Question

为什么这段代码会抛出CloneNotSupportedException？

public class Car {
    private static Car car = null;

    private void car() {
    }

    public static Car GetInstance() {
        if (car == null) {
            car = new Car();
        }
        return car;
    }

    public static void main(String arg[]) throws CloneNotSupportedException {
        car = Car.GetInstance();
        Car car1 = (Car) car.clone();
        System.out.println(car.hashCode());// getting the hash code
        System.out.println(car1.hashCode());
    }
}

Answer 1

如果您正在克隆单例对象，那么您违反了Singleton的设计原则。

默认情况下，clone方法受到保护：protected native Object clone() throws CloneNotSupportedException;

如果您的Car扩展了另一个支持克隆的类，则可能会违反单例的设计原则。因此，绝对肯定100％确定单身实际上是单身，我们必须添加我们自己的clone()方法，如果有人试图创建，则抛出CloneNotSupportedException。下面是我们的覆盖克隆方法。

 @Override
    protected Object clone() throws CloneNotSupportedException {
        /*
         * Here forcibly throws the exception for preventing to be cloned
         */
        throw new CloneNotSupportedException();
        // return super.clone();
    }

请找到以下代码块来处理Clone for Singleton类或通过取消注释代码来避免克隆。

public class Car  implements Cloneable {

    private static Car car = null;

    private void Car() {
    }

    public static Car GetInstance() {
        if (car == null) {
            synchronized (Car.class) {
                   if (car == null) {
                car = new Car();
                   }
            }
        }
        return car;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        /*
         * Here forcibly throws the exception for preventing to be cloned
         */
     //   throw new CloneNotSupportedException();
        return super.clone();
    }

    public static void main(String arg[]) throws CloneNotSupportedException {
        car = Car.GetInstance();
        Car car1 = (Car) car.clone();
        System.out.println(car.hashCode());// getting the hash code
        System.out.println(car1.hashCode());
    }
}

Answer 2

public class Car implements Cloneable {
    private static Car car = null;

    public static Car GetInstance() {
        if (car == null) {
            car = new Car();
        }
        return car;
    }

    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

Car car = Car.GetInstance();
Car car1 = (Car) car.clone();
System.out.println(car.hashCode());
System.out.println(car1.hashCode());

<强>输出：

1481395006
2027946171

克隆Singleton对象

2 个答案: