单例模式实现；

灭霸所有单例模式，克隆、序列化、反射机制破坏7种单例模式；

枚举实现单例；

 破坏单例模式测试代码

import java.io.Serializable;

public class Singleton implements Cloneable, Serializable {
    private static volatile Singleton singleton;

    private Singleton() {
        if (null != singleton) {
            throw new RuntimeException();
        }
    }

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

    /**
     * 防止克隆攻击
     *
     * @return
     * @throws CloneNotSupportedException
     */
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    private Object readResolve() {
        return getInstance();
    }
}

public class Test1 {
    public static void main(String[] args) throws CloneNotSupportedException {
        Singleton singleton = Singleton.getInstance();
        Singleton singleton1 = (Singleton) singleton.clone();
        Singleton singleton2 = Singleton.getInstance();
        System.out.println(singleton.hashCode());
        System.out.println(singleton1.hashCode());
        System.out.println(singleton2.hashCode());
    }
}

public class Test2 {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Singleton singleton = Singleton.getInstance();
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("d:\\xttblog.obj"));
        oos.writeObject(singleton);
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("d:\\xttblog.obj")));
        Singleton singleton1 = (Singleton) ois.readObject();
        System.out.println(singleton.hashCode());
        System.out.println(singleton1.hashCode());
        System.out.println(singleton == singleton1);
    }
}

public class Test3 {
    public static void main(String[] args) throws ReflectiveOperationException {
        Singleton singleton1 = Singleton.getInstance();
        Class cls = Singleton.class;
        Constructor<Singleton> constructor = cls.getDeclaredConstructor();
        constructor.setAccessible(true);
        Singleton singleton = constructor.newInstance();
        System.out.println(singleton.hashCode());
        System.out.println(singleton1.hashCode());
        System.out.println(singleton == singleton1);
    }
}