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copied!<p>My understanding is that the reason <code>wrapper</code> is used is that reading an immutable object (all fields are final) is an atomic operation.</p> <p>If <code>wrapper</code> is null, then it's not yet an immutable object, and we have to fall into the <code>synchronized</code> block.</p> <p>If <code>wrapper</code> is not-null then we are <em>guaranteed</em> to have a fully constructed <code>value</code> object, so we can return it.</p> <p>In your code, the <code>null</code> check could be done without actually reading the referenced object, and so not triggering the atomicity of the operation. I.e. the operation could have initialized <code>helperWrapper</code> in preparation for passing the result of <code>new Helper()</code> to the constructor, but the constructor has not been called yet.</p> <p>Now I would presume that the subsequent code in your example would read <code>return helperWrapper.value;</code> which <em>should</em> trigger an atomic reference read, guaranteeing that the constructor completes, but it's entirely possible ('<em>semantics of some programming languages</em>') that the compiler is allowed to optimize that to not perform an atomic read, and thus it would return an incompletely initialized <code>value</code> object under the exact right circumstances.</p> <p>Performing the barrier using a local variable and a reference copy forces the read and write to be atomic, and ensures that the code is thread-safe.</p> <p>I believe the key understanding is that immutable reference reads and writes are atomic, so assignment to another variable is atomic, but null testing might not be.</p>

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