Singleton with Arguments in Java

Question

I was reading the Singleton article on Wikipedia and I came across this example   public class Singleton          Private constructor prevents instantiation from other classes     private Singleton                      SingletonHolder is loaded on the first execution of Singleton getInstance           or the first access to SingletonHolder INSTANCE  not before              private static class SingletonHolder            private static final Singleton INSTANCE   new Singleton               public static Singleton getInstance             return SingletonHolder INSTANCE            While I really like the way this Singleton behaves  I can t see how to adapt it to incorporate arguments to the constructor   What is the preferred way to do this in Java   Would I have to do something like this   public class Singleton       private static Singleton singleton   null        private final int x       private Singleton int x            this x   x             public synchronized static Singleton getInstance int x            if singleton    null  singleton   new Singleton x           return singleton            Thanks     Edit   I think I have started a storm of controversy with my desire to use Singleton   Let me explain my motivation and hopefully someone can suggest a better idea   I am using a grid computing framework to execute tasks in parallel   In general  I have something like this      AbstractTask implements Serializable public class Task extends AbstractTask       private final ReferenceToReallyBigObject object       public Task ReferenceToReallyBigObject object                this object   object             public void run                    Do some stuff with the object  which is immutable             What happens is that even though I merely pass a reference to my data to all of the tasks  when the tasks are serialized  the data gets copied over and over   What I want to do is share the object among all of the tasks   Naturally  I might modify the class like so      AbstractTask implements Serializable public class Task extends AbstractTask       private static ReferenceToReallyBigObject object   null       private final String filePath       public Task String filePath                this filePath   filePath             public void run                 synchronized this                        if object    null                                ObjectReader reader   new ObjectReader filePath                   object   reader read                                        Do some stuff with the object  which is immutable             As you can see  even here I have the issue that passing a different file path means nothing after the first one is passed   This is why I like the idea for a store which was posted in the answers   Anyhow  rather than including the logic for loading the file in the run method  I wanted to abstract this logic into a Singleton class   I will not provide yet another example  but I hope you get the idea   Please let me hear your ideas for a more elegant way to accomplish what I am trying to do   Thank you again

User · Answer

Singletons are generally considered to be anti-patterns and shouldn't be used. They do not make code easy to test.

A singleton with an argument makes no sense anyway - what would happen if you wrote:

Singleton s = SingletonHolder.getInstance(1);
Singleton t = SingletonHolder.getInstance(2); //should probably throw IllegalStateException

Your singleton is also not thread-safe as multiple threads can make simultaneous calls to getInstance resulting in more than one instance being created (possibly with different values of x).

User · Answer

If you want to create a Singleton class serving as a Context  a good way is to have a configuration file and read the parameters from the file inside instance      If the parameters feeding the Singleton class are got dynamically during the running of your program  simply use a static HashMap storing different instances in your Singleton class to ensure that for each parameter s   only one instance is created

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I m scared to post this as an answer  but I don t understand why nobody think about this  maybe this answer was also given already I just didn t understand it   public class example        private volatile static example instance       private String string      private int iInt   -1    any number you know you don t want to use here    private example            In case someone uses the private method to create a new Instance     if  instance    null         throw new RuntimeException  Use getInstance   method to get the single instance of this class                  public synchronized static example getIsntance        if instance    null         instance   new example              return instance       public void methodDoingWork        if checkInit             DoSome              private boolean checkInit        boolean filled    this string    null   amp  amp   this iInt    -1       return filled         public void setString String string        if this string    null         this string   string       else        throw new RuntimeException  You try to override an already setValue                  public void setiInt int iInt        if this iInt    -1         this iInt   iInt       else        throw new RuntimeException  You try to override an already setValue                  Since the getInstance   returns the same Instance everytime  I think this could work  If this is wrong to much I will delete it  I m just interested in this topic

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Surprised that no one mentioned how a logger is created retrieved  For example  below shows how Log4J logger is retrieved      Retrieve a logger named according to the value of the name parameter  If the named logger already exists  then the existing instance will be returned  Otherwise  a new instance is created  public static Logger getLogger String name    There are some levels of indirections  but the key part is below method which pretty much tells everything about how it works  It uses a hash table to store the exiting loggers and the key is derived from name  If the logger doesn t exist for a give name  it uses a factory to create the logger and then adds it to the hash table   69   Hashtable ht      258  public 259  Logger getLogger String name  LoggerFactory factory    260      System out println  getInstance   name    called     261    CategoryKey key   new CategoryKey name   262       Synchronize to prevent write conflicts  Read conflicts  in 263       getChainedLevel method  are possible only if variable 264       assignments are non-atomic  265    Logger logger  266 267    synchronized ht    268      Object o   ht get key   269      if o    null    270        logger   factory makeNewLoggerInstance name   271        logger setHierarchy this   272        ht put key  logger   273        updateParents logger   274        return logger  275        else if o instanceof Logger    276        return  Logger  o  277

