What is so bad about singletons

Question

The singleton pattern is a fully paid up member of the GoF s patterns book  but it lately seems rather orphaned by the developer world  I still use quite a lot of singletons  especially for factory classes  and while you have to be a bit careful about multithreading issues  like any class actually   I fail to see why they are so awful   Stack Overflow especially seems to assume that everyone agrees that Singletons are evil  Why   Please support your answers with  facts  references  or specific expertise

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Recent article on this subject by Chris Reath at Coding Without Comments.

Note: Coding Without Comments is no longer valid. However, The article being linked to has been cloned by another user.

http://geekswithblogs.net/AngelEyes/archive/2013/09/08/singleton-i-love-you-but-youre-bringing-me-down-re-uploaded.aspx

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When you write code using singletons  say  a logger or a database connection  and afterwards you discover you need more than one log or more than one database  you   re in trouble   Singletons make it very hard to move from them to regular objects   Also  it   s too easy to write a non-thread-safe singleton   Rather than using singletons  you should pass all the needed utility objects from function to function  That can be simplified if you wrap all them into a helper object  like this   void some class  some function parameters  service provider amp  srv        srv get lt error logger gt    log  Hi there         this- gt another function some other parameters  srv

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Paraphrased from Brian Button    They are generally used as a global instance  why is that so bad  Because you hide the dependencies of your application in your code  instead of exposing them through the interfaces  Making something global to avoid passing it around is a code smell  They violate the single responsibility principle  by virtue of the fact that they control their own creation and lifecycle  They inherently cause code to be tightly coupled  This makes faking them out under test rather difficult in many cases  They carry state around for the lifetime of the application  Another hit to testing since you can end up with a situation where tests need to be ordered which is a big no no for unit tests  Why  Because each unit test should be independent from the other

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Singleton is a pattern and can be used or abused just like any other tool   The bad part of a singleton is generally the user  or should I say the inappropriate use of a singleton for things it is not designed to do   The biggest offender is using a singleton as a fake global variable

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It blurs the separation of concerns   Supposed that you have a singleton  you can call this instance from anywhere inside your class  Your class is no longer as pure as it should be  Your class will now no longer operate on its members and the members that it receives explicitly  This will create confusion  because the users of the class don t know what is the sufficient information the class needs  The whole idea of encapsulation is to hide the how of a method from the users  but if a singleton is used inside the method  one will have to know the state of the singleton in order to use the method correctly  This is anti-OOP

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Some counterpoints from the author   You are stuck if you need to make the class not single in the future Not at all - I was in this situation with a single database connection singleton that I wanted to turn into a connection pool  Remember that every singleton is accessed through a standard method   MyClass instance  This is similar to the signature of a factory method  All I did was update the instance method to return the next connection from the pool - no other changes required  That would have been far harder if we had NOT been using a singleton   Singletons are just fancy globals Can t argue with that but so are all static fields and methods - anything that is accessed from the class rather than an instance is essentially global and I dont see so much pushback on the use of static fields   Not saying that Singletons are good  just pushing back at some of the  conventional wisdom  here

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See Wikipedia Singleton pattern     It is also considered an anti-pattern by some people  who feel that it is overly used  introducing unnecessary limitations in situations where a sole instance of a class is not actually required  1  2  3  4    References  only relevant references from the article      Alex Miller  Patterns I hate  1  Singleton  July 2007   Scott Densmore  Why singletons are evil  May 2004   Steve Yegge  Singletons considered stupid  September 2004   J B  Rainsberger  IBM  Use your singletons wisely  July 2001

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Singletons solve one  and only one  problem   Resource Contention   If you have some resource that    1  can only have a single instance  and    2  you need to manage that single instance    you need a singleton   There aren t many examples  A log file is the big one  You don t want to just abandon a single log file  You want to flush  sync and close it properly  This is an example of a single shared resource that has to be managed   It s rare that you need a singleton  The reason they re bad is that they feel like a global and they re a fully paid up member of the GoF Design Patterns book   When you think you need a global  you re probably making a terrible design mistake

