Singleton and Exception

Question

Whats the best way to design a singleton class that could throw an exception?

Here I have a Singleton (using Bill Pugh's method, documented in Wiki for Singleton).

    private static class SingletonObjectFactoryHolder{     //1           private static final ObjectFactory INSTANCE = new ObjectFactory();     }      private ObjectFactory() throws Exception{     //2             //create the factory     }       public static ObjectFactory getInstance(){     //3         return SingletonObjectFactoryHolder.INSTANCE;     } 

If an exception is thrown at 2, I would like to propagate it to the caller. However, I can't throw an exception from line 1.

So, is my only option to return a null object if the singleton object wasn't created correctly?

Thanks

P.S I do realize that this Singleton can break if its loaded via different classloaders or if loaded reflexively, but it's good enough for my purpose.

//UPDATE

I am curious, can I not rearrange my design as below to throw exceptions?

Also, I don't need any synchronization (the classloader guarantees that the static inner class will only loaded once and only when getInstance() is called). Thus, thread-safe and lazily-instantiated?

 private static class SingletonObjectFactoryHolder{         //1              public static ObjectFactory getInstance() throws Exception{          return new ObjectFactory();            }  }   private ObjectFactory() throws Exception{         //2         //create the factory  }    public static ObjectFactory getInstance(){         //3     return SingletonObjectFactoryHolder.getInstance();  } 

Thanks again.

Answer 1

Use a static initializer and rethrow the Exception as ExceptionInInitializerError. Click the link to read the Javadoc, you'll see that it suits exactly for this particular functional requirement: handling exceptions during static initialization. A singleton is in fact nothing less or more than a statically and lazily initialized global object.

private static class SingletonObjectFactoryHolder{     private static final ObjectFactory INSTANCE;     static {         try {             INSTANCE = new ObjectFactory();         } catch (Exception e) {             throw new ExceptionInInitializerError(e);         }     } } 

No need for double checked locking idiom which is considered an anti-pattern and in some circumstances even unsafe.