I am a Java programmer who is new to the corporate world. Recently I've developed an application using Groovy and Java. All through the code I wrote used quite a good number of statics. I was asked by the senior technical lot to cut down on the number of statics used. I've googled about the same, and I find that many programmers are fairly against using static variables.
I find static variables more convenient to use. And I presume that they are efficient too (please correct me if I am wrong), because if I had to make 10,000 calls to a function within a class, I would be glad to make the method static and use a straightforward Class.methodCall() on it instead of cluttering the memory with 10,000 instances of the class, right?
Moreover statics reduce the inter-dependencies on the other parts of the code. They can act as perfect state holders. Adding to this I find that statics are widely implemented in some languages like Smalltalk and Scala. So why is this opposition to statics prevalent among programmers (especially in the world of Java)?
PS: please do correct me if my assumptions about statics are wrong.
Static variables represent global state. That's hard to reason about and hard to test: if I create a new instance of an object, I can reason about its new state within tests. If I use code which is using static variables, it could be in any state - and anything could be modifying it.
I could go on for quite a while, but the bigger concept to think about is that the tighter the scope of something, the easier it is to reason about. We're good at thinking about small things, but it's hard to reason about the state of a million line system if there's no modularity. This applies to all sorts of things, by the way - not just static variables.
Its not very object oriented:
One reason statics might be considered "evil" by some people is they are contrary the object-oriented paradigm. In particular, it violates the principle that data is encapsulated in objects (that can be extended, information hiding, etc). Statics, in the way you are describing using them, are essentially to use them as a global variable to avoid dealing with issues like scope. However, global variables is one of the defining characteristics of procedural or imperative programming paradigm, not a characteristic of "good" object oriented code. This is not to say the procedural paradigm is bad, but I get the impression your supervisor expects you to be writing "good object oriented code" and you're really wanting to write "good procedural code".
There are many gotchyas in Java when you start using statics that are not always immediately obvious. For example, if you have two copies of your program running in the same VM, will they shre the static variable's value and mess with the state of each other? Or what happens when you extend the class, can you override the static member? Is your VM running out of memory because you have insane numbers of statics and that memory cannot be reclaimed for other needed instance objects?
Object Lifetime:
Additionally, statics have a lifetime that matches the entire runtime of the program. This means, even once you're done using your class, the memory from all those static variables cannot be garbage collected. If, for example, instead, you made your variables non-static, and in your main() function you made a single instance of your class, and then asked your class to execute a particular function 10,000 times, once those 10,000 calls were done, and you delete your references to the single instance, all your static variables could be garbage collected and reused.
Prevents certain re-use:
Also, static methods cannot be used to implement an interface, so static methods can prevent certain object oriented features from being usable.
Other Options:
If efficiency is your primary concern, there might be other better ways to solve the speed problem than considering only the advantage of invocation being usually faster than creation. Consider whether the transient or volatile modifiers are needed anywhere. To preserve the ability to be inlined, a method could be marked as final instead of static. Method parameters and other variables can be marked final to permit certain compiler optimiazations based on assumptions about what can change those variables. An instance object could be reused multiple times rather than creating a new instance each time. There may be compliler optimization switches that should be turned on for the app in general. Perhaps, the design should be set up so that the 10,000 runs can be multi-threaded and take advantage of multi-processor cores. If portablity isn't a concern, maybe a native method would get you better speed than your statics do.
If for some reason you do not want multiple copies of an object, the singleton design pattern, has advantages over static objects, such as thread-safety (presuming your singleton is coded well), permitting lazy-initialization, guaranteeing the object has been properly initialized when it is used, sub-classing, advantages in testing and refactoring your code, not to mention, if at some point you change your mind about only wanting one instance of an object it is MUCH easier to remove the code to prevent duplicate instances than it is to refactor all your static variable code to use instance variables. I've had to do that before, its not fun, and you end up having to edit a lot more classes, which increases your risk of introducing new bugs...so much better to set things up "right" the first time, even if it seems like it has its disadvantages. For me, the re-work required should you decide down the road you need multiple copies of something is probably one of most compelling reasons to use statics as infrequently as possible. And thus I would also disagree with your statement that statics reduce inter-dependencies, I think you will end up with code that is more coupled if you have lots of statics that can be directly accessed, rather than an object that "knows how to do something" on itself.
Evil is a subjective term.
You don't control statics in terms of creation and destruction. They live at the behest of the program loading and unloading.
Since statics live in one space, all threads wishing to use them must go through access control that you have to manage. This means that programs are more coupled and this change is harder to envisage and manage (like J Skeet says). This leads to problems of isolating change impact and thus affects how testing is managed.
These are the two main issues I have with them.
No. Global states are not evil per se. But we have to see your code to see if you used it properly. It is quite possible that a newbie abuses global states; just like he would abuses every language feature.