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Singletons are NOT bad  It s only bad when you make something globally unique that isn t globally unique   However  there are  application scope services   think about a messaging system that makes components interact  - this CALLS for a singleton  a  MessageQueue  - class that has a method  SendMessage         You can then do the following from all over the place   MessageQueue Current SendMessage new MailArrivedMessage         And  of course  do   MessageQueue Current RegisterReceiver this    in classes that implement IMessageReceiver

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There is nothing inherently wrong with the pattern  assuming it is being used for some aspect of your model which is truly single   I believe the backlash is due to its overuse which  in turn  is due to the fact that it s the easiest pattern to understand and implement

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Vince Huston has these criteria  which seem reasonable to me      Singleton should be considered only if all three of the following criteria are satisfied          Ownership of the single instance cannot be reasonably assigned   Lazy initialization is desirable   Global access is not otherwise provided for          If ownership of the single instance  when and how initialization occurs  and global access are not issues  Singleton is not sufficiently interesting

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A pattern emerges when several people  or teams  arrives at similar or identical solutions  A lot of people still use singletons in their original form or using factory templates  good discussion in Alexandrescu s Modern C   Design   Concurrency and difficulty in managing the lifetime of the object are the main obstacles  with the former easily managed as you suggest   Like all choices  Singleton has its fair share of ups and downs  I think they can be used in moderation  especially for objects that survive the application life span  The fact that they resemble  and probably are  globals have presumably set off the purists

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The Singleton  ndash  the anti-pattern  by Mark Radford  Overload Journal  57  ndash  Oct 2003  is a good read about why Singleton is regarded an anti-pattern  The article also discusses two alternatives design approaches for replacing Singleton

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I d like to address the 4 points in the accepted answer  hopefully someone can explain why I m wrong    Why is hiding dependencies in your code bad  There are already dozens of hidden dependencies  C runtime calls  OS API calls  global function calls   and singleton dependencies are easy to find  search for instance        Making something global to avoid passing it around is a code smell   Why isn t passing something around to avoid making it a singleton a code smell    If you re passing an object through 10 functions in a call stack just to avoid a singleton  is that so great  Single Responsibility Principle  I think this is a bit vague and depends on your definition of responsibility  A relevant question would be  why does adding this specific  responsibility  to a class matter  Why does passing an object to a class make it more tightly coupled than using that object as a singleton from within the class  Why does it change how long the state lasts  Singletons can be created or destroyed manually  so the control is still there  and you can make the lifetime the same as a non-singleton object s lifetime would be    Regarding unit tests    not all classes need to be unit tested not all classes that need to be unit tested need to change the implementation of the singleton if they do need be unit tested and do need to change the implementation  it s easy to change a class from using a singleton to having the singleton passed to it via dependency injection

