从另一个java进程启动的java进程没有＆没有按预期工作

Question

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.

Answer 1

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!