Question

I’m trying to learn about singleton classes and how they can be used in an application to keep it thread safe. Let’s suppose you have an singleton class called IndexUpdater whose reference is obtained as follows:

 public static synchronized IndexUpdater getIndexUpdater() {
    if (ref == null)
        // it's ok, we can call this constructor
        ref = new IndexUpdater();
    return ref;
}

private static IndexUpdater ref;

Let’s suppose there are other methods in the class that do the actual work (update indicies, etc.). What I’m trying to understand is how accessing and using the singleton would work with two threads. Let’s suppose in time 1, thread 1 gets a reference to the class, through a call like this IndexUpdater iu = IndexUpdater.getIndexUpdater(); Then,
in time 2, using reference iu, a method within the class is called iu.updateIndex by thread 1. What would happen in time 2, a second thread tries to get a reference to the class. Could it do this and also access methods within the singleton or would it be prevented as long as the first thread has an active reference to the class. I’m assuming the latter (or else how would this work?) but I’d like to make sure before I implement.

Thank you,

Elliott

Answer 1

Since getIndexUpdater() is a synchronized method, it only prevents threads from accessing this method (or any method protected by the same synchronizer) simultaneously. So it could be a problem if other threads are accessing the object’s methods at the same time. Just keep in mind that if a thread is running a synchronized method, all other threads trying to run any synchronized methods on the same object are blocked.

More info on:
http://download.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html