如何通知守护进程改变行为

Question

I've written a Java application that is launched as a daemon (I daemonize redirecting stderr and stdout and closing stdin though bash). However, occasionally I would like to be able to message this application and inform it to change certain parts of its behavior. I need to be able to message the application from a terminal, so anything that requires a graphical utility is a no-go.

The change in behaviour is fairly simple. I need a toggle for the state of one thread in my application, and a way to gracefully close the application.

What are my options in achieving this? I know I could have a thread in the process that listens on a socket of some sort for messages, but this seems like overkill for the needs I have.

I'm not too familiar with Signals on Linux/Unix, so I'd like to ask if I could simply set up a custom handler for some signals, and have my code execute when the process receives a signal.

Are there any other options that I simply don't know about or haven't thought about?

Answer 1

Signals may be the easiest. You have SIGUSR1 and SIGUSR2 available for application use and you can write a handler for them to manage your toggle. Generally you don't want to do much in a handler so you can set a switch and your main loop (or whatever) needs to check the switch and act accordingly. Similarly the receipt of the signal could be programmed to read a config file that you changed.

Beyond that you can use any available IPC (fifos, sockets, MQs) but you are going to either have thread blocking on them or somehow incorporate them a select statement (or whatever the java equivalent of that is).

Answer 2

Personally, I like to use OS signals for this.

Use java.lang.Runtime.addShutdownHook and send a SIGTERM, SIGINT or SIGHUP.

Be aware that some signals are reserved for internal use, check out the docs for the JVM you are using.

The SignalHandler is not part of the supported, public interface, so your mileage may vary.

If you are instantiating a JVM from C code, simply set signal handlers (in C) before the JVM and you will be fine.

Answer 3

You basically need a small client that communicates with your application (server). So the default solution should be to use some IPC mechanism. Putting an IPC mechanism is a one time effort and is worth the trouble because it scales well with the requirements. Using signals for IPC is not recommended. I think sockets is a good way to go.