I think it's too obvious to say that a singleton is an anti-pattern as there are tons of articles about that (singleton being an anti-pattern). However, more often than not, the question is how to define global things without a singleton; and the answer to that is not obvious for many of us. There are several examples: a database connection pool, a repository, a configuration map, etc. They all naturally seem to be "global"; but what do we do with them?
I assume you already know what a singleton is and why it's an anti-pattern. If not, I recommend you read this StackOverflow thread: What is so bad about singletons?
Now that we agree it's a bad deal, what do we do if we need to, let's say, have access to a database connection pool in many different places within the application? We simply need something like this:
Later in at, say, the JAX-RS REST method, we need to retrieve something from the database:
We need that
Database.INSTANCE to be a singleton, right? We need it to be globally available so that any MVC controller can have direct access to it. Since we all understand and agree that a singleton is an evil thing, what do we replace it with?
A dependency injection is the answer.
We need to make this database connection pool dependency of the controller and ensure it's provided through a constructor. However, in this particular case, for JAX-RS, we can't do it through a constructor thanks to its ugly architecture. But we can create a
ServletContextListener, instantiate a
Database in its
contextInitialized() method, and add that instance as an attribute of
servletContext. Then, inside the controller, we retrieve the servlet context by adding the
javax.ws.rs.core.Context annotation to a setter and using
getAttribute() on it. This is absolutely terrible and procedural, but it's better than a singleton.
A proper object-oriented design would pass an instance of
Database to all objects that may need it through their constructors.
Nonetheless, what do we do if there are many dependencies? Do we make a 10-argument constructor? No, we don't. If our objects really need 10 dependencies to do their work, we need to break them down into smaller ones.
That's it. Forget about singletons; never use them. Turn them into dependencies and pass them from object to object through the operator