如何保持多线程Java / Scala应用程序运行？

Question

I'm pretty new to Scala/Java and have to build a service that will have at least two sub-services running: a socket-based subscriber that listens for messages to kick off workers, and a web server that will serve a status page for those workers.

I can get these things to run, but after both start the whole process immediately exits with code 0.

I did some research to learn about user threads vs daemon threads in Java, as well as threading in general, so now my approach is basically this:

val webServerThread = new Thread(WebServer(config)).start()
val subscriberThread = new Thread(Subscriber(config)).start()
val aliveThread = new Thread(keepAlive(true)).start()

The third thread simply contains a while(true){} block to leave a user thread up.

There has to be a smarter way of doing this, but I don't know what it is and seems impossible to discover. How do http server's stay running, for example? Is there a while(true) loop underneath every framework out there?

Any help would be appreciated.

Answer 1

The run() method of the thread would have to be an endless loop, until some condition occurs and you exit the loop.

To wait for a thread to exit, the way to do that is as follows:

final Runnable runnable = new Runnable() {
    public void run() {
        // do stuff
    }
};

final Thread thread = new Thread(runnable);
thread.start();
try {
    thread.join();
} catch (final InterruptedException e) {
    // deal with exception
}

Obviously, that will only wait for one thread. It depends on your scenario whether this makes sense. Alternatively, you could use a ThreadPoolExecutor , invoke the shutdown() or shutdownNow() method, and use awaitTermination for it to stop.

Answer 2

So for all services that have to stay running, say a web server or something, is there some code somewhere that just is basically while(shouldRun) {//nothing} ?

No. There is never any reason to have a JRE thread that does nothing, and a thread that uses 100% CPU while accomplishing nothing would be even worse.

Pretty much every thread in a program should sit in a loop, waiting for something to do .* Eg, An I/O thread that waits to receive and process input from some external source, a pool thread that waits for tasks to perform, a scheduler thread that waits until it's time to perform the next scheduled task.

A web service must have at least one thread that sits in a loop and waits to handle incoming connections. I don't remember how to write that without doing some research first because there are so many open-source web servers out there: There's no reason to write your own except for practice. There is even one built-in to the Oracle JRE.** In pseudo code, it might look like this:

while (! time_to_shut_down) {
    connection = WaitForIncomingConnection();
    clientThreadPool.handle(connection);
}

I can get these things to run, but after both start the whole process immediately exits with code 0.

I do not know why your program won't stay running. I am not familiar with Scala or, with the WebServer class or the Subscriber class.

What is config ? Maybe somebody would be able to help you if you would ammend your question to show how you create the configuration object.

---

*One exception to that rule would be a compute-thread in a program that performs a single, massive computation and then exits.

**See https://stackoverflow.com/a/3732328/801894 . The server.start(); call in that example is what kicks off the service thread. And, notice that the main() thread terminates right after it starts the server thread.