by Miško Hevery
Since I have gotten lots of love/hate mail on the Singletons are Pathological Liars and Where Have All the Singletons Gone I feel obliged to to do some root cause analysis.
Lets get the definition right. There is Singleton the design pattern (Notice the capital "S" as in name of something) and there is a singleton as in one of something (notice the lower case "s"). There is nothing wrong with having a single instance of a class, lots of reasons why you may want to do that. However, when I complain about the Singletons, I complain about the design pattern. Specifically: (1) private constructor and (2) global instance variable which refers to the singleton. So from now on when I say Singleton, I mean the design (anti)pattern.
I would say that at this point most developers recognize that global state is harmful to your application design. Singletons have global instance variable which points to the singleton. The instance is global. The trouble with global variables is that they are transitive. It is not just the global variable marked with static which is global but any other variable/object which is reachable by traversing the object graph. All of it is global! Singletons, usually are complex objects which contain a lot of state. As a result all of the state of Singleton is global as well. I like to say that "Singletons are global state in sheep's clothing." Most developers agree that global state is bad, but they love their Singletons.
The moment you traverse a global variable your API lies about its true dependencies (see: Singletons are Pathological Liars) The root problem is not the Singleton design pattern, the root problem here is the global reference to singleton. But the moment you get rid of the global variable you get rid of the Singleton design pattern. So from my point of view blaming Singletons or blaming global state is one and the same. You can't have a Singleton design pattern and at the same time not have the global state.
Someone pointed out that any design pattern can be abused. I agree, but with Singleton design pattern, I don't know how I can possibly use it in a good way. The global reference and hence the global state is ever so present. Now, in my line of work I don't see too much global state in classical sense of the word, but I see a lot of global state masquerading as Singletons. Hence, I complain about Singletons. If I would complain about global state no one would care, as that is old news.
Now, there is one kind of Singleton which is OK. That is a singleton where all of the reachable objects are immutable. If all objects are immutable than Singleton has no global state, as everything is constant. But it is so easy to turn this kind of singleton into mutable one, it is very slippery slope. Therefore, I am against these Singletons too, not because they are bad, but because it is very easy for them to go bad. (As a side note Java enumeration are just these kind of singletons. As long as you don't put state into your enumeration you are OK, so please don't.)
The other kind of Singletons, which are semi-acceptable are those which don't effect the execution of your code, They have no "side effects". Logging is perfect example. It is loaded with Singletons and global state. It is acceptable (as in it will not hurt you) because your application does not behave any different whether or not a given logger is enabled. The information here flows one way: From your application into the logger. Even thought loggers are global state since no information flows from loggers into your application, loggers are acceptable. You should still inject your logger if you want your test to assert that something is getting logged, but in general Loggers are not harmful despite being full of state.
So the root cause is "GLOBAL STATE!" Keep in mind that global state is transitive, so any object which is reachable from a global variable is global as well. It is not possible to have a Singleton and not have a global state. Therefore, Singleton design patter can not be used in "the right way." Now you could have a immutable singleton, but outside of limited use as enumerations, they have little value. Most applications are full of Singletons which have lots of global state, and where the information flows both directions.