如何利用反射破壞單例模式

1.什麼是單例模式

單例模式必須滿足以下兩點要求

• 私有化構造函數
• 全局唯一的共有訪問點

2.懶漢式單例

public class Lazy {

    private static Lazy instance;

    private Lazy(){}

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

}

3.餓漢式單例

public class Hungry {

    private static final Hungry instance = new Hungry();

    private Hungry() {
    }

    public static Hungry getInstance() {
        return instance;
    }

}

4.利用反射破壞單例

public class SingletonDestroyer {

    public static void main(String[] args) throws Exception {

        Lazy lazyInstance = Lazy.getInstance();

        Constructor constructor = Lazy.class.getDeclaredConstructor();

        constructor.setAccessible(true);

        Lazy lazyInstanceReflect = (Lazy) constructor.newInstance();

        System.out.println(lazyInstance  == lazyInstanceReflect); // false

    }
}