Global states are absolute necessity. We cannot avoid global states. We cannot avoid reasoning about global states. - If we care to understand our application semantics.
People who try to get rid of global states for the sake of it, inevitably end up with a much more complex system - and the global states are still there, cleverly/idiotically disguised under many layers of indirections; and we still have to reason about global states, after unwrapping all the indirections.
Like the Spring people who lavishly declare global states in xml and think somehow it's superior.
#Jon Skeet if I create a new instance of an object now you have two things to reason about - the state within the object, and the state of the environment hosting the object.
If you are using the ‘static’ keyword without the ‘final’ keyword, this should be a signal to carefully consider your design. Even the presence of a ‘final’ is not a free pass, since a mutable static final object can be just as dangerous.
I would estimate somewhere around 85% of the time I see a ‘static’ without a ‘final’, it is WRONG. Often, I will find strange workarounds to mask or hide these problems.
Please don’t create static mutables. Especially Collections. In general, Collections should be initialized when their containing object is initialized and should be designed so that they are reset or forgotten about when their containing object is forgotten.
Using statics can create very subtle bugs which will cause sustaining engineers days of pain. I know, because I’ve both created and hunted these bugs.
If you would like more details, please read on…
Why Not Use Statics?
There are many issues with statics, including writing and executing tests, as well as subtle bugs that are not immediately obvious.
Code that relies on static objects can’t be easily unit tested, and statics can’t be easily mocked (usually).
If you use statics, it is not possible to swap the implementation of the class out in order to test higher level components. For example, imagine a static CustomerDAO that returns Customer objects it loads from the database. Now I have a class CustomerFilter, that needs to access some Customer objects. If CustomerDAO is static, I can’t write a test for CustomerFilter without first initializing my database and populating useful information.
And database population and initialization takes a long time. And in my experience, your DB initialization framework will change over time, meaning data will morph, and tests may break. IE, imagine Customer 1 used to be a VIP, but the DB initialization framework changed, and now Customer 1 is no longer VIP, but your test was hard-coded to load Customer 1…
A better approach is to instantiate a CustomerDAO, and pass it into the CustomerFilter when it is constructed. (An even better approach would be to use Spring or another Inversion of Control framework.
Once you do this, you can quickly mock or stub out an alternate DAO in your CustomerFilterTest, allowing you to have more control over the test,
Without the static DAO, the test will be faster (no db initialization) and more reliable (because it won’t fail when the db initialization code changes). For example, in this case ensuring Customer 1 is and always will be a VIP, as far as the test is concerned.
Executing Tests
Statics cause a real problem when running suites of unit tests together (for example, with your Continuous Integration server). Imagine a static map of network Socket objects that remains open from one test to another. The first test might open a Socket on port 8080, but you forgot to clear out the Map when the test gets torn down. Now when a second test launches, it is likely to crash when it tries to create a new Socket for port 8080, since the port is still occupied. Imagine also that Socket references in your static Collection are not removed, and (with the exception of WeakHashMap) are never eligible to be garbage collected, causing a memory leak.
This is an over-generalized example, but in large systems, this problem happens ALL THE TIME. People don’t think of unit tests starting and stopping their software repeatedly in the same JVM, but it is a good test of your software design, and if you have aspirations towards high availability, it is something you need to be aware of.
These problems often arise with framework objects, for example, your DB access, caching, messaging, and logging layers. If you are using Java EE or some best of breed frameworks, they probably manage a lot of this for you, but if like me you are dealing with a legacy system, you might have a lot of custom frameworks to access these layers.
If the system configuration that applies to these framework components changes between unit tests, and the unit test framework doesn’t tear down and rebuild the components, these changes can’t take effect, and when a test relies on those changes, they will fail.
Even non-framework components are subject to this problem. Imagine a static map called OpenOrders. You write one test that creates a few open orders, and checks to make sure they are all in the right state, then the test ends. Another developer writes a second test which puts the orders it needs into the OpenOrders map, then asserts the number of orders is accurate. Run individually, these tests would both pass, but when run together in a suite, they will fail.
Worse, failure might be based on the order in which the tests were run.
In this case, by avoiding statics, you avoid the risk of persisting data across test instances, ensuring better test reliability.
Subtle Bugs
If you work in high availability environment, or anywhere that threads might be started and stopped, the same concern mentioned above with unit test suites can apply when your code is running on production as well.
When dealing with threads, rather than using a static object to store data, it is better to use an object initialized during the thread’s startup phase. This way, each time the thread is started, a new instance of the object (with a potentially new configuration) is created, and you avoid data from one instance of the thread bleeding through to the next instance.