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Monopoly is the devil and singletons with non-readonly mutable state are the  real  problem     After reading Singletons are Pathological Liars as suggested in jason s answer I came across this little tidbit that provides the best presented example of how singletons are often misused      Global is bad because          a  It causes namespace conflict   b  It exposes the state in a unwarranted fashion         When it comes to Singletons         a  The explicit OO way of calling them  prevents the conflicts  so point a  is not an issue   b  Singletons without state are  like factories  are not a problem  Singletons with state can again fall in two categories  those which are immutable or write once and read many  config property files   These are not bad  Mutable Singletons  which are kind of reference holders are the ones which you are speaking of       In the last statement he s referring to the blog s concept of  singletons are liars    How does this apply to Monopoly   To start a game of monopoly  first    we establish the rules first so everybody is on the same page everybody is given an equal start at the beginning of the game only one set of rules is presented to avoid confusion the rules aren t allowed to change throughout the game   Now  for anybody who hasn t really played monopoly  these standards are ideal at best  A defeat in monopoly is hard to swallow because  monopoly is about money  if you lose you have to painstakingly watch the rest of the players finish the game  and losses are usually swift and crushing  So  the rules usually get twisted at some point to serve the self-interest of some of the players at the expense of the others   So you re playing monopoly with friends Bob  Joe  and Ed  You re swiftly building your empire and consuming market share at an exponential rate  Your opponents are weakening and you start to smell blood  figuratively   Your buddy Bob put all of his money into gridlocking as many low-value properties as possible but his isn t receiving a high return on investment the way he expected  Bob  as a stroke of bad luck  lands on your Boardwalk and is excised from the game   Now the game goes from friendly dice-rolling to serious business  Bob has been made the example of failure and Joe and Ed don t want to end up like  that guy   So  being the leading player you  all of a sudden  become the enemy  Joe and Ed start practicing under-the-table trades  behind-the-back money injections  undervalued house-swapping and generally anything to weaken you as a player until one of them rises to the top    Then  instead of one of them winning  the process starts all over  All of a sudden  a finite set of rules becomes a moving target and the game degenerates into the type of social interactions that would make up the foundation of every high-rated reality TV show since Survivor  Why  because the rules are changing and there s no consensus on how why what they re supposed to represent  and more importantly  there s no one person making the decisions  Every player in the game  at that point  is making his her own rules and chaos ensues until two of the players are too tired to keep up the charade and slowly give up   So  if a rulebook for a game accurately represented a singleton  the monopoly rulebook would be an example of abuse    How does this apply to programming   Aside from all of the obvious thread-safety and synchronization issues that mutable singletons present    If you have one set of data  that is capable of being read manipulated by multiple different sources concurrently and exists during the lifetime of the application execution  it s probably a good time to step back and ask  am I using the right type of data structure here    Personally  I have seen a programmer abuse a singleton by using it as some sort of twisted cross-thread database store within an application  Having worked on the code directly  I can attest that it was a slow  because of all the thread locks needed to make it thread-safe  and a nightmare to work on  because of the unpredictable intermittent nature of synchronization bugs   and nearly impossible to test under  production  conditions  Sure  a system could have been developed using polling signaling to overcome some of the performance issues but that wouldn t solve the issues with testing and  why bother when a  real  database can already accomplish the same functionality in a much more robust scalable manner   A Singleton is only an option if you need what a singleton provides  A write-one read-only instance of an object  That same rule should cascade to the object s properties members as well

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Some counterpoints from the author   You are stuck if you need to make the class not single in the future Not at all - I was in this situation with a single database connection singleton that I wanted to turn into a connection pool  Remember that every singleton is accessed through a standard method   MyClass instance  This is similar to the signature of a factory method  All I did was update the instance method to return the next connection from the pool - no other changes required  That would have been far harder if we had NOT been using a singleton   Singletons are just fancy globals Can t argue with that but so are all static fields and methods - anything that is accessed from the class rather than an instance is essentially global and I dont see so much pushback on the use of static fields   Not saying that Singletons are good  just pushing back at some of the  conventional wisdom  here

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Recent article on this subject by Chris Reath at Coding Without Comments.

Note: Coding Without Comments is no longer valid. However, The article being linked to has been cloned by another user.

http://geekswithblogs.net/AngelEyes/archive/2013/09/08/singleton-i-love-you-but-youre-bringing-me-down-re-uploaded.aspx

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It s not that singletons themselves are bad but the GoF design pattern is  The only really argument that is valid is that the GoF design pattern doesn t lend itself in regards to testing  especially if tests are run in parallel   Using a single instance of an class is a valid construct as long as you apply the following means in code    Make sure the class that will be used as a singleton implements an interface  This allows stubs or mocks to be implemented using the same interface Make sure that the Singleton is thread-safe   That s a given  The singleton should be simple in nature and not overly complicated  During the runtime of you application  where singletons need to be passed to a given object  use a class factory that builds that object and have the class factory pass the singleton instance to the class that needs it  During testing and to ensure deterministic behavior  create the singleton class as separate instance as either the actual class itself or a stub mock that implements its behavior and pass it as is to the class that requires it  Don t use the class factor that creates that object under test that needs the singleton during test as it will pass the single global instance of it  which defeats the purpose    We ve used Singletons in our solutions with a great deal of success that are testable ensuring deterministic behavior in parallel test run streams

