Question

I have a class, called counter. It looks like this:

public class Counter {
  private int count;

  public Counter() {
    count = 1;
  }

  public int getCount(){
    return count;
  }

  public void incrementCount(){
    count++;
  }

I want to share a single instance of this between every user of a tomcat application.
So user 1 and user 2 would both see getCount() as the same value.
Assume for this that there is a technical reason why I can’t store and read from a database.

Any advice?

Thanks.

Answer 1

Just create one and store it in the application scope during server’s startup.

@WebListener
public class Config implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent event) {
        event.getServletContext().setAttribute("counter", new Counter());
    }

    // ...
}

This way it’s available in every servlet as follows:

Counter counter = (Counter) getServletContext().getAttribute("counter");
// ...

And in every JSP as follows:

<p>The count is ${counter.count}</p>

See also:

• What’s the difference between static methods and applying singleton pattern?
• Synchronized singleton pattern

---

Unrelated to the concrete problem, your counter is not threadsafe. I’d suggest to use AtomicInteger instead of int.

public class Counter {
  private AtomicInteger count = new AtomicInteger();

  public Counter() {
    count.incrementAndGet();
  }

  public int getCount(){
    return count.get();
  }

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