Java: Example for unsafe publication [duplicate]

Question

After reading "Java concurrent in practice" and "OSGI in practice" I found a specific subject very interesting; Safe Publication. The following is from JCIP:

To publish an object safely, both the reference to the object and the object's state must be made visible to other threads at the same time. A properly constructed object can be safely published by:

• Initializing an object reference from a static initializer.
• Storing a reference to it into a volatile field.
• Storing a reference to it into a final field.
• Storing a reference to it into a field that is properly guarded by a (synchronized) lock.

My first question: how many java developers are aware of this (problem)? How many real world java applications are really following this, AND is this really a real problem? I have a feeling that 99% of the implemented JVMs out there are not that "evil", i.e. a thread is not guaranteed (in fact its practical (almost) "impossible") to see stale data just because the reference is not following the "safe publication idiom" above.

Answer 1

Proportionally, it's probably fair to say that very few programmers sufficiently understand synchronization and concurrency. Who knows how many server applications there are out there right now managing financial transactions, medical records, police records, telephony etc etc that are full of synchronization bugs and essentially work by accident, or very very occasionally fail (never heard of anybody get a phantom phone call added to their telephone bill?) for reasons that are never really looked into or gotten to the bottom of.

Object publication is a particular problem because it's often overlooked, and it's a place where it's quite reasonable for compilers to make optimisations that could result in unexpected behaviour if you don't know about it: in the JIT-compiled code, storing a pointer, then incrementing it and storing the data is a very reasonable thing to do. You might think it's "evil", but at a low level, it's really how you'd expect the JVM spec to be. (Incidentally, I've heard of real-life programs running in JRockit suffering from this problem-- it's not purely theoretical.)

If you know that your application has synchronization bugs but isn't misbehaving in your current JVM on your current hardware, then (a) congratulations; and (b), now is the time to start "walking calmly towards the fire exit", fixing your code and educating your programmers before you need to upgrade too many components.