Singleton pattern vs Static Class (a
class, having all static methods) is another interesting questions, which I
missed while blogging about Interview questions on Singleton pattern
in Java. Since both Singleton pattern and static class provides
good accessibility, and they share some similarities e.g. both can be used
without creating object and both provide only one instance, at very high level
it looks that they both are intended for same task. Because of high level
similarities, interviewer normally ask questions like, Why you use Singleton instead of Static Methods, or Can you replace
Singleton with static class, and what
are differences between Singleton pattern and static in Java. In order to
answer these question, it’s important to remember fundamental difference
between Singleton pattern and static class, former gives you an Object, while later just
provide static methods. Since an object is always much more capable than a
method, it can guide you when to use Singleton pattern vs static methods.
In this Java article we will learn, where to use Singleton pattern in
Java, and when static class is better alternative. By the way, JDK has examples
of both singleton and static, and that too very intelligently e.g. java.lang.Math is a final class with full of static methods, on the other
hand java.lang.Runtime is a Singleton class in Java. For those who are not
familiar with Singleton design pattern or static class, static class is a Java class, which only
contains static methods, good examples of static class is java.lang.Math,which
contains lots of utility methods for various maths function e.g. sqrt(). While Singleton classes are those,
which has only one instance during application life cycle like java.lang.Runtime.
When to use Static Class in place of Singleton in Java
Indeed there are some situations, where static classes makes sense than
Singleton. Prime example of this is java.lang.Math which is
not Singleton, instead a class with all static methods. Here are few situation
where I think using static class over Singleton pattern make sense:
1) If your Singleton is not maintaining any state, and just providing
global access to methods, than consider using static class, as static methods are
much faster than Singleton, because of static binding during compile
time. But remember its not advised to maintain state inside static class, especially
in concurrent environment, where it could lead subtle race conditions when modified
parallel by multiple threads without adequate synchronization.
You can also choose to use static method, if you need to combine bunch of
utility method together. Anything else, which requires singles access to some
resource, should use Singleton design pattern.
Difference between Singleton vs Static in Java
This is answer of our second interview question about Singleton over
static. As I said earlier, fundamental difference between them is, one
represent object while other represent a method. Here are few more differences
between static and singleton in Java.
1) Static class provides better performance than Singleton pattern, because
static methods are bonded on compile time.
2) One more difference between Singleton and static is, ability to
override. Since static methods in Java cannot be overridden, they leads to inflexibility. On the other hand, you can
override methods defined in Singleton class by extending it.
3) Static classes are hard to mock and consequently hard to test than
Singletons, which are pretty easy to mock and thus easy to test. It’s easier to
write JUnit test for Singleton than
static classes, because you can pass mock object whenever Singleton is
expected, e.g. into constructor or as method arguments.
4) If your requirements needs to maintain state than Singleton pattern is
better choice than static class, because
maintaining state in later case is
nightmare and leads to subtle bugs.
5) Singleton classes can be lazy loaded if its an heavy object,
but static class doesn't have such advantages and always eagerly loaded.
6) Many Dependency Injection framework
manages Singleton quite well e.g. Spring, which makes using them very easy.
These are some differences between static class and singleton pattern,
this will help to decide between two, which situation arises. In next section
we will when to choose Singleton pattern over static class in Java.
Advantage of Singleton Pattern over Static Class in Java
Main advantage of Singleton over static is that former is more object
oriented than later. With Singleton, you can use Inheritance and Polymorphism to extend a base
class, implement an interface and capable of providing different
implementations. If we talk about java.lang.Runtime, which is a
Singleton in Java, call to getRuntime() method return
different implementations based on different JVM, but guarantees only one
instance per JVM, had java.lang.Runtime an static class, it’s not
possible to return different implementation for different JVM.
That’s all on difference between Singleton and static class in Java. When
you need a class with full OO capability , chose Singleton, while if you just
need to store bunch of static methods together, than use static class.
Other Java Design Pattern Tutorials from Javarevisited Blog
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12 comments:
Good article! As you say the principal reason is the better performance of static classes and inheritance or polymorphism aren't neccesaries.
I Disagree with following :
2) One more difference between Singleton and static is, ability to override. Since static methods in Java cannot be overridden, they leads to inflexibility. On the other hand, you can override methods defined in Singleton class by extending it.
Here the author say you can override methods defined in Singleton.Isn't it breaking Singleton.
Hi Javin Gr8 article , I want to add two things...
1) Singleton class can be extended. Polymorphism can save a lot of repetition.
2) A Singleton class can implement an interface, which can come in handy when you want to separate implementation from API.
3)Singleton can be extended. Static not.
4)Singleton creation may not be threadsafe if it isn't implemented properly. Static not.
5)Singleton can be passed around as an object. Static not.
6)Singleton can be garbage collected. Static not.
7)Singleton object stores in Heap but, static object stores in stack
8)We can clone the object of Singleton but, we can not clone the static class object
9)Singleton class follow the OOP(object oriented principles) but not static class
10)Another advantage of a singleton is that it can easily be serialized, which may be necessary if you need to save its state to disc, or send it somewhere remotely.
The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces (or derive from useful base classes, although that's less common IME), so you can pass around the singleton as if it were "just another" implementation.
@Saral, Good points. On same note, Static class are good for utility classes, which doesn't maintain state and not required to be extended.
I would agree with Anand in that most definitions consider extending a Singleton as breaking the definition... Likewise for cloning and serialization...
Also, since one can use/pass around between threads the Singleton instance, any mutable state it might have makes the object susceptible to race conditions as well...
The fact that "a Singleton follows the OOP principles" is utterly irrelevant since in order to really implement the pattern we have to renounce most of the OOP principles and flexibility anyway...
Moreover, having static classes/methods means less parameters to a method that needs this (static or singleton) functionality, eventually less injection, etc. so basically reduced "interfaces" which is normally good. However, this "lack of dependencies" might tend to hide things, introduce magic behavior and inflexible design. Hence, we get to the only situation and strong point (that Saral made and) that could favor a Singleton: implementing an interface and being passed around as just one of the possible implementations...
I cant understand the discussion. Singleton it is a design pattern with a specific purpose, having just on single instance of a class. You can achieve that with different forms in Java: Enumerator and static classes. Within these forms you can use: Lazy Initialization, Eager Initialization, static block Initialization and the Enum way.
So, its not a question of difference, but when to use one or another or both.
@Jose Cruz, Thanks for your comment, yes ultimately it comes down to, when to use Singleton and Static class. There are situations like implementing Interface, being part of a type hierarchy where Singleton fits better than static classes, but if you just have some utility methods, wrapping them in static class is better.
I can think of a situation when many classloaders are around and we wish to make a class singleton, in that case having a static class will not solve the problem as the class may have been loaded by number of classloaders, this happen all the time in web application servers. To protect this behaviour, classloader safe implementation of Singleton class is a must. Just think about a STATIC ConnectionPool object in a web server, if each of the application using its own classloader may potentially create its own connection pool object!!
Hello Javin, What is difference between Singleton and Factory pattern? Doesn't Singleton is a kind of factory e.g. getInstance() method returns an instance of Singleton rather than client creating instance using new() operator? doesn't this is similar to Factory method pattern? I would say Singleton is a combination of Factory method + job to keep Singleton as Singleton
good post
Good Post..Thx..
Really Good post... Thanks!
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