3、用私有构造器或者枚举类型强化SIngleton属性

结论

单元素的枚举类型已经成为实现Singleton的最佳方法

Singleton模式

常见的Singleton有两种方式，第一种：

public class Elvis {
    //被定义为final的，不可改变的
    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();
    }
}
           

第二种方式：

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

    private Elvis() {
    }
    //定义为静态方法
    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();
    }
}
           

让Singleton变成可序列化的

为了让反序列化的时候只创建一个对象

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();
    }
}
           

使用枚举实现单例模式

无偿提供了序列化的机制，绝对防止多次实例化

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();
    }
}