枚举实现单例模式

枚举可能是实现单例的最佳方式

public enum EnumInstance {
    INSTANCE;
    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public static EnumInstance getInstance() {
        return INSTANCE;
    }
}

public class Test {
    public static void main(String[] args) throws Exception {
        EnumInstance instance = EnumInstance.getInstance();
        instance.setData(new Object());
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("singleton_file"));
        oos.writeObject(instance);

        File file = new File("singleton_file");
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file));

        EnumInstance newInstance = (EnumInstance) ois.readObject();
        System.out.println(instance.getData());
        System.out.println(newInstance.getData());
        System.out.println(instance.getData() == newInstance.getData());
    }
}

java.lang.Object@7291c18f
java.lang.Object@7291c18f
true

以上说明枚举类扛过了序列化破坏

这时，反射蠢蠢欲动，要来试下

public static void main(String[] args) throws Exception {
    Class<EnumInstance> objectClass = EnumInstance.class;
    Constructor<EnumInstance> constructor = objectClass.getDeclaredConstructor();
    // 通过反射，将权限打开
    constructor.setAccessible(true);
}

直接报异常 NoSuchMethodException，无法获取枚举类的无参构造器

反射踩到铁钉子了，由此可见，枚举类天然地不惧序列化以及反射破坏

查看 Enum 源码，发现其只有一个构造器

protected Enum(String name, int ordinal) {
    this.name = name;
    this.ordinal = ordinal;
}

反射贼心不死，继续在犯罪的边缘试探，直接拿带参构造器来处理

public static void main(String[] args) throws Exception {
    Class<EnumInstance> objectClass = EnumInstance.class;
    Constructor<EnumInstance> constructor = objectClass.getDeclaredConstructor(String.class, int.class);
    // 通过反射，将权限打开
    constructor.setAccessible(true);
    EnumInstance enumInstance = constructor.newInstance("Geely", 66);
}

执行，继续报错：

这下就真的 GG 了，你再牛，也牛不过 jdk 的规定吧