Double check locking is broken. Since initialized is a primitive, it may not require it to be volatile to work, however nothing prevents initialized being seen as true to the non-syncronized code before instance is initialized. EDIT: To clarify the above answer, the original question asked about using a boolean to control the double check … Read more
The volatile prevents memory writes from being re-ordered, making it impossible for other threads to read uninitialized fields of your singleton through the singleton’s pointer. Consider this situation: thread A discovers that uniqueInstance == null, locks, confirms that it’s still null, and calls singleton’s constructor. The constructor makes a write into member XYZ inside Singleton, … Read more
Synchronization by itself would be enough in this case if the first check was within synchronized block (but it’s not and one thread might not see changes performed by another if the variable were not volatile). Volatile alone would not be enough because you need to perform more than one operation atomically. But beware! What … Read more
Double-checking locking now works in Java as well as C# (the Java memory model changed and this is one of the effects). However, you have to get it exactly right. If you mess things up even slightly, you may well end up losing the thread safety. As other answers have stated, if you’re implementing the … Read more
A good resource for understanding why volatile is needed comes from the JCIP book. Wikipedia has a decent explanation of that material as well. The real problem is that Thread A may assign a memory space for instance before it is finished constructing instance. Thread B will see that assignment and try to use it. … Read more