Sharing Objects - Java Concurrency in Practice - page 38

Listing 3.3. Thread-safe Mutable Integer Holder.
3.1.2. Nonatomic 64-bit Operations
When a thread reads a variable without synchronization, it may see a stale value, but at least
it sees a value that was actually placed there by some thread rather than some random value.
This safety guarantee is called out-of-thin-air safety .
Out-of-thin-air safety applies to all variables, with one exception: 64-bit numeric variables
( double and long ) that are not declared volatile (see Section 3.1.4 ) . The Java Memory
Model requires fetch and store operations to be atomic, but for nonvolatile long and
double variables, the JVM is permitted to treat a 64-bit read or write as two separate 32-bit
operations. If the reads and writes occur in different threads, it is therefore possible to read a
nonvolatile long and get back the high 32 bits of one value and the low 32 bits of another. [3]
Thus, even if you don't care about stale values, it is not safe to use shared mutable long
and double variables in multithreaded programs unless they are declared volatile or
guarded by a lock.