Question

I’m not used to working with synchronized. Does the following snippet look right?

public void setNewSessionListener(NewSessionListener newSessionListener) {
    if (this.newSessionListener != null)
        synchronized (this.newSessionListener) {
            this.newSessionListener = newSessionListener;
        }
    else
        this.newSessionListener = newSessionListener;
}

More specifically do I need to perform a null check? I have a gut feeling there is something fundamentally wrong with that code.

Answer 1

There are two mistakes. The first one is that if you access a field which requires synchronization, you always have to access it with the same lock held. Also you have to check if the field is null and write to the field in the same sychronized block, because otherwise when you write something to the field, it may already be not null.

The second one is that it is best to sychronize on something that doesn’t change, in other words, on a static final field or on the instance itself. For example, you can create a lock object specifically for this purpose:

private static final Object LOCK = new Object();

And then you will write:

synchronized (LOCK) {
    if (this.newSessionListener == null) this.newSessionListener = newSessionListener;
}