I have a static variable that I would like to be unique per thread.
Is this the case for all static variables? Or can it not be guaranteed. That is, will threads occasionally update the static variable's value in the main memory, or keep it to themselves?
If this cannot be guaranteed, is there any type of variable in Java that is both static and thread-unique? Something essentially global to a thread, but hidden from other threads?
I think what you are looking for is Java's ThreadLocal.
This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable.
Mind you, if you do thread pooling this may cause problems for you since you may think you are getting a new thread, in terms of it starting a new process, but what is happening is that you are reusing a thread that finished working on other data and thus has left-overs and these are hard to debug when they occur in the wild.
Here is a tutorial on using ThreadLocal.
static variables are shared among threads.
If you want your variable to be unique per thread, you may want to use ThreadLocal:
// Atomic integer containing the next thread ID to be assigned private static final AtomicInteger nextId = new AtomicInteger(0); // Thread local variable containing each thread's ID private static final ThreadLocal<Integer> threadId = new ThreadLocal<Integer>() { @Override protected Integer initialValue() { return nextId.getAndIncrement(); } }; 
volatile. Essentially, a thread is allowed to cache it for a while and I'm sure of the guarantees around it.
ThreadLocalis for. Although I would be curious to know what you are storing in this static variable and why you believe you need a unique one for each thread.