Java内存模型中的final字段

Question

Could you explain how the value of fy could be seen 0 instead of 4? Would that be because other thread writes updates the value to 0 from 4? This example is taken from jls https://docs.oracle.com/javase/specs/jls/se8/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
            } 
        } 
    }

Answer 1

Assuming that we have two threads started, like this:

new Thread(FinalFieldExample::writer).start(); // Thread #1
new Thread(FinalFieldExample::reader).start(); // Thread #2

We might observe our program's actual order of operations to be the following:

1. Thread #1 writes x = 3 .
2. Thread #1 writes f = ... .
3. Thread #2 reads f and finds that it is not null .
4. Thread #2 reads fx and sees 3 .
5. Thread #2 reads fy and sees 0 , because y does not appear to be written yet.
6. Thread #1 writes y = 4 .

In other words, Threads #1 and #2 are able to have their operations interleave in a way such that Thread #2 reads fy before Thread #1 writes it.

Note also that the write to the static field f was allowed to be reordered so that it appears to happen before the write to fy . This is just another consequence of the absence of any kind of synchronization. If we declared f as also volatile , this reordering would be prevented.

---

There's some talk in the comments about writing to final fields with reflection, which is true. This is discussed in §17.5.3 :

In some cases, such as deserialization, the system will need to change the final fields of an object after construction. final fields can be changed via reflection and other implementation-dependent means.

It's therefore possible in the general case for Thread #2 to see any value when it reads fx .

There's also a more conventional way to see the default value of a final field, by simply leaking this before the assignment:

class Example {
    final int x;

    Example() {
        leak(this);
        x = 5;
    }

    static void leak(Example e) { System.out.println(e.x); }

    public static void main(String[] args) { new Example(); }
}

I think that if FinalFieldExample 's constructor was like this:

static FinalFieldExample f;

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

Thread #2 would be able to read fx as 0 as well.

This is from §17.5 :

An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields.

The more technical sections of specification for final contain wording like that as well.

Answer 2

Could you explain how the value of fy could be seen 0 instead of 4?

In Java, one of the important optimizations performed by the compiler/JVM is the reordering of instructions. As long as it doesn't violate the language specifications, the compiler is free to reorder all instructions for efficiency reasons. During object construction, it is possible for an object to be instantiated, the constructor to finish, and its reference published before all of the fields in the object have been properly initialized.

However, Java language says that if a field is marked as final then it must be properly initialized by the time the constructor finishes. To quote from the section of the Java language specs you reference . Emphasis is mine.

An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields .

So by the time the FinalFieldExample is constructed and assigned to f , the x field must be properly initialized to 3 however the y field may or may not have been properly initialized. So if thread1 makes the call to writer() and then thread2 makes the call to reader() and sees f as not null, y could be 0 (not yet initialized) or 4 (initialized).