java Singleton - prevent multiple creation through reflection

Question

I have a singleton like this.

public class BookingFactory {      private final static BookingFactory instance;      static {         instance = new BookingFactory();     }      public static BookingFactory getInstance() {         return instance;     }      private BookingFactory() {         System.out.println("Object is created.");     } }  public class Test  {     BookingFactory instance = BookingFactory.getInstance();     instance = BookingFactory.getInstance();      Class<?> clazz = Class.forName("com.test.BookingFactory");      Constructor pvtConstructor = clazz.getDeclaredConstructors()[0];      // Set its access control     pvtConstructor.setAccessible(true);      // Invoke Private Constructor     BookingFactory notSingleton = (BookingFactory) pvtConstructor.newInstance(null); } 

When I run this, I saw more than one printout message. Is there any way to prevent this singleton from being instantiated more than once from this reflection?

Thanks.

Answer 1

Try using an enum. enums make for good Singletons.

public static enum BookingFactory {     INSTANCE;     public static BookingFactory getInstance() {         return INSTANCE;     } } 

You can't create an enum via reflection.

The getInstance() method is superfluous but makes it easier to run your test, throwing the following exception:

java.lang.IllegalArgumentException: Cannot reflectively create enum objects     at java.lang.reflect.Constructor.newInstance(Constructor.java:530)     at MultiSingletonTest.main(MultiSingletonTest.java:40) 

Oh look, someone already gave the enum answer. Posting anyway for more completeness.