When a thread dies, a static object doesn’t get reset or garbage collected. Imagine you have a thread called “EmailCustomers”, and when it starts it populates a static String collection with a list of email addresses, then begins emailing each of the addresses. Lets say the thread is interrupted or canceled somehow, so your high availability framework restarts the thread. Then when the thread starts up, it reloads the list of customers. But because the collection is static, it might retain the list of email addresses from the previous collection. Now some customers might get duplicate emails.
An Aside: Static Final
The use of “static final” is effectively the Java equivalent of a C #define, although there are technical implementation differences. A C/C++ #define is swapped out of the code by the pre-processor, before compilation. A Java “static final” will end up memory resident in the JVM's class memory, making it (usually) permanent in ram. In that way, it is more similar to a “static const” variable in C++ than it is to a #define.
Summary
I hope this helps explain a few basic reasons why statics are problematic up. If you are using a modern Java framework like Java EE or Spring, etc, you may not encounter many of these situations, but if you are working with a large body of legacy code, they can become much more frequent.
There are 2 main problems with static variables:
Thread Safety - static resources are by definition not thread-safe
Code Implicity - You do not know when a static variables is instantiated and whether or not it will be instantiated before another static variable
Summarising few basic Advantages & Disadvantages of using Static methods in Java:
Advantages:
Globally accessible i.e. not tied with any particular object instance.
One instance per JVM.
Can be accessed by using class name (No object require).
Contains a single value applicable to all instances.
Load up on JVM startup and dies when JVM shuts down.
They doesn't modify state of Object.
Disadvantages:
Static members are always part of memory whether they are in use or not.
You can not control creation and destruction of static variable. Usefully they have been created at program loading and destroyed when program unload (or when JVM shuts down).
You can make statics thread safe using synchronize but you need some extra efforts.
If one thread change value of a static variable that can possibly break functionality of other threads.
You must know “static“ before using it.
You cannot override static methods.
Serialization doesn't work well with them.
They don't participate in runtime polymorphism.
There is a memory issue (to some extent but not much I guess) if a large number of static variables/methods are used. Because they will not be Garbage Collected until program ends.
Static methods are hard to test too.
Static variables are generally considered bad because they represent global state and are therefore much more difficult to reason about. In particular, they break the assumptions of object-oriented programming. In object-oriented programming, each object has its own state, represented by instance (non-static) variables. Static variables represent state across instances which can be much more difficult to unit test. This is mainly because it is more difficult to isolate changes to static variables to a single test.
That being said, it is important to make a distinction between regular static variables (generally considered bad), and final static variables (AKA constants; not so bad).
Since no one* has mentioned it: concurrency. Static variables can surprise you if you have multiple threads reading and writing to the static variable. This is common in web applications (e.g., ASP.NET) and it can cause some rather maddening bugs. For example, if you have a static variable that is updated by a page, and the page is requested by two people at "nearly the same time", one user may get the result expected by the other user, or worse.
statics reduce the inter-dependencies on the other parts of the code. They can act as perfect state holders
I hope you're prepared to use locks and deal with contention.
*Actually, Preet Sangha mentioned it.
if I had to make 10,000 calls to a function within a class, I would be
glad to make the method static and use a straightforward
class.methodCall() on it instead of cluttering the memory with 10,000
instances of the class, Right?
You have to balance the need for encapsulating data into an object with a state, versus the need of simply computing the result of a function on some data.
Moreover statics reduce the inter-dependencies on the other parts of the code.
So does encapsulation. In large applications, statics tend to produce spaghetti code and don't easily allow refactoring or testing.
The other answers also provide good reasons against excessive use of statics.
In my opinion it's hardly ever about performance, it's about design. I don't consider the use of static methods wrong as apposed of the use of static variables (but I guess you are actually talking about method calls).
It's simply about how to isolate logic and give it a good place. Sometimes that justifies using static methods of which java.lang.Math is a good example. I think when you name most of your classes XxxUtil or Xxxhelper you'd better reconsider your design.
I have just summarized some of the points made in the answers. If you find anything wrong please feel free to correct it.
Scaling: We have exactly one instance of a static variable per JVM. Suppose we are developing a library management system and we decided to put the name of book a static variable as there is only one per book. But if system grows and we are using multiple JVMs then we dont have a way to figure out which book we are dealing with?
Thread-Safety: Both instance variable and static variable need to be controlled when used in multi threaded environment. But in case of an instance variable it does not need protection unless it is explicitly shared between threads but in case of a static variable it is always shared by all the threads in the process.
Testing: Though testable design does not equal to good design but we will rarely observe a good design that is not testable. As static variables represent global state and it gets very difficult to test them.
Reasoning about state: If I create a new instance of a class then we can reason about the state of this instance but if it is having static variables then it could be in any state. Why? Because it is possible that the static variable has been modified by some different instance as static variable is shared across instances.
Serialization: Serialization also does not work well with them.
