Question

I have a concurrent server where a user chooses an option to display. Every time any option is selected a counter increments displaying only the total for all options selected. Individual counters are NOT required for different options; only total is required.

After referencing the JAVA API the most sensible method appeared to be using atomic integers and everything worked perfectly as expected.

I produced the following:

ServerProtocol.java

public class ServerProtocol {

private static final int ANOTHER = 2;
private static final int OPTIONS = 3;

case OPTIONS:
    if (theInput.equals("1")) {
        theOutput = "computer program description here  -- Another? Y or N";

        DownloadCounter counter = new DownloadCounter();
        counter.incrementCount();
        System.out.println(counter);

        TrackDownloads clientDetails = new TrackDownloads();
        clientDetails.trackLocalhostAddress();
        state = ANOTHER;
case ANOTHER:

DownloadCounter.java

import java.util.concurrent.atomic.AtomicInteger;

public class DownloadCounter {

    private static AtomicInteger count = new AtomicInteger(0);

    public void incrementCount() {
        count.incrementAndGet();
    }

    @Override
    public String toString() {
        return "Total Downloads =" + count;
    }
}

The problem being that my lecturer has turned around and said I cant use atomic integers!

They have supplied me with some code that I must include in my project. That is:

 int tmp = yourCounter;
     try {
         Thread.sleep(5000);
     } catch (InterruptedException ex) {
         System.out.println("sleep interrupted");
     }
  yourCounter = tmp + 1;

The problem is I don’t seem to be able to use this counter concurrently. I have so far produced the following:

ServerProtocol.java

public class ServerProtocol {

private static final int ANOTHER = 2;
private static final int OPTIONS = 3;
private static int yourCounter;

case OPTIONS:
    if (theInput.equals("1")) {
        theOutput = "computer program description here  -- Another? Y or N";

        int tmp = yourCounter;
        try {
            Thread.sleep(5000);
        } catch (InterruptedException ex) {
             System.out.println("sleep interrupted");
        }
        yourCounter = tmp + 1;

        System.out.println("Download Total " + yourCounter);

        TrackDownloads clientDetails = new TrackDownloads();
        clientDetails.trackLocalhostAddress();
        state = ANOTHER;
case ANOTHER:    

With the above method, when 2 or clients request the option simultaneously, the yourCounter variable only increments once. I’m expecting that only a single client is granted access to the variable at once to ensure the counter remains accurate. Any ideas?

Thanks in advance.

Answer 1

You have to use the synchronized keyword. Each access to the counter should be in a synchronized block that locks the same object: either a static synchronized method of the class, or a synchronized block that explicitly locks TheClass.class.