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The Perfect Singleton

From time to time I met Java programmers that are not really sure how they should implement Singleton pattern properly. (if you don’t have an idea what Singleton is just try Wikipedia: Singleton pattern).

And I’m not talking about proper implementation in threaded environment. But using the most common implementation you can find over internet you can easily create as many singletons as you like.

Just imagine you have the following common singleton implementation:

public final class NonSafeSingleton implements Serializable {

    private static final NonSafeSingleton INSTANCE = new NonSafeSingleton();

    private NonSafeSingleton() {}

    public static NonSafeSingleton getInstance() {
        return INSTANCE;
    }
}

Now concentrate on Serializable word. Think for one more second… Yes you’re right. If you send this stuff over RMI you’ll get second instance. It should even be enough to do some in memory serialization and de-serialization and kaboom! You’ve just blown away general Singleton contract. That’s not very nice. But how to fix that? Generally there are two ways I use:

  1. The hard way (or you use 1.4 or older Java)
    You need to implement readResolve method in your Singleton class. This small thing is used to override what serialization mechanism has created. What you return there will be used instead of data that came from serialization (for details check: Serializable Javadoc). Just return your instance here:

    ...
        protected Object readResolve() throws ObjectStreamException {
            return INSTANCE;
        }
    ...
  2. The easy way (Yes, I’m using 1.5 or newer)
    Change your singleton class to enum and remove private constructor and getInstance method. Yes it’s really that simple. You get this for free then.

    public enum SafeSingleton implements Serializable {
    
        INSTANCE;
    }

Just keep this in mind when implementing next Singleton. It can make your life easier if you use RMI heavily.

Reference: The Perfect Singleton from our JCG partner Marek Piechut at the Development world stories.

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One comment

  1. What is instance control?

    Instance control basically refers to single instance of the class OR singleton design pattern .

    Java 1.5 onwards we should always prefer ENUM to create singleton instance. It is absolutely safe . JVM guarantees that. All earlier mechanical of controlling instance to single are already broken.

    So in any interview you can confidently say ENUM= provides perfect singleton implementation .

    Now what is readResolve?

    readResolve is nothing but a method provided in serializable class . This method is invoked when serialized object is deserialized. Through readResolve method you can control how instance will be created at the time of deserialization.Lets try to understand some code

    public class President {

    private static final President singlePresident = new President();

    private President(){

    }

    }

    This class generates single instance of President class. singlePresident is static instance and same will be used across.

    But do you see it breaks anywhere?

    Please visit http://efectivejava.blogspot.in/ for more details on this

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