Creation and destruction: Creation and destruction of static variables can not be controlled. Usually they are created and destroyed at program loading and unloading time. It means they are bad for memory management and also add up the initialization time at start up.
But what if we really need them?
But sometimes we may have a genuine need of them. If we really feel the need of many static variables that are shared across the application then one option is to make use of Singleton Design pattern which will have all these variables. Or we can create some object which will have these static variable and can be passed around.
Also if the static variable is marked final it becomes a constant and value assigned to it once cannot be changed. It means it will save us from all the problems we face due to its mutability.
Seems to me that you're asking about static variables but you also point out static methods in your examples.
Static variables are not evil - they have its adoption as global variables like constants in most cases combined with final modifier, but as it said don't overuse them.
Static methods aka utility method. It isn't generally a bad practice to use them but major concern is that they might obstruct testing.
As a example of great java project that use a lot of statics and do it right way please look at Play! framework. There is also discussion about it in SO.
Static variables/methods combined with static import are also widely used in libraries that facilitate declarative programming in java like: make it easy or Hamcrest. It wouldn't be possible without a lot of static variables and methods.
So static variables (and methods) are good but use them wisely!
Static variables most importantly creates problem with security of data (any time changed,anyone can change,direct access without object, etc.)
For further info read this
Thanks.
It might be suggested that in most cases where you use a static variable, you really want to be using the singleton pattern.
The problem with global states is that sometimes what makes sense as global in a simpler context, needs to be a bit more flexible in a practical context, and this is where the singleton pattern becomes useful.
Yet another reason: fragility.
If you have a class, most people expect to be able to create it and use it at will.
You can document it's not the case, or protect against it (singleton/factory pattern) - but that's extra work, and therefore an additional cost.
Even then, in a big company, chances are someone will try at some point to use your class without fully paying attention to all the nice comments or the factory.
If you're using static variables a lot, that will break. Bugs are expensive.
Between a .0001% performance improvement and robustness to change by potentially clueless developers, in a lot of cases robustness is the good choice.
I find static variables more convenient to use. And I presume that they are efficient too (Please correct me if I am wrong) because if I had to make 10,000 calls to a function within a class, I would be glad to make the method static and use a straightforward class.methodCall() on it instead of cluttering the memory with 10,000 instances of the class, Right?
I see what you think, but a simple Singleton pattern will do the same without having to instantiate 10 000 objects.
static methods can be used, but only for functions that are related to the object domain and do not need or use internal properties of the object.
ex:
public class WaterContainer {
private int size;
private int brand;
...etc
public static int convertToGallon(int liters)...
public static int convertToLiters(int gallon)...
}
The issue of 'Statics being evil' is more of an issue about global state. The appropriate time for a variable to be static, is if it does not ever have more than one state; IE tools that should be accessible by the entire framework and always return the same results for the same method calls are never 'evil' as statics. As to your comment:
I find static variables more convenient to use. And I presume that they are efficient too
Statics are the ideal and efficient choice for variables/classes that do not ever change.
The problem with global state is the inherent inconsistency that it can create. Documentation about unit tests often address this issue, since any time there is a global state that can be accessed by more than multiple unrelated objects, your unit tests will be incomplete, and not 'unit' grained. As mentioned in this article about global state and singletons, if object A and B are unrelated (as in one is not expressly given reference to another), then A should not be able to affect the state of B.
There are some exceptions to the ban global state in good code, such as the clock. Time is global, and--in some sense--it changes the state of objects without having a coded relationship.
My $.02 is that several of these answers are confusing the issue, rather than saying "statics are bad" I think its better to talk about scoping and instances.
What I would say is that a static is a "class" variables - it represenst a value that is shared across all instances of that class. Typically it should be scoped that way as well (protected or private to class and its instances).
If you plan to put class-level behavior around it and expose it to other code, then a singleton may be a better solution to support changes in the future (as #Jessica suggested). This is because you can use interfaces at the instance/singleton level in ways that you can not use at the class level - in particular inheritance.
Some thoughts on why I think some of the aspects in other answers are not core to the question...
Statics are not "global". In Java scoping is controlled separately from static/instance.
Concurrency is no less dangerous for statics than instance methods. It's still state that needs to be protected. Sure you may have 1000 instances with an instance variable each and only one static variable, but if the code accessing either isn't written in a thread-safe way you are still screwed - it just may take a little longer for you to realize it.
Managing life cycle is an interesting argument, but I think it's a less important one. I don't see why its any harder to manage a pair of class methods like init()/clear() than the creation and destroying of a singleton instance. In fact, some might say a singleton is a little more complicated due to GC.
PS, In terms of Smalltalk, many of its dialects do have class variables, but in Smalltalk classes are actually instances of Metaclass so they are really are variables on the Metaclass instance. Still, I would apply the same rule of thumb. If they are being used for shared state across instances then ok. If they are supporting public functionality you should look at a Singleton. Sigh, I sure do miss Smalltalk....
