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copied!<p>On a single-CPU machine there is no such thing as <em>"at the same time"</em> - the operating system (OS) schedules each thread to be executed for a short period of time, before moving onto the next thread.</p> <p>On multi-CPU (SMP) machines, two CPUs can be executing code simultaneously, but the <a href="http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#36930" rel="nofollow">Java Language Specification</a> (JLS) provides guarantees about <code>volatile</code> fields so you can still write correct multi-threaded code.</p> <p>From the programmer's point of view, there's no lock involved when dealing with <code>volatile</code> fields. Ignoring the uninteresting case where both threads are reading the field (spoiler: they see the same value) we have two cases:</p> <ol> <li>one thread reads, the other updates: the reader will either see the old or the new value, depending on the order in which the two threads are scheduled by the OS</li> <li>both threads update: the field will end up holding the value of whichever thread was scheduled most recently</li> </ol> <p>It's important to note that:</p> <ul> <li>even if the field is of type <code>long</code>, occupying 8 bytes of memory, the JLS guarantees that, with <code>volatile</code> fields, readers will never see (for example) 4 bytes of the "old" value and 4 bytes of the "new" - it's all-or-nothing</li> <li><strong>all</strong> threads scheduled after the update will see the new value <strong>immediately</strong></li> </ul> <p>Neither of these are necessarily true for non-<code>volatile</code> fields.</p>

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