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How do javax.ejb.Singletons work?
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created by <a href="https://community.jboss.org/people/jhn134910">John Lee</a> in <i>EJB3</i> - <a href="https://community.jboss.org/message/804718#804718">View the full discussion</a>
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<div class="jive-rendered-content"><p>Are singletons implemented using synchronized methods?</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>I have an EJB singleton, say:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>@Singleton</p><p>public void MySingleton {</p><p>     ...</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>     public void receive(final String string) {</p><p>          x.callMyBlockingMethod(string);</p><p>     }</p><p>}</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Consider the scenario, where this is running in JBOSS standalone. This singleton shall be called from multiple threads, with names Thread-1, Thread-2 and Thread-3 in that order. Thread-1 calls the blocking method which doesn't return for 10 seconds. Thread-2 arrives at the receive method shortly after Thread-1, followed shortly by Thread-3 and both wait for the lock to be released by Thread-1. Is there anything in the EJB standard regarding singletons that guarantees that Thread-2 shall aquire the lock before Thread-3 once Thread-1 releases it?</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>From my testing with synchronized methods I have verified that synchronized methods provide no such guarantee.</p><p>I'm going to test this singleton behaviour but if my test shows that Thread-2 is indeed processed before Thread-3 and order is maintained, it shall not tell me that this is guaranteed behaviour, and is merely a consequence of using a particular OS, JVM or EJB container implementation. So does @Singleton offer any guarantees?</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>If it does, are there any gotchas about using this behavior to queue up client requests against singleton where they wait to acquire the lock?</p></div>
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