原型模式对单例模式的破坏

• 利用反射调用单例类的 clone() 方法，获得单例类的多个实例；

public class HungrySingleton implements Cloneable{

    private final static HungrySingleton hungrySingleton;

    static {
        hungrySingleton = new HungrySingleton();
    }

    private HungrySingleton() {}

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

}

public class Test {
    public static void main(String[] args) throws Exception{
        HungrySingleton hungrySingleton = HungrySingleton.getInstance();

        Method method = HungrySingleton.class.getDeclaredMethod("clone");
        method.setAccessible(true);
        HungrySingleton cloneHungrSingleton = (HungrySingleton)method.invoke(hungrySingleton);

        System.out.println(hungrySingleton);
        System.out.println(cloneHungrSingleton);
        System.out.println(hungrySingleton == cloneHungrSingleton);
    }
}

输出：

designpattern.creational.prototype.singletonattack.HungrySingleton@5b6f7412
designpattern.creational.prototype.singletonattack.HungrySingleton@27973e9b
false

解决方案

• 在 clone() 方法中直接调用 getInstance() 方法；

public class HungrySingleton implements Cloneable{

    private final static HungrySingleton hungrySingleton;

    static {
        hungrySingleton = new HungrySingleton();
    }

    private HungrySingleton() {}

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return getInstance();
    }

}

public class Test {
    public static void main(String[] args) throws Exception{
        HungrySingleton hungrySingleton = HungrySingleton.getInstance();

        Method method = HungrySingleton.class.getDeclaredMethod("clone");
        method.setAccessible(true);
        HungrySingleton cloneHungrSingleton = (HungrySingleton)method.invoke(hungrySingleton);

        System.out.println(hungrySingleton);
        System.out.println(cloneHungrSingleton);
        System.out.println(hungrySingleton == cloneHungrSingleton);
    }
}

输出：

designpattern.creational.prototype.singletonattack.HungrySingleton@5b6f7412
designpattern.creational.prototype.singletonattack.HungrySingleton@5b6f7412
true