There are two main questions in your post.
First, about static variables.
Static variables are completelly unnecesary and it's use can be avoided easily. In OOP languajes in general, and in Java particularlly, function parameters are pased by reference, this is to say, if you pass an object to a funciont, you are passing a pointer to the object, so you dont need to define static variables since you can pass a pointer to the object to any scope that needs this information. Even if this implies that yo will fill your memory with pointers, this will not necesary represent a poor performance because actual memory pagging systems are optimized to handle with this, and they will maintain in memory the pages referenced by the pointers you passed to the new scope; usage of static variables may cause the system to load the memory page where they are stored when they need to be accessed (this will happen if the page has not been accesed in a long time). A good practice is to put all that static stuf together in some little "configuration clases", this will ensure the system puts it all in the same memory page.
Second, about static methods.
Static methods are not so bad, but they can quickly reduce performance. For example, think about a method that compares two objects of a class and returns a value indicating which of the objects is bigger (tipical comparison method) this method can be static or not, but when invoking it the non static form will be more eficient since it will have to solve only two references (one for each object) face to the three references that will have to solve the static version of the same method (one for the class plus two, one for each object). But as I say, this is not so bad, if we take a look at the Math class, we can find a lot of math functions defined as static methods. This is really more eficient than putting all these methods in the class defining the numbers, because most of them are rarelly used and including all of them in the number class will cause the class to be very complex and consume a lot of resources unnecesarilly.
In concluson: Avoid the use of static variables and find the correct performance equilibrium when dealing with static or non static methods.
PS: Sorry for my english.
There's nothing wrong with static variables per se. It's just the Java syntax that's broken. Each Java class actually defines two structures- a singleton object which encapsulates static variables, and an instance. Defining both in the same source block is pure evil, and results in a code that's hard to read. Scala did that right.
everything (can:) have its purpose, if you have bunch of threads that needs to share/cache data and also all accessible memory (so you dont split into contexts within one JVM) the static is best choice-> of course you can force just one instance, but why?
i find some of the comments in this thread evil, not the statics ;)
Static variables are not good nor evil. They represent attributes that describe the whole class and not a particular instance. If you need to have a counter for all the instances of a certain class, a static variable would be the right place to hold the value.
Problems appear when you try to use static variables for holding instance related values.
a) Reason about programs.
If you have a small- to midsize-program, where the static variable Global.foo is accessed, the call to it normally comes from nowhere - there is no path, and therefore no timeline, how the variable comes to the place, where it is used. Now how do I know who set it to its actual value? How do I know, what happens, if I modify it right now? I have grep over the whole source, to collect all accesses, to know, what is going on.
If you know how you use it, because you just wrote the code, the problem is invisible, but if you try to understand foreign code, you will understand.
b) Do you really only need one?
Static variables often prevent multiple programs of the same kind running in the same JVM with different values. You often don't foresee usages, where more than one instance of your program is useful, but if it evolves, or if it is useful for others, they might experience situations, where they would like to start more than one instance of your program.
Only more or less useless code which will not be used by many people over a longer time in an intensive way might go well with static variables.
All the answers above show why statics are bad. The reason they are evil is because it gives the false impression that you are writing object oriented code, when in fact you are not.
That is just plain evil.
There are plenty of good answers here, adding to it,
Memory:
Static variables are live as long as the class loader lives[in general till VM dies], but this is only in-case of bulk objects/references stored as static.
Modularization:
consider concepts like IOC, dependencyInjection, proxy etc.. All are completely against tightly coupling/static implementations.
Other Con's: Thread Safety, Testability
I've played with statics a lot and may I give you a slightly different answer--or maybe a slightly different way to look at it?
When I've used statics in a class (Members and methods both) I eventually started to notice that my class is actually two classes sharing responsibility--there is the "Static" part which acts a lot like a singleton and there is the non-static part (a normal class). As far as I know you can always separate those two classes completely by just selecting all the statics for one class and non-statics for the other.
This used to happen a lot when I had a static collection inside a class holding instances of the class and some static methods to manage the collection. Once you think about it, it's obvious that your class is not doing "Just one thing", it's being a collection and the doing something completely different.
Now, let's refactor the problem a little: If you split your class up into one class where everything is static and another which is just a "Normal Class" and forget about the "Normal Class" then your question becomes purely Static class vs Singleton which is addressed in length here (and probably a dozen other questions).
Static fields are de facto GC roots (see the How Garbage Collection Works section earlier in this chapter), which means they are never garbage-collected! For convenience alone, static fields and collections are often used to hold caches or share state across threads. Mutable static fields need to be cleaned up explicitly. If the developer does not consider every possibility (a near certainty), the cleanup will not take place, resulting in a memory leak. This sort of careless programming means that static fields and collections have become the most common cause of memory leaks!
