Design Pattern #5: Singleton Pattern

These are my notes about the book Head First Design Patterns.

Definition:

• Ensure one and only one object is instantiated for a given class, and provides a global point of access.

• Example usage: global resources like connection/thread pools

• For resource intensive objects, can implemented in lazy manner.

Java implementation:

Use a private constructor.

/**
 * Created by jsun on 11/14/2015 AD.
 */
public class Singleton {

    private static Singleton uniqueInstance = null;

    private Singleton(){};

    // NOT THREAD SAFE!
    public static Singleton getInstance() { // others have to call this method to get instance
        if (uniqueInstance == null){
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }

    // other methods below

}

To be thread safe

• Use synchronized. Expensive (can decrease performance by 100x) and we actually only need to synchronize during the first time.

public static synchronized Singleton getInstance()

• Create eagerly

JVM will ensure instance created before any thread access.

public class Singleton {

    private static Singleton uniqueInstance = new Singleton();

    private Singleton(){};

    public static Singleton getInstance() {
        return uniqueInstance;
    }

}

• Double checked locking: only synchronize if instance is not created yet.

public class Singleton {

    private volatile static Singleton uniqueInstance = new Singleton();

    private Singleton(){};

    public static Singleton getInstance() {

        if (uniqueInstance == null){
            synchronized (Singleton.class){
                if (uniqueInstance == null){ // check again
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }

}

Scala:

• Scala has native support for singleton: Object.