Values read in Multithreading environment

Question

I was going through the JLS documentation on Thread and Locks http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.5 .

class FinalFieldExample { 
final int x;
int y; 
static FinalFieldExample f;

public FinalFieldExample() {
    x = 3; 
    y = 4; 
} 

static void writer() {
    f = new FinalFieldExample();
} 

static void reader() {
    if (f != null) {
        int i = f.x;  // guaranteed to see 3  
        int j = f.y;  // could see 0
    } 
} 
}

I am confused with above example (ex no 17.5-1) mentioned in the section as to how f.y could be seen as zero. The Reader Threads will either read the object f as null in which case it will not execute anything or it will read the object f with some reference. If the object f has a reference then the constructor must have completed its execution even though Multiple Writer Threads are running so that a reference could be assigned to f and if the constructor has executed then f.y should be seen as 4.

In what condition can f.y =0 be possible?

Thanks

Answer 1

In what condition can f.y =0 be possible?

The Java memory model allows the JIT compiler to reorder initialization of non final fields outside the constructor. The x field is final so it must be initialized by the JVM but y is not final. So there is the possibility that the FinalFieldExample is allocated and set on static FinalFieldExample f but the initialization of the y field has not been completed.

To quote from 17.5-1:

Because the writer method writes f after the object's constructor finishes, the reader method will be guaranteed to see the properly initialized value for f.x: it will read the value 3. However, f.y is not final; the reader method is therefore not guaranteed to see the value 4 for it.

Because f.y is not final there is no guarantee that it has been set by the time the constructor finishes and the static f is assigned. So there is a race condition created and the reader may see y as 3 or 0 depending on this race.