In short, never use mutable static fields—use only constants. If you think you need mutable static fields, think about it again, and then again! There's always a more appropriate technique.
I think excessive uses of global variables with static keyword will also leads to memory leakage at some point of instance in the applica
From my point of view static variable should be only read only data or variables created by convention.
For example we have a ui of some project, and we have a list of countries, languages, user roles, etc. And we have class to organize this data. we absolutely sure that app will not work without this lists. so the first that we do on app init is checking this list for updates and getting this list from api (if needed). So we agree that this data is "always" present in app. It is practically read only data so we don't need to take care of it's state - thinking about this case we really don't want to have a lot of instances of those data - this case looks a perfect candidate to be static.
I have an application that has several classes used for storing application-wide settings (locations of resources, user settings, and such). Right now these classes are just full of static fields and methods, but I never instantiate them.
Someone suggested I make them Singletons, What's the case for/against?
I consider the Singleton pattern to be the most inappropriately applied design pattern. In ~12 years of software development I'd say I've maybe seen 5 examples that were appropriate.
I worked on a project where we had a system monitoring service that modeled our system with a System class (not to be confused with Java's built-in System class) that contained a list of Subsystems each with a list of Components and so on. The designer made System a Singleton. I asked "Why is this a Singleton?" Answer: "Well, there is only one system." "I know, but, why did you make it a Singleton? Couldn't you just instantiate one instance of a normal class and pass it to the classes that need it?" "It was easier to just call getInstance() everywhere instead of passing it around." "Oh..."
This example is typical: Singletons are often misused as a convenient way to access a single instance of a class, rather than to enforce a unique instance for technical reasons. But this comes at a cost. When a class depends on getInstance(), it is forever bound to the Singleton implementation. This makes it less testable, reusable, and configurable. It violates a basic rule I follow and that probably has a common name in some design principles essay somewhere: classes should not know how to instantiate their dependencies. Why? Because it hardcodes classes together. When a class calls a constructor, it is bound to an implementation. getInstance() is no different. The better alternative is to pass an interface into the class, and something else can do the constructor/getInstance()/factory call. This is where dependency injection frameworks like Spring come in, though they are not necessary (just really nice to have).
So when is it appropriate to use a Singleton? In that rare case where instantiating more than one of something would literally ruin the application. I'm not talking about instantiating two Earths in a solar system app - that's just a bug. I mean where there is some underlying hardware or software resource that will blow up your app if you call/allocate/instantiate it more than once. Even in this case, classes that use the Singleton should not know it is a Singleton. There should be one and only one call to getInstance() that returns an interface that is then passed to constructors/setters of classes that need it. I guess another way of saying it is that you should use a Singleton for its "singleness" and not for its "globally accessibleness".
By the way, on that project I mentioned where System was a Singleton... Well System.getInstance() was laced throughout the code base, along with several other inappropriate Singletons. A year later some new requirements came down: "We are deploying our system to multiple sites and want the system monitoring service to be able to monitor each instance." Each instance... hmmm... getInstance() ain't gonna cut it :-)
Effective Java says:
Singletons typically represent some system component that is intrinsically
unique, such as a video display or file system.
So if your component warrants single instance accross the entire application and it has some state, it makes sense to make it a singleton
In your case, the settings of the application is a good candidate for singleton.
On the other hand, a class can only have static methods if you want to group certain functions together, such as utility classes, examples in jdk are java.util.Arrays java.util.Collections. These have several related methods that act on arrays or collections
Singleton will give you an object reference, that you can use all over your app...
you will use singleton if you want objects and/or polymorphism...
If you don't ever need to instantiate them, I don't see the point of singletons. Well, I should amend that - if these classes cannot be instantiated, then it doesn't make sense to compare them to singletons - the singleton pattern restricts instantiation to one object, and comparing that to something that cannot be instantiated doesn't make sense.
I find my main use of singletons generally involves a class that has static methods that, after maybe preparing an environment, instantiate an instance of themselves. By utilising private constructors and overriding Object.clone() to throw CloneNotSupportedException, no other classes can create a new instance of it, or if they're ever passed an instance of it, they cannot clone it.
I guess I'd say that if your application settings are part of a class that is never instantiated, it's not relevant to say "It should/shouldn't be a singleton."
I think you no need to create signleton class.
just make this class constructor private.
Like Math class in java.
public final class Math {
/**
* Don't let anyone instantiate this class.
*/
private Math() {}
//static fields and methods
}
Singletons often show up in situations like you're describing- you have some sort of global data that needs to live in some place, and you'll only ever need one, preferably you want to make sure that you can only have one.
