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copied!<p>Singleton exceptions are supposed to be a performance optimization. The thinking is that you eliminate the cost of populating the stack trace when creating an exception which is usually the most expensive part of exceptions.</p> <p>If the exception were sufficiently unique and descriptive and the stacktrace was not going to be used, then a singleton could potentially be effective. You could design the app such that a specific exception type with a specific message always means that an exception orgininated from a specific location. Then the stack trace would be irrelevant.</p> <p>The <a href="http://java.sun.com/developer/JDCTechTips/2003/tt0422.html#2" rel="noreferrer">April 22, 2003 Tech Tips article</a> describes a scenario where you reuse an exception. In this case, they are trying to game the garbage collector by reducing the number of objects created. If you skipped the <code>populateStackTrace()</code> call, you would not have to worry about threading.</p> <p>Generally, if the performance impact of an exception is causing problems, that is a sign that exceptions are being used for application logic and an error-code should be used instead.</p> <p>In newer JVM's (1.4+, I believe), this "optimization" can be done automatically by the JVM when running in "-server" mode. This hotspot optimization can be controlled by the option <code>-XX:+OmitStackTraceInFastThrow</code>.</p> <p><em>Personally, I would recommend against using the singleton exception [anti-]pattern.</em></p>

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