3.用私有构造器或者枚举类型强化Singleton属性

在我们通常的单例方法中,通常有两种方法来击穿单例,反射和序列化

使用单元素枚举可以有效的解决这两个问题(在最后)

反射击穿单例

这是一个普通的单例的例子

  public class Singleton {
    private static final Singleton SINGLETON = new Singleton();

    public static Singleton getSingleton(){
        return SINGLETON;
    }
}

用反射击穿

public class App {
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
         
        Class<?> clazz = Singleton.class;
        Constructor<?> c = clazz.getDeclaredConstructor(null);
        c.setAccessible(true);
         // 单例对象
        Singleton singleton = (Singleton) c.newInstance();
        // 反射创建的对象
        Singleton singleton1 = Singleton.getSingleton();

        // 结果是false 证明并不是同一个对象
        System.out.println(singleton == singleton1);
    }
}

如何防止反射击穿呢?

我们在构造方法中进行一次判断

public class Singleton {
    private static final Singleton SINGLETON = new Singleton();

    private Singleton() {
        synchronized (Singleton.class) {
            if (SINGLETON != null){
                throw new RuntimeException("试图破坏单例模式");
            }
        }
    }

    public static Singleton getSingleton() {
        return SINGLETON;
    }
}

public class App {
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, NoSuchFieldException {
        // 单例对象
        Singleton singleton1 = Singleton.getSingleton();
        // 利用反射创建对象
        Class<?> clazz = Singleton.class;
        Constructor<?> c = clazz.getDeclaredConstructor(null);
        c.setAccessible(true);
        // 到这里的时候会报错
        Singleton singleton = (Singleton) c.newInstance();
        //比较
        System.out.println(singleton == singleton1);
    }
}

序列化击穿单例

经过序列化再反序列化的对象已经和原对象是两个对象了(这里我就不举例子了)可以看看本文最后的链接

如果需要实例化,我们必须添加一个方法.

    private Object readResolve(){
        return SINGLETON;
    }

引用一下文档的说明

This writeReplace method is invoked by serialization if the method
exists and it would be accessible from a method defined within the
class of the object being serialized. Thus, the method can have private,
protected and package-private access. Subclass access to this method
follows java accessibility rules.
Classes that need to designate a replacement when an instance of it
is read from the stream should implement this special method with the
exact signature.
ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
This readResolve method follows the same invocation rules and
accessibility rules as writeReplace.

简单来说反序列化时可以用这个方法返回的对象替换反序列化对象

枚举单例
枚举单例十分简单,并且无法被反射和序列化(自己就能序列化)击穿

public enum EnumSingleton {
    SINGLETON;
    
    public void doSomeThing(){
        // do what you what to do
    }
}

EnumSingleton.SINGLETON.doSomeThing();

作者的话

单元素的枚举类型已经成为实现Singleton的最佳方法

参考
https://my.oschina.net/u/3441184/blog/884767