Question

Right now, I have a piece of code that contacts another server asking if an item is in a list, and returns a boolean value based on that returned value.

The code goes like so:

public boolean checkIfOnline(int accountId) {
    //First loop is incase if someone is already checking. Second is for the checking that this account is doing.
    while (isCheckingIfOnline) {
        try {
            Thread.sleep(1);
        } catch (InterruptedException ex) {
        }
    }
    isCheckingIfOnline = true;
    sendCheckIfOnline(accountId);
    while (isCheckingIfOnline) {
        try {
            Thread.sleep(1);
        } catch (InterruptedException ex) {
        }
    }
    return onlineResponse;
}

The onlineResponse and isCheckingIfOnline are changed within a method that handles what the other server returns, and this is the method I’ve thrown together to have the system wait for the other server to respond. Obviously, this is very flawed, as when this method gets called often, it’ll slow down the system since it only allows for one query at a time, when it should allow for multiple queries to be executed simultaneously.

What other method could I use that accomplishes what the above code does, but allows for more than one query to run at once?

Edit: To clarify even more, checkIfOnline takes an account ID, and asks another server is that account ID is on a list, which that other server responds to the current server if the account ID is or is not on the list.

Answer 1

Sounds like you would want to make use of the ExecutorService in Java 6+.

The ExecutorService requires you to submit to it a class that implements Callable. When you submit a Callable to a ES, you receive back a Future that you can use to do a number of things, including cancelling the process or getting a result from a completed process.

It’s a little hard for me to understand exactly what you are trying to achieve with your code and why you’re threading that particular part. That being said, if you want to achieve concurrency there, you’d have to:

1. submit a Callable to the ES that does the online checks & query;
2. provide a way for the Callable to notify the your application that it has finished it’s execution.

It will not be sufficient to simply submit the task and call Future.get() on it because whatever thread makes that call will be suspended until the task is completed.

You’d need to either allow the Callable to invoke a callback, or thread the class that performs the submission of the task and allow it to sit and wait for the future.get() method to return a result.

Good luck 🙂