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One rather bad thing about singletons is that you can t extend them very easily  You basically have to build in some kind of decorator pattern or some such thing if you want to change their behavior  Also  if one day you want to have multiple ways of doing that one thing  it can be rather painful to change  depending on how you lay out your code   One thing to note  if you DO use singletons  try to pass them in to whoever needs them rather than have them access it directly    Otherwise if you ever choose to have multiple ways of doing the thing that singleton does  it will be rather difficult to change as each class embeds a dependency if it accesses the singleton directly   So basically   public MyConstructor Singleton singleton        this singleton   singleton      rather than   public MyConstructor         this singleton   Singleton getInstance        I believe this sort of pattern is called dependency injection and is generally considered a good thing   Like any pattern though    Think about it and consider if its use in the given situation is inappropriate or not    Rules are made to be broken usually  and patterns should not be applied willy nilly without thought

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The singleton pattern is not a problem in itself  The problem is that the pattern is often used by people developing software with object-oriented tools without having a solid grasp of OO concepts  When singletons are introduced in this context they tend to grow into unmanageable classes that contain helper methods for every little use   Singletons are also a problem from a testing perspective  They tend to make isolated unit-tests difficult to write  Inversion of control  IoC  and dependency injection are patterns meant to overcome this problem in an object-oriented manner that lends itself to unit testing   In a garbage collected environment singletons can quickly become an issue with regard to memory management   There is also the multi-threaded scenario where singletons can become a bottleneck as well as a synchronization issue

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Misko Hevery  from Google  has some interesting articles on exactly this topic    Singletons are Pathological Liars has a unit testing example that illustrates how singletons can make it difficult to figure out dependency chains and start or test an application  It is a fairly extreme example of abuse  but the point that he makes is still valid   Singletons are nothing more than global state  Global state makes it so your objects can secretly get hold of things which are not declared in their APIs  and  as a result  Singletons make your APIs into pathological liars   Where have all the Singletons Gone makes the point that dependency injection has made it easy to get instances to constructors that require them  which alleviates the underlying need behind the bad  global Singletons decried in the first article

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This is what I think is missing from the answers so far   If you need one instance of this object per process address space  and you are as confident as you can be that this requirement will not change   you should make it a singleton   Otherwise  it s not a singleton   This is a very odd requirement  hardly ever of interest to the user  Processes and address space isolation are an implementation detail  They only impact on the user when they want to stop your application using kill or Task Manager   Apart from building a caching system  there aren t that many reasons why you d be so certain that there should only be on instance of something per process  How about a logging system  Might be better for that to be per-thread or more fine-grained so you can trace the origin of messages more automatically  How about the application s main window  It depends  maybe you ll want all the user s documents to be managed by the same process for some reason  in which case there would be multiple  main windows  in that process

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Here is one more thing about singletons which nobody said yet   In most cases  singletonity  is a detail of implementation for some class rather than characteristic of its interface  Inversion of Control Container may hide this characteristic from class users  you just need to mark your class as a singleton  with  Singleton annotation in Java for example  and that s it  IoCC will do the rest  You don t need to provide global access to your singleton instance because the access is already managed by IoCC  Thus there is nothing wrong with IoC Singletons   GoF Singletons in opposite to IoC Singletons are supposed to expose  singletonity  in the interface through getInstance   method  and so that they suffer from everything said above

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Firstly a class and its collaborators should firstly perform their intended purpose rather than focusing on dependents  Lifecycle management  when instances are created and when they go out of scope  should not be part of the classes responsibility  The accepted best practice for this is to craft or configure a new component to manage dependencies using dependency injection    Often software gets more complicated it makes sense to have multiple independent instances of the Singleton class with different state  Committing code to simply grab the singleton is wrong in such cases  Using Singleton getInstance   might be ok for small simple systems but it doesn t work scale when one might need a different instance of the same class   No class should be thought of as a singleton but rather that should be an application of it s usage or how it is used to configure dependents  For a quick and nasty this does not matter - just luke hard coding say file paths does not matter but for bigger applications such dependencies need to be factored out and managed in more appropriate way using DI   The problems that singleton cause in testing is a symptom of their hard coded single usage case environment  The test suite and the many tests are each individual and separate something that is not compatible with hard coding a singleton

