Immutability and reordering

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The confusion I think you have here is what the author meant by safe publication. He was referring to the safe publication of a non-null Resource, but you seem to get that.

Your question is interesting – is it possible to return a null cached value of resource?

Yes.

The compiler is allowed to reorder the operation like such

public static Resource getInstance(){
   Resource reordered = resource;
   if(resource != null){
       return reordered;
   }
   return (resource = new Resource());
}

This doesn’t violate the rule of sequential consistency but can return a null value.

Whether or not this is the best implementation is up for debate but there is no rules to prevent this type of reordering.

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