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    <p>Yes, it is correct. volatile protects only that object reference, but nothing else.</p> <p>No, putting an element to a <code>volatile</code> HashMap will not create a happens-before relationship, not even with a <code>ConcurrentHashMap</code>.</p> <p>Actually <code>ConcurrentHashMap</code> does not hold lock for read operations (e.g. <code>containsKey()</code>). See <a href="http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="nofollow">ConcurrentHashMap</a> Javadoc.</p> <p><strong>Update</strong>:</p> <p>Reflecting your updated question: you have to synchronize on the object you put into the CHM. I recommend to use a <strong>container object</strong> instead of directly storing the <code>Object</code> in the map:</p> <pre><code>public class ObjectContainer { volatile boolean isSetupDone = false; Object o; } static ConcurrentHashMap&lt;String, ObjectContainer&gt; containers = new ConcurrentHashMap&lt;String, ObjectContainer&gt;(); public Object getInstance(String groupId) { ObjectContainer oc = containers.get(groupId); if (oc == null) { // it's enough to sync on the map, don't need the whole class synchronized(containers) { // double-check not to overwrite the created object if (!containers.containsKey(groupId)) oc = new ObjectContainer(); containers.put(groupId, oc); } else { // if another thread already created, then use that oc = containers.get(groupId); } } // leave the class-level sync block } // here we have a valid ObjectContainer, but may not have been initialized // same doublechecking for object initialization if(!oc.isSetupDone) { // now syncing on the ObjectContainer only synchronized(oc) { if(!oc.isSetupDone) { oc.o = new String("typically a more complicated operation")); oc.isSetupDone = true; } } } return oc.o; } </code></pre> <p>Note, that at <em>creation</em>, at most one thread may create <code>ObjectContainer</code>. But at <em>initialization</em> each groups may be initialized in parallel (but at most 1 thread per group). </p> <p>It may also happen that <code>Thread T1</code> will create the ObjectContainer, but <code>Thread T2</code> will initialize it.</p> <p>Yes, it is worth to keep the <code>ConcurrentHashMap</code>, because the map reads and writes will happen at the same time. But <code>volatile</code> is not required, since the map object itself will not change.</p> <p>The sad thing is that the double-check does not always work, since the compiler may create a bytecode where it is reusing the result of <code>containers.get(groupId)</code> (that's not the case with the volatile isSetupDone). That's why I had to use <code>containsKey</code> for the double-checking.</p>
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    1. POWith double-checked locking, does a put to a volatile ConcurrentHashMap have happens-before guarantee?
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    1. POWith double-checked locking, does a put to a volatile ConcurrentHashMap have happens-before guarantee?
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    1. COHi GaborSch, Thank you for your suggestion; that looks really great. Just one question, though: when you say "that's not the case with the volatile isSetupDone", do you mean it we should make `isSetupDone` `volatile`like so: `volatile boolean isSetupDone = false;`
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      1. USChristian
    2. CO@Christian Exactly. Sorry, it's my mistake, going to update that line. I meant `isSetupDone` to be `volatile`.
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