The simplest thing you can do is a static variable:
public class Globals{
public static final int ACONSTANT=1;
This is fine, and ensures that you'll have only one, and you have no instantiation issues. The main downside, of course, is that it's often inconvenient to have your data baked in. It also runs into loading issue if, for example, your string is actually built, by, say, building something from an outside resource (there's also a gotcha with primitives - if your static final is an int, say, classes that depend on it compile it inline, which means that recompiling your constants may not replace the constants in the app - e.g., given public class B{ int i = Globals.ACONSTANT; } changing Globals.ACONSTANT and recompiling only Globals will leave B.i still =1.)
Building your own singleton is the next simplest thing to do, and is often fine (though look up discussions on problems inherent in singleton loading, e.g. double-checked locking).
These sorts of issues are a big reason why many apps are built using Spring, Guice, or some other framework that manages resource loading.
So basically:
Statics
Good: easy to code, code is clear and simple
bad: Not configurable- you've got to recompile to change your values, may not be workable if your global data requires complex initialization
Singletons fix some of this, Dependency Injection frameworks fix and simplify some of the issues involved in singleton loading.
Since your class is holding global settings, a pro for a singleton could be that you have more control about creation of the singleton. You could read a configuration file during object creation.
In other cases if methods are static there would be no benefit like in javas Math class which has only static members.
A more obvious need for singletons is when you implement factories as singletons, because you can interchange different implementations of this factory.
Sometimes what people think belong as Singleton objects really can be private members of a class. Other times they should be unique global variables. Depends on what the design needs them to be.
If there should be one, and exactly one instance of an object: use a Singleton. By that, I mean if the program should HALT if there's more than one object. A good example is if you're designing a video game that only supports rendering to a single output device, ever. Trying to opening the same device again (shame on you for hardcoding!) would be forbidden. IMHO a case like that often means you shouldn't be using classes in the first place. Even C allows you to trivially encapsulating such a problem without the complexity of making a Singleton class, and still maintain the elements of OO that apply to a singleton. When you're stuck in a language like Java/C# , the singleton pattern is what you've got to work with, unless purely static members will do the trick on their own. You can still simulate the other way through those.
If it's merely a case of interfacing objects, you probably should think more object oriented. Here's another example: let's say our game engines rendering code needs to interface resource and input managers, so it can do it's job. You could make those singletons and do sth like ResourceManager.getInstance().getResource(name). Or you could create an application class (e.g. GameEngine) that has ResourceManager and InputManager as private members. Then have the GameEngine pass these as necessary to methods of the rendering code. Such as r.render(resourcemanager);.
For singletons—can easily be accessed from anywhere, it's like a global variable, but there can only be one copy of it.
Against singletons—many uses of singletons can be solved by encapsulating it within in a parent object and passing a member object to another member objects methods.
Sometimes just using a stupid global variable is the right thing. Like using GOTO or a compounded (and/or) conditional statement instead of writing the same error handling code N times with copy and paste.
Code smarter, not harder.
You should use singletons for modularization.
Imagine the following entities in singleton:
Printer prt;
HTTPInfo httpInfo;
PageConfig pgCfg;
ConnectionPool cxPool;
Case 1.
Imagine if you did not do that, but a single class to hold all the static fields/methods.
Then you would have one big pool of static to deal with.
Case 2.
In your current case, you did segregate them into their appropriate classes but as static references. Then there would be too much noise, because every static property is now available to you. If you do not need the information, especially when there is a lot of static information, then you should restrict a current scope of the code from seeing unneeded information.
Prevention of data clutter helps in maintenance and ensure dependencies are restricted. Somehow having a sense of what is or is not available to me at my current scope of coding helps me code more effectively.
Case 3 Resource identity.
Singletons allow easy up-scaling of resources. Let us say you now have a single database to deal with and therefore, you place all its settings as static in the MyConnection class. What if a time came when you are required to connect to more than one database? If you had coded the connection info as a singleton, the code enhancement would comparative much simpler.
Case 4 Inheritance.
Singleton classes allow themselves to be extended. If you had a class of resources, they can share common code. Let's say you have a class BasicPrinter which is instantiable as singleton. Then you have a LaserPrinter which extends BasicPrinter.
If you had used static means, then your code would break because you would not be able to access BasicPrinter.isAlive as LaserPrinter.isAlive. Then your single piece of code would not be able to manage different types of printers, unless you place redundant code.
If you are coding in Java, you could still instantiate a totally static content class and use the instance reference to access its static properties. If someone should do that, why not just make that a singleton?
Of course, extending singleton classes have their issues beyond this discussion but there are simple ways to mitigate those issues.
Case 5 Avoid information grandstanding.
There are so few pieces of info that needs to be made globally available like the largest and smallest integers. Why should Printer.isAlive be allowed to make a grandstand? Only a very restricted set of information should be allowed to make a grandstand.
