Correct way of making a singleton a Spring bean

Question

I am converting a singleton to a Spring bean, so that if the singleton fails to initialize, then entire web application\'s spring context doesn\'t load properly.

Th

Answer 1

Why are you using singleton pattern on the first place? Just let Spring create bean for you (with default singleton scope) and... use it. Of course always somebody might create the bean by hand, but this was never a problem in my case.

Dependency injection and Spring-managed bean lifecycle will ease your life significantly (just see how many pitfalls you can avoid). Also note that exceptions thrown from c-tor or @PostContruct method will propagate and cause application context startup to fail as well.

UPDATE: I get your point. This is what came in to my mind:

@Service
public class Singleton {

    private static AtomicReference INSTANCE = new AtomicReference();

    public Singleton() {
        final Singleton previous = INSTANCE.getAndSet(this);
        if(previous != null)
            throw new IllegalStateException("Second singleton " + this + " created after " + previous);
    }

    public static Singleton getInstance() {
        return INSTANCE.get();
    }

}

And let Spring do its job. You can use DI when possible and Singleton.getInstance() where you have to.

Also there are more hard-core solutions like compile-time AspectJ weaving and injecting Spring beans basically to everything.