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It is easily  ab used as a global variable  Classes that depend on singletons are relatively harder to unit test in isolation

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Some coding snobs look down on them as just a glorified global  In the same way that many people hate the goto statement there are others that hate the idea of ever using a global  I have seen several developers go to extraordinary lengths to avoid a global because they considered using one as an admission of failure  Strange but true   In practice the Singleton pattern is just a programming technique that is a useful part of your toolkit of concepts  From time to time you might find it is the ideal solution and so use it  But using it just so you can boast about using a design pattern is just as stupid as refusing to ever use it because it is just a global

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Singletons are bad from a purist point of view   From a pratical point of view  a singleton is a trade-off developing time vs complexity   If you know your application won t change that much they are pretty OK to go with  Just know that you may need to refactor things up if your requirements change in an unexpected way  which is pretty OK in most cases    Singletons sometimes also complicate unit testing

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The problems with singletons is the issue of increased scope and therefore coupling  There is no denying that there are some of situations where you do need access to a single instance  and it can be accomplished other ways   I now prefer to design around an inversion of control  IoC  container and allow the the lifetimes to be controlled by the container  This gives you the benefit of the classes that depend on the instance to be unaware of the fact that there is a single instance  The lifetime of the singleton can be changed in the future  Once such example I encountered recently was an easy adjustment from single threaded to multi-threaded   FWIW  if it a PIA when you try to unit test it then it s going to PIA when you try to debug  bug fix or enhance it

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I think the confusion is caused by the fact that people don t know the real application of the Singleton pattern  I can t stress this enough  Singleton is not a pattern to wrap globals  Singleton pattern should only be used to guarantee that one and only one instance of a given class exists during run time    People think Singleton is evil because they are using it for globals  It is because of this confusion that Singleton is looked down upon  Please  don t confuse Singletons and globals  If used for the purpose it was intended for  you will gain extreme benefits from the Singleton pattern

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A singleton gets implemented using a static method  Static methods are avoided by people who do unit testing because they cannot be mocked or stubbed  Most people on this site are big proponents of unit testing  The generally most accepted convention to avoid them is using the inversion of control pattern

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Singletons are also bad when it comes to clustering  Because then  you do not have  exactly one singleton  in your application anymore   Consider the following situation  As a developer  you have to create a web application which accesses a database  To ensure that concurrent database calls do not conflict each other  you create a thread-save SingletonDao   public class SingletonDao          songleton s static variable and getInstance   method etc  omitted     public void writeXYZ               synchronized                      some database writing operations                        So you are sure that only one singleton in your application exists and all database go through this one and only SingletonDao  Your production environment now looks like this    Everything is fine so far   Now  consider you want to set up multiple instances of your web application in a cluster  Now  you suddenly have something like this     That sounds weird  but now you have many singletons in your application  And that is exactly what a singleton is not supposed to be  Having many objects of it  This is especially bad if you  as shown in this example  want to make synchronized calls to a database   Of course this is an example of a bad usage of a singleton  But the message of this example is  You can not rely that there is exactly one instance of a singleton in your application - especially when it comes to clustering

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Because they are basically object oriented global variables  you can usually design your classes in such a way so that you don t need them

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Singletons aren t evil  if you use it properly  amp  minimally  There are lot of other good design patterns which replaces the needs of singleton at some point   amp  also gives best results   But some programmers are unaware of those good patterns  amp  uses the singleton for all the cases which makes the singleton evil for them

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Off the top of my head    They enforce tight-coupling  If your singleton resides on a different assembly than its user  the using assembly cannot ever function without the assembly containing the singleton  They allow for circular dependencies  e g   Assembly A can have a singleton with a dependency on Assembly B  and Assembly B can use Assembly A s singleton  All without breaking the compiler