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Modification of Singleton pattern that uses Bill Pugh s initialization on demand holder idiom  This is thread safe without the overhead of specialized language constructs  i e  volatile or synchronized    public final class RInterfaceHL                   Private constructor prevents instantiation from other classes              private RInterfaceHL                       R REPL  read-evaluate-parse loop  handler              private static RMainLoopCallbacks rloopHandler   null                  SingletonHolder is loaded  and the static initializer executed          on the first execution of Singleton getInstance   or the first         access to SingletonHolder INSTANCE  not before              private static final class SingletonHolder                           Singleton instance  with static initializer                      private static final RInterfaceHL INSTANCE   initRInterfaceHL                            Initialize RInterfaceHL singleton instance using rLoopHandler from            outer class                          return RInterfaceHL instance                     private static RInterfaceHL initRInterfaceHL                 try                   return new RInterfaceHL rloopHandler                 catch  REngineException e                       a static initializer cannot throw exceptions                    but it can throw an ExceptionInInitializerError                 throw new ExceptionInInitializerError e                                                   Prevent instantiation                      private SingletonHolder                                       Get singleton RInterfaceHL                          return RInterfaceHL singleton                      public static RInterfaceHL getInstance                 return SingletonHolder INSTANCE                                   Return the singleton instance of RInterfaceHL  Only the first call to        this will establish the rloopHandler                  param rloopHandler                   R REPL handler supplied by client          return RInterfaceHL singleton instance         throws REngineException                    if REngine cannot be created             public static RInterfaceHL getInstance RMainLoopCallbacks rloopHandler              throws REngineException           RInterfaceHL rloopHandler   rloopHandler           RInterfaceHL instance   null           try               instance   SingletonHolder getInstance              catch  ExceptionInInitializerError e                    rethrow exception that occurred in the initializer                so our caller can deal with it             Throwable exceptionInInit   e getCause                throw new REngineException null  exceptionInInit getMessage                        return instance                        org rosuda REngine REngine high level R interface              private REngine rosudaEngine   null                  Construct new RInterfaceHL  Only ever gets called once by          link SingletonHolder initRInterfaceHL                   param rloopHandler                   R REPL handler supplied by client          throws REngineException                    if R cannot be loaded              private RInterfaceHL RMainLoopCallbacks rloopHandler              throws REngineException               tell Rengine code not to die if it can t            load the JRI native DLLs  This allows            us to catch the UnsatisfiedLinkError            ourselves         System setProperty  jri ignore ule    yes             rosudaEngine   new JRIEngine new String      --no-save     rloopHandler

User · Answer

Singleton is  of course  an  anti-pattern   assuming a definition of a static with variable state    If you want a fixed set of immutable value objects  then enums are the way to go  For a large  possibly open-ended set of values  you can use a Repository of some form - usually based on a Map implementation  Of course  when you are dealing with statics be careful with threading  either synchronise sufficiently widely or use a ConcurrentMap either checking that another thread hasn t beaten you or use some form of futures

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In your example you are not using a singleton  Notice that if you do the following  assuming that the Singleton getInstance was actually static    Singleton obj1   Singleton getInstance 3   Singleton obj2   Singleton getInstance 4     Then the obj2 x s values is 3  not 4   If you need to do this  make it a plain class  If the number of values is small and fixed  you can consider using an enum  If you are having problem with excessive object generation  which is usually not the case   then you can consider caching values  and check sources or get help with that  as it is obvious how to build caches without the danger of memory leaks    You also might want to read this article as singletons can be very easily overused

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This is not quite a singleton  but may be something that could fix your problem   public class KamilManager      private static KamilManager sharedInstance              This method cannot be called before calling KamilManager constructor or else      it will bomb out        return         public static KamilManager getInstanceAfterInitialized         if sharedInstance    null          throw new RuntimeException  You must instantiate KamilManager once  before calling this method         return sharedInstance       public KamilManager Context context  KamilConfig KamilConfig          Set whatever you need to set here then call    s  haredInstance   this

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If we take the problem as  how to make singleton with state   then it is not necessary to pass the state as constructor parameter   I agree with the posts that initialize the states or using set method after getting the singleton instance   Another question is  is it good to have singleton with state

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I think this is a common problem  Separating the  initialization  of the singleton from the  get  of the singleton might work  this example uses a variation of double checked locking    public class MySingleton        private static volatile MySingleton INSTANCE        SuppressWarnings  UnusedAssignment       public static void initialize              final SomeDependency someDependency             MySingleton result   INSTANCE           if  result    null                throw new IllegalStateException  The singleton has already                          been initialized                        synchronized  MySingleton class                result   INSTANCE               if  result    null                    INSTANCE   result   new MySingleton someDependency                                       public static MySingleton get             MySingleton  result   INSTANCE           if  result    null                throw new IllegalStateException  The singleton has not been                          initialized  You must call initialize      before                          calling get                        return result

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Another reason Singletons are an anti-pattern is that if written according to recommendations  with private constructor  they are very hard to subclass and configure to use in certain unit tests   Would be required in maintaining legacy code  for example

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Use getters and setters to set the variable and make the default constructor private  Then use   Singleton getInstance   setX value

