枚舉實現(xiàn)單例模式

前面我們說到序列化和反序列化以及反射對單例都是有破壞的嗦哆，下面我們介紹一種更加優(yōu)雅的實現(xiàn)燕鸽，也是effective java中推薦的實現(xiàn)方式，枚舉實現(xiàn)單例模式敬拓。話不多說我們直接看代碼吧邻薯。

public enum EnumInstance {
    INSTANCE{
        protected  void printTest(){
            System.out.println("redstar Print SingletonTest");
        }
    };
    protected abstract void printTest();
    private Object data;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }
    public static EnumInstance getInstance(){
        return INSTANCE;
    }

}

然后我們看看測試類

public class SingletonTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
    
    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());
    }

這里返回的結果是true，然后我們測試一下反射去獲取這個對象乘凸。

public class SingletonTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        Class objectClass = EnumInstance.class;
        Constructor constructor = objectClass.getDeclaredConstructor(String.class,int.class);
        constructor.setAccessible(true);
        EnumInstance instance = (EnumInstance) constructor.newInstance("redstar",666);
}

運行結果報異常

Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
at java.lang.reflect.Constructor.newInstance(Constructor.java:417)

我們點擊進去查看一下417行的代碼

        if ((clazz.getModifiers() & Modifier.ENUM) != 0)
            throw new IllegalArgumentException("Cannot reflectively create enum objects");
        ConstructorAccessor ca = constructorAccessor; 

從這里我們可以看到jdk底層就為我們對反射進行處理了厕诡。