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POJava Memory Model and final fields
 

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    <p>I recently got confused when i referred to JMM for guarantees associated with "final". Here is an excerpt and example from JMM</p> <p>Figure 4 gives an example that demonstrates how final fields compare to normal fields. The class FinalFieldExample has a final int field x and a non-final int field y. One thread might execute the method writer(), and another might execute the method reader(). Because writer() writes f after the object’s constructor finishes, the reader() 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</p> <pre><code>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 } } } </code></pre> <p>My confusion is that is an object 'Obj' has final and non-final fields is fully initialized and is being referenced by a Thread 'T', T will only see correct values for final fields ? What about non-final fields that are not mutated after construction. I understand that if they are mutated after construction thread 'T' might not see new value ( unless the field is a volatile ). But i am what if the field is non-final and non-volatile and is not mutated after construction ?</p> <p>How does JVM implements guarantees associated with 'final' ? E.g for volatile there are memory barriers.</p>
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    1. USsnegi
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    1. COAnswer here: http://stackoverflow.com/questions/10301061/is-this-a-safe-publication-of-object/10301179#10301179
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    2. COthere's a bug in this code - after the `if(f!=null)` check, `f` could appear to be null again. the code should use a local variable to cache `f`
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      1. USZhongYu
    3. COhttp://stackoverflow.com/a/15342485/2031799 - this is how final fields are implemented. In theory this example might break, but not in practice.
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      1. USMikhail
 

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