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I m not going to comment on the good evil argument  but I haven t used them since Spring came along  Using dependency injection has pretty much removed my requirements for singleton  servicelocators and factories  I find this a much more productive and clean environment  at least for the type of work I do  Java-based web applications

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My answer on how Singletons are bad is always   they are hard to do right   Many of the foundational components of languages are singletons  classes  functions  namespaces and even operators   as are components in other aspects of computing  localhost  default route  virtual filesystem  etc    and it is not by accident  While they cause trouble and frustration from time to time  they also can make a lot of things work a LOT better   The two biggest screw ups I see are  treating it like a global  amp  failing to define the Singleton closure   Everyone talks about Singleton s as globals  because they basically are  However  much  sadly  not all  of the badness in a global comes not intrinsically from being global  but how you use it  Same goes for Singletons  Actually more so as  single instance  really doesn t need to mean  globally accessible   It is more a natural byproduct  and given all the bad that we know comes from it  we shouldn t be in such a hurry to exploit global accessibility  Once programmers see a Singleton they seem to always access it directly through its instance method  Instead  you should navigate to it just like you would any other object  Most code shouldn t even be aware it is dealing with a Singleton  loose coupling  right    If only a small bit of code accesses the object like it is a global  a lot of harm is undone  I recommend enforcing it by restricting access to the instance function   The Singleton context is also really important  The defining characteristic of a Singleton is that there is  only one   but the truth is it is  only one  within some kind of context namespace  They are usually one of  one per thread  process  IP address or cluster  but can also be one per processor  machine  language namespace class loader whatever  subnet  Internet  etc   The other  less common  mistake is to ignore the Singleton lifestyle  Just because there is only one doesn t mean a Singleton is some omnipotent  always was and always will be   nor is it generally desirable  objects without a begin and end violate all kinds of useful assumptions in code  and should be employed only in the most desperate of circumstances   If you avoid those mistakes  Singletons can still be a PITA  bit it is ready to see a lot of the worst problems are significantly mitigated  Imagine a Java Singleton  that is explicitly defined as once per classloader  which means it needs a thread safety policy   with defined creation and destruction methods and a life cycle that dictates when and how they get invoked  and whose  instance  method has package protection so it is generally accessed through other  non-global objects  Still a potential source of trouble  but certainly much less trouble   Sadly  rather than teaching good examples of how to do Singletons  We teach bad examples  let programmers run off using them for a while  and then tell them they are a bad design pattern

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Too many people put objects which are not thread safe in a singleton pattern  I ve seen examples of a DataContext  LINQ nbsp to nbsp SQL  done in a singleton pattern  despite the fact that the DataContext is not thread safe and is purely a unit-of-work object

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Singleton is not about single instance   Unlike other answers I don t want to talk about what is wrong with Singletons but to show you how powerful and awesome they are when used right    Problem  Singleton can be a challenge in multi-threading environment Solution  Use a single threaded bootstrap process to initialize all the   dependencies of your singleton  Problem  It is hard to mock singletons  Solution  Use method Factory pattern for mocking        You can map MyModel to TestMyModel class that inherits it  everywhere when MyModel will be injected you will get TestMyModel instread   - Problem  Singletons can cause memory leaks as they never disposed     Solution  Well  dispose them  Implement a callback in your app to properly dispose a singletons  you should remove any data linked to them and finally  remove them from the Factory    As I stated at the title singleton are not about single instance    Singletons improves readability  You can look at your class and see what singleton it injected to figure out what is it s dependencies  Singletons improves maintenance  Once you removed a dependency from a class you just deleted some singleton injection  you don t need to go and edit a big link of other classes that just moved your dependency around This is smelly code for me  Jim Burger  Singletons improves memory and performance  When some thing happens in your application  and it takes a long chain of callbacks to deliver  you are wasting memory and performance  by using Singleton you are cutting the middle man  and improve your performance and memory usage by avoiding unnecessary local variables allocations

[design-patterns] What is so bad about singletons?

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