Double Checked Locking in Java and Why it was broken before JDK 5?

Double checked locking pattern is one of the interesting topics on Java Interviews. Earlier, it was asked to see if Java developer can write code using synchronized block or not and now it ask to gauge the candidate's understanding of concurrency, volatile and synchronization in Java. One of the simplest ways to write thread-safe Singleton was to make the getInstance() method synchronized but prior to JDK 1.6, a simple uncontented synchronization block was expensive and that lead many developers to write the getInstance() method of Singleton class using double-checked locking idiom. This was one of the clever idiom of that time which only uses synchronization when the Singleton object is created as seen in the following code and thus improves the performance of getInstance() method, which is used to retrieve the Singleton object.

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