How to force a single thread server?

[tobiascapin]

Hi, I'm using nanohttpd as local REST api server that has to serve a single client at once.
I don't want to have multiple threading due to the fact I need to process the request exacltly in the receive order and I see with the standard implementation sometimes the requests order are mixed.
I tried to add a synchronized block into the server method but it didn't help, then I'm trying to understand if I can force a single-thread custom AsyncRunner but I didn't find documentation about it.

Is it possibile to define a single threaded AsyncRunner?
Do you have any other idea about?

[liushuai42]

Try this code, all the clients will be served in the same thread.

public class SingleThreadAsyncRunner extends NanoHTTPD.DefaultAsyncRunner {
    private final ExecutorService executor = Executors.newSingleThreadExecutor(r -> {
        Thread t = new Thread(r);
        t.setDaemon(true);
        t.setName("Single Thread Request Processor");
        return t;
    });

    @Override
    public void exec(NanoHTTPD.ClientHandler clientHandler) {
        executor.execute(clientHandler);
    }
}

public class MyNanoServer extends NanoHTTPD {
    public MyNanoServer(int port) {
        this(null, port);
    }

    public MyNanoServer(String hostname, int port) {
        super(hostname, port);

        setAsyncRunner(new SingleThreadAsyncRunner());
    }
}

PS.

I tried to add a synchronized block into the server method but it didn't help

May be you locked the wrong OBJECT. I tried to add the synchronized keyword to the method public synchronized Response serve(IHTTPSession session) which works as expected.

public class MyNanoServer extends NanoHTTPD {
    public MyNanoServer(int port) {
        this(null, port);
    }

    public MyNanoServer(String hostname, int port) {
        super(hostname, port);
    }

    @Override
    public synchronized Response serve(IHTTPSession session) {
        try {
            Thread.sleep(3000); // a long time to wait request
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        return super.serve(session);
    }
}

[tbadalov]

I tried to add a synchronized block into the server method but it didn't help

Synchronized doesn't guarantee fairness, i.e. preservation of order. As a quick alternative, try ReentrantLock with fairness enabled.