinated, Apr 26 '15 at 18:57

score 1 · Answer 4 · answered Apr 22 '15 at 01:09

1

You can use a shutdown hook which launches your application again. Please note the downside of this - your JVM will never stop running, until you force-kill it.

http://wrapper.tanukisoftware.com/doc/english/qna-shutdown.html

answered Apr 22 '15 at 01:09

kaykay

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Do you mind sharing what's at that link? I hesitate to open external links – Caffeinated Apr 22 '15 at 18:47

score 1 · Answer 5 · edited May 23 '17 at 12:22

1

Without going into the reason why you would want to so such a thing :-), a "robust" way of doing this would be to execute your "parent" program or batch script in:

Windows service to execute java programs as detailed in Install java program as a windows service: Alternative to JavaService?
3PP schedulers
If you are on Linux, you could use the init daemons or Cron schedulers.

The above approaches even withstand computer restarts!

edited May 23 '17 at 12:22

Community

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1

answered Apr 29 '15 at 19:06

KayDK

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Curious , What are `3PP schedulers` ? – Caffeinated Apr 29 '15 at 19:12
1

3PP stands for 3rd Party Publishers :-) i.e Software developed by others. As an example, check http://quartz-scheduler.org/, one of the most famous schedulers for java applications – KayDK Apr 29 '15 at 19:17

score 1 · Answer 6 · answered May 03 '15 at 16:45

You should checkout monit. It provides lot of utilities than just restarting a program.

For e.g.

It checks if your program is consuming more memory (possibly due to memory leakage).
Monitor for file changes.
Restart program in case if it crashes.
Script based support for customizing startup options.

score 1 · Answer 7 · answered May 04 '15 at 00:04

1

It sounds like you'd like to run your application as a Service ( windows ) or Daemon ( Unix ).

In Java you typically use a service wrapper for this. I use Apache Commons Daemon for this.

Commons Daemon provides ProcRun.exe on Windows for running your Java application as a service. You can even rename the executable to that of your program. On Unix you have JSvc for restarting your Java application. I write a thin generic start-up API in my main class which is called by both executables.

This is the only way I believe will do this reliably if you need to do this in production ( and not as an exercise of some sort ).

answered May 04 '15 at 00:04

kervin

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I think this is interesting . it says `but in those cases where the application is not interactive (server applications) there is currently no portable way to notify the Virtual Machine of its imminent shutdown.` - though it's still a little foggy how I'd use it. So you have created some code that restarts your own programs ? – Caffeinated May 04 '15 at 01:47
1

@Coffee Commons Daemon works by providing a small native executable that can run the JVM or host jvm.dll in its own process space. It then calls into your application at various API methods, including when it catches System Signals ( E.g. CTRL+C ). The result is an application at can run as a service (even interactively). Many production Java Server Applications use this approach. – kervin May 04 '15 at 01:59
How does JSvc compare to the `supervise` tool ? I looked up something called daemontools, and found this thing called `supervise' ? curious if we you knew about that – Caffeinated Jun 07 '15 at 23:50

score 0 · Answer 8 · answered May 01 '15 at 16:11

0

In Windows the way to do this is to run your program as a service. It allows among other things the capability to start your program at system start up (with no user logged on) and to restart your program if it shuts down. The Windows Resource kits provided a program called SrvAny.exe that allows you to run any program as a service.

https://support.microsoft.com/en-us/kb/137890

answered May 01 '15 at 16:11

JJF

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Hmm ? Not sure I see what you mean – Caffeinated May 02 '15 at 19:32

Is there a way to dynamically restart a HelloWorld Java program which loops print outputs?

8 Answers8