Testing initialization safety of final fields

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I wrote the spec. The TL; DR version of this answer is that just because it may see 0 for y, that doesn’t mean it is guaranteed to see 0 for y.

In this case, the final field spec guarantees that you will see 3 for x, as you point out. Think of the writer thread as having 4 instructions:

r1 = <create a new TestClass instance>
r1.x = 3;
r1.y = 4;
f = r1;

The reason you might not see 3 for x is if the compiler reordered this code:

r1 = <create a new TestClass instance>
f = r1;
r1.x = 3;
r1.y = 4;

The way the guarantee for final fields is usually implemented in practice is to ensure that the constructor finishes before any subsequent program actions take place. Imagine someone erected a big barrier between r1.y = 4 and f = r1. So, in practice, if you have any final fields for an object, you are likely to get visibility for all of them.

Now, in theory, someone could write a compiler that isn’t implemented that way. In fact, many people have often talked about testing code by writing the most malicious compiler possible. This is particularly common among the C++ people, who have lots and lots of undefined corners of their language that can lead to terrible bugs.

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