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A singleton with parameters is not a singleton  statement is not completely correct  We need to analyze this from the application perspective rather than from the code perspective    We build singleton class to create a single instance of an object in one application run  By having a constructor with parameter  you can build flexibility into your code to change some attributes of your singleton object every time you run you application  This is not a violation of Singleton pattern  It looks like a violation if you see this from code perspective    Design Patterns are there to help us write flexible and extendable code  not to hinder us writing good code

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Couldn t we do something like this   public class Singleton        private int x          Private constructor prevents instantiation from other classes     private Singleton                      SingletonHolder is loaded on the first execution of Singleton getInstance           or the first access to SingletonHolder INSTANCE  not before              private static class SingletonHolder            private static final Singleton INSTANCE   new Singleton               public static Singleton getInstance int x            Singleton instance   SingletonHolder INSTANCE          instance x   x          return instance

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I think you need something like a factory to have objects with various parameters instantiated and reused  It could be implemented by using a synchronized HashMap or ConcurrentHashMap map a parameter  an Integer for an example  to your  singleton  parameterizable class    Although you might get to the point where you should use regular  non-singleton classes instead  for example needing 10 000 differently parametrized singleton    Here is an example for such store   public final class UsefulObjFactory        private static Map lt Integer  UsefulObj gt  store           new HashMap lt Integer  UsefulObj gt          public static final class UsefulObj           private UsefulObj int parameter                   init                   public void someUsefulMethod                    some useful operation                      public static UsefulObj get int parameter            synchronized  store                UsefulObj result   store get parameter               if  result    null                    result   new UsefulObj parameter                   store put parameter  result                             return result                      To push it even further  the Java enums can be also considered  or used as  parametrized singletons  although allowing only a fixed number static variants   However  if you need a distributed1 solution  consider some lateral caching solution  For example  EHCache  Terracotta  etc   1 in the sense of spanning multiple VMs on probably multiple computers

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The reason you can t make sense of how to accomplish what you re trying to do is probably that what you re trying to do doesn t really make sense   You want to call getInstance x  with different arguments  but always return the same object   What behavior is it you want when you call getInstance 2  and then getInstance 5    If you want the same object but for its internal value to be different  which is the only way it s still a singleton  then you don t need to care about the constructor at all  you just set the value in getInstance   on the object s way out   Of course  you understand that all your other references to the singleton now have a different internal value   If you want getInstance 2  and getInstance 5  to return different objects  on the other hand  you re not using the Singleton pattern  you re using the Factory pattern

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You can also use the Builder pattern if you want to show that some parameters are mandatory       public enum EnumSingleton        INSTANCE       private String name     Mandatory     private Double age   null     Not Mandatory      private void build SingletonBuilder builder            this name   builder name          this age   builder age                Static getter     public static EnumSingleton getSingleton             return INSTANCE             public void print             System out println  Name   name      age    age               public static class SingletonBuilder            private final String name     Mandatory         private Double age   null     Not Mandatory          private SingletonBuilder              name   null                     SingletonBuilder String name                this name   name                     public SingletonBuilder age double age                this age   age              return this                     public void build                EnumSingleton INSTANCE build this                          Then you could create instantiate parametrized it as follow   public static void main String   args        new EnumSingleton SingletonBuilder  nico   age 41  build        EnumSingleton getSingleton   print

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I ll make my point very clear  a singleton with parameters is not a singleton   A singleton  by definition  is an object you want to be instantiated no more than once  If you are trying to feed parameters to the constructor  what is the point of the singleton   You have two options  If you want your singleton to be initialized with some data  you may load it with data after instantiation  like so   SingletonObj singleton   SingletonObj getInstance    singleton init paramA  paramB      init the object with data   If the operation your singleton is performing is recurring  and with different parameters every time  you might as well pass the parameters to the main method being executed   SingletonObj singleton   SingletonObj getInstance    singleton doSomething paramA  paramB      pass parameters on execution   In any case  instantiation will always be parameter-less  Otherwise your singleton is not a singleton

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You can add a configurable initialization method in order to separate instantiation from getting   public class Singleton       private static Singleton singleton   null      private final int x       private Singleton int x            this x   x             public static Singleton getInstance             if singleton    null                throw new AssertionError  You have to call init first                       return singleton             public synchronized static Singleton init int x            if  singleton    null                           in my opinion this is optional  but for the purists it ensures                that you only ever get the same instance when you call getInstance             throw new AssertionError  You already initialized me                       singleton   new Singleton x           return singleton             Then you can call Singleton init 123  once to configure it  for example in your app startup

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In spite of what some may assert  here is a singleton with parameters in constructor  public class Singleton        private static String aParameterStored       private static final Singleton instance   new Singleton  Param to set         private Singleton                do nothing            private Singleton String param            aParameterStored   param             public static Singleton getInstance             return instance                           stuff you would like the singleton do             The singleton pattern say     ensure that only one instance of the singleton class ever exists provide global access to that instance    which are respected with this example   Why not directly set the property   It s textbook case to show how we can get a singleton having constructor with parameter but it could be useful in some situations  For example in inheritance cases to force the singleton to set some superclass properties

[java] Singleton with Arguments in Java

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