結論

單元素的列舉型別已經成為實現Singleton的最佳方法

Singleton模式

常見的Singleton有兩種方式，第一種：

public class Elvis {
    //被定義為final的，不可改變的
    public static final Elvis INSTANCE = new Elvis();

    private Elvis() {
    }

    public void leaveTheBuilding() {
        System.out.println("Whoa baby, I'm outta here!");
    }

    // This code would normally appear outside the class!
    public static void main(String[] args) {
        Elvis elvis = Elvis.INSTANCE;
        elvis.leaveTheBuilding();
    }
}

第二種方式：

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

    private Elvis() {
    }
    //定義為靜態方法
    public static Elvis getInstance() {
        return INSTANCE;
    }

    public void leaveTheBuilding() {
        System.out.println("Whoa baby, I'm outta here!");
    }

    // This code would normally appear outside the class!
    public static void main(String[] args) {
        Elvis elvis = Elvis.getInstance();
        elvis.leaveTheBuilding();
    }
}

讓Singleton變成可序列化的

為了讓反序列化的時候只建立一個物件

public class Elvis {
    public static final Elvis INSTANCE = new Elvis();

    private Elvis() {
    }

    public void leaveTheBuilding() {
        System.out.println("Whoa baby, I'm outta here!");
    }

    private Object readResolve() {
        // Return the one true Elvis and let the garbage collector
        // take care of the Elvis impersonator.
        return INSTANCE;
    }

    // This code would normally appear outside the class!
    public static void main(String[] args) {
        Elvis elvis = Elvis.INSTANCE;
        elvis.leaveTheBuilding();
    }
}

使用列舉實現單例模式

無償提供了序列化的機制，絕對防止多次例項化

public enum Elvis {
    INSTANCE;

    public void leaveTheBuilding() {
        System.out.println("Whoa baby, I'm outta here!");
    }

    // This code would normally appear outside the class!
    public static void main(String[] args) {
        Elvis elvis = Elvis.INSTANCE;
        elvis.leaveTheBuilding();
    }
}