Volatile or synchronized for primitive type?

Question

In Java, assignment is atomic if the size of the variable is less than or equal to 32 bits but is not if more than 32 bits.

What (volatile/synchronized) would be more efficient to use in case of double or long assignment?

Like,

  volatile double x = y;

synchronized is not applicable with primitive argument. How do I use synchronized in this case? Of course I don't want to lock my class, so this should not be used.

Answer 1

What are you trying to do? The synchronized and volatile keywords are mechanisms in Java which can be used to ensure that consistent values are observed by different threads reading the same data. In particular they allow you to reason about happens-before relations in your programs.

You simply cannot avoid using one of volatile or synchronized in order to properly access non-final fields in a multi-threaded program. That said, the main reason that you are likely to require synchronized over volatile is the requirement for using atomic compare and set operations (i.e. it will not be any performance consideration). For example, in a multi-threaded program:

volatile int i = 0;

public void foo() { 
    if (i == 0) i = i + 1;
}

The above code is inherently unsafe, even though the variable's declaration as being volatile means that reads and writes are flushed to main memory - the only safe implementation of such a method would be something like:

int i = 0;

public synchronized void foo() {
    if (i == 0) i = i + 1;
}

So which should you prefer? Well, if you have multiple threads modifying a field dependent on that field's value (i.e. compare-and set), then synchronized is the only safe solution.

It's also worth saying: the performance overhead of synchronized is not a problem (in the overwhelming majority of cases). Synchronization-performance issues are usually due to unnecessary code bottlenecks, deadlocks or livelocks and can be mitigated if necessary. Any pure clock-cycles overhead will be dwarfed by other things you application does: file IO, database queries, remoting etc.

Answer 2

If you find locking on the object itself too heavy, then synchronized is the way to go. Prior to Java 1.5 volatile may have been a good choice, but now volatile can have a very large impact by forcing instruction ordering on the method where the assignment happens. Create a separate object (private final Object X_LOCK = new Object();) and synchronize on it when setting or getting the value of that double. This will give you a fine level of control over the locking, which it seems that you need.

In the new concurrency package there are more options, such as AtomicReference which may be a good replacement for volatile if you really need to avoid synchronization.