Double Checked Locking_96501859

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Gumtree : Double-Checked Locking

Created by Tony Lam, last modified on Aug 05, 2008

Double-Checked Locking is a technique for thread-safe object creation within it's owner object.  This is useful for avoiding an instance of variable gets created twice in the multi threading environment. The following example shows how double checked locking works with the singleton pattern:

    // Ensure thread-safe access to this variable
    private volatile static Singleton uniqueInstance;

    private Singleton() {
        super();
    }

    public static Singleton getInstance() {
        // General check
        if (uniqueInstance == null) {
            // Class / static level syncrhonization
            synchronized (Singleton.class) {
                // Double check
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
    }
}

Note the second check only works with Java 5 or above, due the JVM memory handling model.

Document generated by Confluence on Apr 01, 2015 00:11

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