Develop Reference

How do I create a daemon which executes TimerTasks?

I need to create a daemon in Java which periodically retrieves data via HTTP and stores that in a database.
When the main thread starts up, it reads the data sources and poll intervals from a configuration file and creates a TimerTask with the appropriate interval for each data source. In addition, it calls Runtime.getRuntime().addShutdownHook() to add a shutdown hook which performs any cleanup needed before shutdown. After that, the main thread has nothing else to do.
The daemon is intended for use in a classic Unix environment, i.e. controlled with a start/stop script, though it should be portable to other OSes (say, Windows with SrvAny).
How would I go about this? If I just let the main thread exit, will the TimerTask instances keep the VM running until all of them have been cancelled? If not, how would I accomplish this?

Threads in Java have a flag to indicate if they should keep the jvm alive or not. This flag is called "daemon": the jvm will exit when only daemon threads are running.
The thread started by Timer is not a daemon thread by default, so it will keep the jvm alive, which is what you want. If you wanted the jvm to exit, you could create the timer with new Timer(true) - this would set the daemon flag. https://docs.oracle.com/javase/10/docs/api/java/util/Timer.html#%3Cinit%3E(boolean)

It depends on the Timer on which the TimerTask was scheduled: if that Timer was created not to run its tasks as daemons, a pending TimerTask will keep the VM alive even after the main thread has finished its work. This is the case for all Timer constructors which do not take a boolean argument, or where the boolean argument is false.
The following seems to work as intended:
package com.example.daemon;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
public class SampleDaemon {
private static Timer testTimer = new Timer(false);
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(new Thread() {
#Override
public void run() {
System.out.println("Received shutdown request!");
if (testTimer != null)
testTimer.cancel();
testTimer = null;
}
});
testTimer.schedule(new TestTimerTask(), new Date(), 2000);
}
private static class TestTimerTask extends TimerTask {
#Override
public void run() {
System.out.println("Still running…");
}
}
}
It prints Still running… every 2 seconds. When the JVM receives a SIGTERM, the program prints Received shutdown request! and exits. This can also be accomplished by sending Ctrl+C from the console. SIGHUP does the same as SIGTERM.
SIGCONT has no effect; SIGUSR1 results in a hard exit (presumably the same as SIGKILL), i.e. without executing the shutdown hook.

A java process launched from another java process without & doesn't work as expected

Suppose a little Java procces whose task is to launch other Java processes. The procedure is similar to the following:
String[] command = { "/bin/sh", "-c", "some.sh" + " &"};
Process pro = Runtime.getRuntime().exec(command);
//rest
This first option works because the &, and this another one doesn't work:
String[] command = { "/bin/sh", "some.sh"};
Process pro = Runtime.getRuntime().exec(command);
//rest
Q: What is the meaning of "it doesn't work"?
A: Both options launch the process but in the second one the child process stops working after a few seconds, however, if I inspect running processes (ps aux | grep some.sh), it is there (but doing nothing). The first option works fine, it lauches process and the child does its task.
I don't understand why when I launch child process without background it appears like active in ps processes list but it isn't doing nothing.

Launching a command in Unix with & at the end implies that it will be followed by another command. I presume that if the process is halted and doing nothing, it is likely because it isn't intelligent enough to realize that another command isn't coming.
Therefore, the reason why the first doesn't close but seems to be doing nothing is precisely because of this added &. I imagine that some.sh ends. Perhaps it shouldn't, but it is.
Please look into Apache Tomcat daemon for information concerning how to create a daemon (under section Unix daemon). In your code, you should create a shutdown variable and shutdown hook so that when your daemon is halted, you can execute code:
private volatile boolean shutdown = false;
...
Runtime.getRuntime().addShutdownHook(new Thread() {
#Override
public void run() {
// What to run on shutdown
shutdown = true;
}
});
Once you have this, perform some action every so often in an infinite loop (using sleep of course or your CPU would be wasted):
while(!shutdowwn) {
// Perform action here every 1000 milliseconds.
Thread.sleep(1000);
}
Apache Tomcat daemon can be run on windows as a service or just as well in Linux/Unix. Hope that helps!

