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).