Best Way to Gracefully Shutdown a Java Command Line Program

Question

Hello,

I'm interested in different approaches to gracefully shutting down a Java command line program. Sending a kill signal is not an option.

I can think of a few different approaches.

1. Open a port an wait for a connection. When one is made, gracefully shutdown.
2. Watch for a file to be created, then shutdown.
3. Read some input from the terminal, such as "execute shutdown"

The third one is not ideal, since there is often program output pumped to the screen. The first one takes too much effort (I'm lazy). Do most programmers use the second option? If not, what else is possible/elegant/simple?

Thanks in advance. Mike

Answer 1

The first two option is simple to implement. You could also use some JMX stuff (I don't know much about that). Tomcat uses the first approach and I applied 1 and 2 in two of my projects.

Answer 2

you could try to use Runtime.getRuntime().addShutdownHook() that should satisfy your requisite. In this way you can register an hook to do cleanups, in order to perfom a gracefull shutdown.

EDIT

http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Runtime.html#addShutdownHook(java.lang.Thread) public void addShutdownHook(Thread hook) 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.

Answer 3

you can try something like this:

Runtime.getRuntime().addShutdownHook(new Thread() {
    public void run() { /*
       my shutdown code here
    */ }
 });

edit:

the shutdown hook will not perform the shutting down of the app. instead, it gives the developer a way to perform any clean-up that he/she wishes at shutdown.

from the JavaDoc for Runtime (a good read if you are planning to use this method):

A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. ...

Answer 4

The benefit of the second option - checking for a file - over the first - listening on a port - is that you have some possibility of security.

You can set the permissions on the directory where the file is created so that only appropriate users can close the program. If you listen on a port any user can connect to it.

Answer 5

I would suggest to use the shutdown hook. It will allow your program do be controlled using standard OS tools. It also does not need any additional access to external resources (disk, ports, whatever).

Answer 6

Say I decide to use the shutdown hook. What is the best method for initiating the shutdown? Do I have to catch the signal in Java? From the experimenting I've done, Ctrl+C kills the program and the shutdown hook is not executed.

At the moment, I favor the second option with file permissions. Thanks for the responses.

Answer 7

If you wanted to go with the socket version, it is very simple to implement. Here's the code:

ServerSocket server = new ServerSocket(8080);
System.out.println("Socket listening!");
server.accept();
System.out.println("Connection received!");

You could easily embed this code in a separate thread that you start with your program, and then have it modify global state to initiate shutdown.

Answer 8

Consider having a JMX component. Then you can attach with JConsole either locally or over the network, and communicate with your component. Then the component can shut down the program properly.

With Java 6 u 10 or later, you can do the same with JVisualVM.