Shutdown hook from UNIX

I am trying to get my Java program to exit gracefully on my unix server. I have a jar file, which I start through a cron job in the morning. Then in the evening, when I want to shut it down, I have a cron job which calls a script that finds the PID and calls kill -9 <PID>. However, it doesn't seem that my shutdown hook is activated when I terminate this way. I also tried kill <PID> (no -9) and I get the same problem. How can I make sure the shutdown hook gets called? Alternatively, perhaps there is a better way to kill my process daily.
class ShutdownHook {
ShutdownHook() {}
public void attachShutDownHook() {
Runtime.getRuntime().addShutdownHook(new Thread() {
#Override
public void run() {
System.out.println("Shut down hook activating");
}
});
System.out.println("Shut Down Hook Attached.");
}
}

You can use code like this on Unix to trap SIGINT (#2) signal:
Signal.handle(new Signal("INT"), new SignalHandler() {
public void handle(Signal sig) {
// Forced exit
System.exit(1);
}
});

kill -9 <pid> sends a KILL signal. This signal cannot be intercepted by the program.
If you call kill <pid>, the TERM signal (15) wil be sent. In that case, the JVM will catch the signal and the shutdown hooks will be executed.

This has nothing to do with the signals the JVM is trapping/receiving but everything to do with the terrible shutdown process of Gnome, which apparently needs to be cooperative not to absolutely shit the bed (and the jdk doesn't have the api for this). If you want to see a even worse consequence of this, try to run:
dbus-monitor --profile --session type='method_call',interface='org.gnome.SessionManager'
on a shell, and logout or restart: it will crash gnome-shell and hang the computer until you login on a TTY and order a restart.
Maybe kdbus will fix this on this case, maybe not. The only thing i know is that shutdownhooks on a java application that is using AWT (not command line) will NEVER run its shutdownhooks on GNOME3. Actually, the VM will always exit with non-zero code (failure) presumably from native code. At least it doesn't hang, although this makes shutdown hooks quite useless
(i've been trying to make a workaround by using dbus-monitor, but as you can see from the example i gave, it's a bit too dangerous too).

Terminate a Thread thats running a jar

I have a class that extends Thread and when started, it runs a jar's Main method. When I terminate the Thread it does not kill the process it started. How do I go about that? Thanks!
Below if the run() method from my custom Thread class.
#Override
public void run() {
if(parent != null) {
MessageConsole console = new MessageConsole(parent.getConsole().getTextPane());
console.redirectOut(new Color(240, 240, 240), null);
console.redirectErr(Color.RED, null);
console.setMessageLines(20);
}
if(oParent != null) {
MessageConsole console = new MessageConsole(oParent.getConsole().getTextPane());
console.redirectOut(new Color(240, 240, 240), null);
console.redirectErr(Color.RED, null);
console.setMessageLines(20);
}
try {
somejar.SomeJar.main(args);
} catch (Exception ex) {
System.err.println("Engine failed to start!\nSuggestion: Restart the application, and if the issue is not fixed reinstall the application.");
}
}

When I terminate the Thread it does not kill the process it started. How do I go about that?
There is no reliable way to do that.
Firstly, there is no reliable way to terminate the thread. The preferred way to try is to call Thread.interrupt() on the thread, but this may be ignored. If you call the deprecated method Thread.stop(), you are liable to destabilize your JVM.
Second, even assuming you kill the JAR thread, you cannot reliably kill any child threads that it created. You cannot reliably distinguish the child threads from other threads created by other parts of your application. And if you could identify them, you cannot reliably kill them.
The best you can do is launch a new JVM as an external process to run the JAR. Under normal circumstances a JVM can reliably kill an external process that it has created. (There are exceptions, e.g. setuid commands, but they shouldn't be an issue here.)

You cannot terminate a thread without terminating the process.
You can signal it to stop e.g. interrupt() or throw an exception, but the only way to get it to stop is for it to return or throw a Throwable from the run() method.

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.

Reference https://aws.amazon.com/blogs/containers/graceful-shutdowns-with-ecs/
import sun.misc.Signal;
import sun.misc.SignalHandler;
public class ExampleSignalHandler {
public static void main(String... args) throws InterruptedException {
final long start = System.nanoTime();
Signal.handle(new Signal("TERM"), new SignalHandler() {
public void handle(Signal sig) {
System.out.format("\nProgram execution took %f seconds\n", (System.nanoTime() - start) / 1e9f);
System.exit(0);
}
});
int counter = 0;
while(true) {
System.out.println(counter++);
Thread.sleep(500);
}
}
}

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.