Monitors in Java two printers

Question

I've implemented a problem with two printers that two printers can not print at the same time, for example printer A is printing and B can not, as easy as it, I did it with Semaphores as follows :

My Printer.class looks like

public class Printer extends Thread {

    Semaphore mutex,multiplex;
    PrinterMachine printerMachine;
    String printer = "";
    public Printer(Semaphore mutex, Semaphore multiplex, PrinterMachine pm) {
        this.multiplex = multiplex;
        this.mutex = mutex;
        printerMachine = pm;
    }
    @Override
    public void run() {
        String printer = "";
        for(;;) {
            try {multiplex.acquire();} catch (InterruptedException e) {}
            try {mutex.acquire();} catch (InterruptedException e) {}
            if(printerMachine.getCanPrintA()) {
                printer = "A";
                printerMachine.setCanPrintA(false); 
            }
            else {
                printer="B";
                printerMachine.setCanPrintB(false); 
            }
            mutex.release();
            try {Thread.sleep(100);} catch (InterruptedException e) {}
            System.out.println(printer);
            if(printer.equals("A")) {
                printerMachine.setCanPrintA(true);
                }
            else {
                printerMachine.setCanPrintB(true);
            }
            try {Thread.sleep(100);} catch (InterruptedException e) {}
            multiplex.release();
        }
    }

}

Then I have a class that shares a variable

class PrinterMachine{
    public volatile Boolean canPrintA = true,canPrintB = true;
.... //Getter and Setter

And then I have my main

public static void main(String[] args) {
        Semaphore mutex = /* COMPLETE */ new Semaphore(1);
        Semaphore multiplex = /* COMPLETE */ new Semaphore(2);
        PrinterMachine pm = new PrinterMachine();

        Printer printers[] = new Printer[10];
        for (int i = 0 ; i<printers.length; i++) {
            printers[i] = new Printer(mutex,multiplex,pm);
            printers[i].start();
        }   
        try {
            Thread.sleep(5000);
        }
        catch(InterruptedException ie) {}
        for (int i = 0 ; i<printers.length; i++) {
            printers[i].stop();
        }
    }

It is working ok, but I'm wondering how do I change my semaphores to work with monitors instead?

---

EDIT

Problem?

I have two printers and I can not print a document (System.out.println()) at the same time, so I did a program with Semaphores to do this, and with that I can not print on A and B printers at the same time, and now I'm trying to instead of using Semaphores do it with Monitors.

Answer 1

The monitor is something that the jvm does for you when you use the good old synchronized code block / method. This would look like this:

Object mutex = new Object(); // it can be any object, even a class.
                             // However, it's preferred to have a separate class only for this.
for (int i = 0 ; i<printers.length; i++) {
    printers[i] = new Printer(mutex, pm); // object sharing the mutex object
    printers[i].start();
}

And within a printer:

public class Printer extends Thread {
    private final Object mutex;
    // ..

    public Printer (Object mutex, ...) {
        this.mutext = mutex;
    }

    @Override
    public void run() {
        synchronized(mutex) { //all Printers synchronize the same object
            System.out.println("A");
        }
    }

The code block above makes sure that only one printer can be in the synchronized(mutex) code block. In your code that was Semaphore mutex = new Semaphore(1); I.e. two printers can not be there at the same time. This is easy, simple, and no shared memory is required.

On the other hand, the Semaphore multiplex = new Semaphore(2); must be a semaphore. You might use your own counter and re-implement Semaphore. That's likely to be buggy and slow because these problems are more complicated than they seem to be. I suggest to use Semaphore(2) when necessary.

Disclaimer: I don't fully understand the problem. Reverse-engineering the solution would lead to misunderstandings.