Code-Bin #997 - Mutual Exclusion Example

Mutual Exclusion Example
Posted by: Semaphore.java_Dispatcher.java | December 5, 2010 @ 9:28am

Java Code

public class Semaphore {
    private int value;

/**
     * In practice, this ArrayList would be declared as
     * ArrayList<Process>.
     */
     private ArrayList queue;

public Semaphore( int value ) {
        this.value = value;

/*
         * In practice, this.queue would be declared with
         * new ArrayList<Process>.
         */
        this.queue = new ArrayList();
    }

/**
     * In practice, the process parameter would be of type Process.
     */
    public static void P( Object process ) {
        this.value--;

if ( this.value < 0 ) {
            // must have been 0 or less before the decrement
            queue.add( process );
        }
    }

/**
     * In practice, the process parameter would be of type Process.
     */
    public static void V( Object process ) {
        this.value++;

if ( this.value <= 0 ) {
            // must have been negative before the increment
            queue.remove( process );
            // a system call I made up
            System.addToReadyQueue( process );
        }
        System.dispatch(); // dispatches the next process on the ready queue
    }

}

public class Dispatcher {
    public static void main( String[] args ) {

Semaphore mutex = new mutex( 1 );
        Process p1 = new Process();
        Process p2 = new Process();

while ( System.notHalt() ) {

// entry section
            mutex.P( p1 ); // mutex.value = 0
            mutex.P( p2 ); // mutex.value = -1, mutex.queue = [p2]

// critical section
            System.dispatch(); // dispatches p1, as p2 is in mutex.queue

// exit section

/*
             * mutex.value = 0, so p2 is taken off mutex.queue and
             * added to System.readyQueue, and the dispatcher runs
             * the next process on the ready queue, which is now p2
             */
            mutex.V( p1 );

/*
             * mutex.value = 1, so nothing happens because
             * mutex.value > 0
             */
            mutex.V( p2 );
        }

}
}

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Author Comments

This is only concept code and will not actually compile or execute.

Rating

4.62 / 8
32 Votes