Are Static Variables Shared Between Threads

Are static variables shared between threads?

There isn't anything special about static variables when it comes to visibility. If they are accessible any thread can get at them, so you're more likely to see concurrency problems because they're more exposed.

There is a visibility issue imposed by the JVM's memory model. Here's an article talking about the memory model and how writes become visible to threads. You can't count on changes one thread makes becoming visible to other threads in a timely manner (actually the JVM has no obligation to make those changes visible to you at all, in any time frame), unless you establish a happens-before relationship.

Here's a quote from that link (supplied in the comment by Jed Wesley-Smith):

Chapter 17 of the Java Language Specification defines the happens-before relation on memory operations such as reads and writes of shared variables. The results of a write by one thread are guaranteed to be visible to a read by another thread only if the write operation happens-before the read operation. The synchronized and volatile constructs, as well as the Thread.start() and Thread.join() methods, can form happens-before relationships. In particular:
Each action in a thread happens-before every action in that thread that comes later in the program's order.
An unlock (synchronized block or method exit) of a monitor happens-before every subsequent lock (synchronized block or method entry) of that same monitor. And because the happens-before relation is transitive, all actions of a thread prior to unlocking happen-before all actions subsequent to any thread locking that monitor.
A write to a volatile field happens-before every subsequent read of that same field. Writes and reads of volatile fields have similar memory consistency effects as entering and exiting monitors, but do not entail mutual exclusion locking.
A call to start on a thread happens-before any action in the started thread.
All actions in a thread happen-before any other thread successfully returns from a join on that thread.

Static Variables and Threads (C)

All the threads of a process share the same address space. Since i is a static variable, it has a fixed address. Its "state" is just the content of the memory at that address, which is shared by all the threads.

The postfix ++ operator increments its argument and yields the value of the argument before the increment. The order in which these are done is not defined. One possible implementation is

copy i to R1
copy R1 to R2
increment R2
copy R2 to i
return R1

If more than one thread is running, they can both be executing these instructions simultaneously or interspersed. Work out for yourself sequences where various results obtain. (Note that each thread does have its own register state, even for threads running on the same CPU, because registers are saved and restored when threads are switched.)

A situation like this where there are different results depending on the indeterministic ordering of operations in different threads is called a race condition, because there's a "race" among the different threads as to which one does which operation first.

Are scoped static variables shared between threads?

Of course it can print 3. It is even the "usual semantics" of this code to do so. Thread 1 initializes it with 1 and increments it, so it is 2. Thread 2 increments it again, so it is 3.

So, yes, scoped static variables are static, i.e., global variables. They are shared by threads.

Of course, the code has a race condition, so the result can possibly be anything, but 3 is a possible result.

Are Static Variables Shared Between Threads