Singleton notes

Singleton是近代有名的Design Pattern,以下介紹各種實做的approach
-Enum approach
Java SE 1.5後多了enum的型態,enum沒有可以access的constructor,不能被extend,根據"Effective Java"指出 single-element enum是最佳實作singleton的方式

public enum Singleton{
  INSTANCE;
}

-static initializer

public final class Singleton {

    private static final Singleton instance = new Singleton();
    //一開始即做靜態的初始
    private Singleton() {
        //初始化的欄位
    }

    public static Singleton getInstance() {
        return instance;
    }
}

-Synchonized approach

public final class Singleton {

    private static final Singleton instance = null; //宣告為null

    private Singleton() {
        //初始化的欄位
    }

    public static synchronized Singleton getInstance() {
        return instance;//確保getinstance()有synchronized避免race condition
    }
}

x Boken Singleton-Double checked locking
此問題詳細介紹如下:
http://en.wikipedia.org/wiki/Double-checked_locking

public  class Singleton {

    private static final Singleton instance = null; //宣告為null

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

-Lazy initialization
1.initialize-on-demand holder class idiom

public final class Singleton {

    private Singleton() {
    }

    private static class SingletonHolder {

        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

2.Double-check idiom with volatile
Java SE 5後新的memory model讓volatile此關鍵字確保所有thread所取得的欄位值是最新被寫入的

    private volatile Helper helper = null;

    public Helper getHelper() {
        if (helper == null) {
            sychonized(this)
            {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
    }