Newsflash: Double-checked locking is still broken

April 12, 2002

Q: In ” Singletons with Needles and Thread ,” you declare the getInstance() method in the Singleton class as synchronized. The drawback to this approach: the getInstance() call is synchronized even if the Singleton exists — thus introducing unneeded overhead.

I suggest the following as a better approach:

public class Singleton {
    private static Singleton instance;
    public static Singleton getInstance() {
        if (null == instance) {
            synchronized(Singleton.class) {
                if (null == instance) {
                        instance = new Singleton();
                        }
                }
        }
        return instance;
    }
}

• “The ‘Double-Checked Locking Is Broken’ Declaration,” David Bacon, et al.
• “Double-Checked Locking: Clever, but Broken,” Brian Goetz (JavaWorld, February 2001)
• “Warning! Threading in a Multiprocessor World,” Allen Holub (JavaWorld, February 2001)
• “Can Double-Checked Locking Be Fixed?,” Brian Goetz (JavaWorld, May 2001)
• “Can ThreadLocal Solve the Double-Checked Locking Problem?,” Brian Goetz (JavaWorld, November 2001)
• “Letters to the Editor,” (JavaWorld):
  • March 2001
  • June 2001
  • July 2001
  • November 2001

Also, consider this alternative:

public class Singleton {
    public final static Singleton INSTANCE = new Singleton();
    private Singleton() {}
}