There is a saying: Think globally, act locally. Equivalently, a programmer should use singletons to think globally but act locally.
Effective Java says:
Singletons typically represent some system component that is intrinsically
unique, such as a video display or file system.
So if your component warrants single instance across the entire application and it has some state, it makes sense to make it a singleton.
(The above nakedly borrowed from naikus)
In many cases the above situation can be handled by a 'utility class' in which all the methods are static. But sometimes a Singleton is still preferred.
The main reason for advocating a Singleton over a Static Utility Class is when there is a non-negligible cost involved in setting it up. For example if your class represents the file system, it will take some initialization, which can be put in the constructor of a Singleton but for a Static Utility Class will have to be called in the Static Initializer. If some executions of your app might never access the file system then with a Static Utility Class you have still paid the cost of initializing it , even though you don't need it. With a Singleton if you never need to instantiate it you never call the initialization code.
Having said all that, Singleton is almost certainly the most-overused design pattern.
I have a project (related to graph algorithms). It is written by someone else.
The code is horrible:
public fields, no getters/setters
huge methods, all public
some classes have over 20 fields
some classes have over 5 constructors (which are also huge)
some of those constructors just leave many fields null
(so I can't make some fields final, because then every second constructor signals errors)
methods and classes rely on each other in both directions
I have to rewrite this into a clean and understandable API.
Problem is: I myself don't understand anything in this code.
Please give me hints on analyzing and understanding such code.
I was thinking, perhaps, there are tools which perform static code analysis
and give me call graphs and things like this.
Oh dear :-) I envy you and not at the same time..ok let's take one thing at a time. Some of these things you can tackle yourself before you set a code analyzing tool loose at it. This way you will gain a better understanding and be able to proceed much further than with a simple tool
public fields, no getters/setters
make everything private. Your rule should be to limit access as much as possible
huge methods, all public
split and make private where it makes sense to do so
some classes have over 20 fields
ugh..the Builder pattern in Effective Java 2nd Ed is a prime candidate for this.
some classes have over 5 constructors (which are also huge)
Sounds like telescoping constructors, same pattern as above will help
some of those constructors just left many fields null
yep it is telescoping constructors :)
methods and classes rely on each other in both directions
This will be the least fun. Try to remove inheritance unless you're perfectly clear
it is required and use composition instead via interfaces where applicable
Best of luck we are here to help
WOW!
I would recommend: write unittests and then start refactoring
* public fields, no getters/setters
start by making them private and 'feel' the resistance on compiler errors as metric.
* huge methods, all public
understand their semantics, try to introdue interfaces
* some classes have over 20 fields
very common in complex appilcations, nothing to worrie
* some classes have over 5 constructors (which are also huge)
replace them by by buider/creator pattern
* some of those constructors just left many fields null
see above answer
* methods and classes rely on each other in both directions
decide whether to to rewrite everything (honestly I faced cased where only 10% of the code was needed)
Well, the clean-up wizard in eclipse will scrape off a noticable percentage of the sludge.
Then you could point Sonar at it and fix everything it complains about, if you live long enough.
For static analysis and call graphs (no graphics, but graph structures), you can use Dependency Finder.
Use an IDE that knows something about refactoring, like IntelliJ. You won't have situations where you move one method and five other classes complain, because IntelliJ is smart enough to make all the required changes.
Unit tests are a must. Someone refactoring without unit tests is like a high-wire performer without a safety net. Get one before you start the long, hard climb.
The answer may be: patience & coffee.
This is the way I would do it:
Start using the code , e.g. from within a main method, as if it were used by the other classes - same arguments, same invocation orders. Do that inside a debugger, as you see each step that this class makes.
Start writing unit tests for that functionality. Once you have reached a reasonable coverage, you will start to notice that this class probably has too many responsibilities.
while ( responsibilities != 1 ) {
Extract an interface which expresses one responsibility of that class.
Make all callers use that interface instead of the concrete type;
Extract the implementation to a separate class;
Pass the new class to all callers using the new interface.
}
Not saying tools like Sonar, FindBugs etc. that some have already mentiones don't help, but there are no magic tricks. Start from something you do understand, create a unit test for it and once it runs green start refactoring piece by piece. Remember to mock dependencies as you go along.
Sometimes it is easier to rewrite something from scratch. Is this 'horrible code' working as intended or full of bugs? It is documented?
In my current project, deleting my predessor's work nearly in its entirety, and rewriting it from scratch, was the most efficient approach. Granted, this was an extreme case of code obfuscation, utter lack of meaningful comments, and utter incompetence, so your mileage may vary.
Though some legacy code might be barely comprehensible, still it can be refactored and improved to legibility in a stepwise fashion. Have you seen Joshua Kerievsky's Refactoring To Patterns book? -- it's good on this.