Singleton design pattern vs Singleton beans in Spring container

Question

As we all know we have beans as singleton by default in Spring container and if we have a web application based on Spring framework then in that case do we really need to implement Singleton design pattern to hold global data rather than just creating a bean through spring   Please bear with me if I m not able to explain what I actually meant to ask

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singleton  in spring is using bean factory get instance  then cache it  which singleton design pattern is strictly  the instance can only be retrieved from static get method  and the object can never be publicly instantiated

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A singleton bean in Spring and the singleton pattern are quite different  Singleton pattern says that one and only one instance of a particular class will ever be created per classloader   The scope of a Spring singleton is described as  per container per bean   It is the scope of bean definition to a single object instance per Spring IoC container  The default scope in Spring is Singleton   Even though the default scope is singleton  you can change the scope of bean by specifying the scope attribute of  lt bean     gt  element    lt bean id      class      scope  prototype    gt

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Singleton scope in spring means single instance in a Spring context     Spring container merely returns the same instance again and again for subsequent calls to get the bean     And spring doesn t bother if the class of the bean is coded  as singleton or not   in fact if the class is coded as singleton whose constructor as private  Spring use BeanUtils instantiateClass  javadoc here  to set the constructor to accessible and invoke it   Alternatively  we can use a factory-method attribute in bean definition like this        lt bean id  exampleBean  class  example Singleton   factory-method  getInstance   gt

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Singleton beans in Spring and classes based on Singleton design pattern are quite different   Singleton pattern ensures that one and only one instance of a particular class will ever be created per classloader where as the scope of a Spring singleton bean is described as  per container per bean   Singleton scope in Spring means that this bean will be instantiated only once by Spring  Spring container merely returns the same instance again and again for subsequent calls to get the bean

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All the answers  so far at least  concentrate on explaining the difference between the design pattern and Spring singleton and do not address your actual question  Should a Singleton design pattern be used or a Spring singleton bean  what is better     Before I answer let me just state that you can do both  You can implement the bean as a Singleton design pattern and use Spring to inject it into the client classes as a Spring singleton bean    Now  the answer to the question is simple  Do not use the Singleton design pattern  Use Spring s singleton bean implemented as a class with public constructor  Why  Because the Singleton design pattern is considered an anti-pattern  Mostly because it complicates testing   And if you don t use Spring to inject it then all classes that use the singleton are now tightly bound to it   and you can t replace or extend it  One can google  Singleton anti-pattern  to get more info on this  e g  Singleton anti-pattern  Using Spring singleton is the way to go  with a the singleton bean implemented NOT as a Singleton design pattern  but rather with a public constructor  so that the Spring singleton bean can easily be tested and classes that use it are not tightly coupled to it  but rather  Spring injects the singleton  as an interface  into all the beans that need it  and the singleton bean can be replaced any time with another implementation without affecting the client classes that use it

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There is a very fundamental difference between the two  In case of Singleton design pattern  only one instance of a class will be created per classLoader while that is not the case with Spring singleton as in the later one shared bean instance for the given id per IoC container is created   For example  if I have a class with the name  SpringTest  and my XML file looks something like this  -    lt bean id  test1  class  com SpringTest  scope  singleton  gt          --some properties here  lt  bean gt       lt bean id  test2  class  com SpringTest  scope  singleton  gt          --some properties here     lt  bean gt    So now in the main class if you will check the reference of the above two it will return false as according to Spring documentation -     When a bean is a singleton  only one shared instance of the bean will   be managed  and all requests for beans with an id or ids matching that   bean definition will result in that one specific bean instance being   returned by the Spring container   So as in our case  the classes are the same but the id s that we have provided are different hence resulting in two different instances being created

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Singleton scope in Spring means that this bean will be instantiated only once by Spring  In contrast to the prototype scope  new instance each time   request scope  once per request   session scope  once per HTTP session    Singleton scope has technically nothing to do with the singleton design pattern  You don t have to implement your beans as singletons for them to be put in the singleton scope

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Spring singleton bean is described as  per container per bean   Singleton scope in Spring means that same object at same memory location will be returned to same bean id  If one creates multiple beans of different ids of the same class then container will return different objects to different ids  This is like a key value mapping where key is bean id and value is the bean object in one spring container  Where as Singleton pattern ensures that one and only one instance of a particular class will ever be created per classloader

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I find  quot per container per bean quot  difficult to apprehend  I would say  quot one bean per bean id in a container quot  Lets have an example to understand it  We have a bean class Sample  I have defined two beans from this class in bean definition  like   lt bean id  quot id1 quot  class  quot com example Sample quot  scope  quot singleton quot  gt           lt property name  quot name quot  value  quot James Bond 001 quot   gt       lt  bean gt       lt bean id  quot id7 quot  class  quot com example Sample quot  scope  quot singleton quot  gt           lt property name  quot name quot  value  quot James Bond 007 quot   gt       lt  bean gt   So when ever I try to get the bean with id  quot id1 quot  the spring container will create one bean  cache it and return same bean where ever refered with id1  If I try to get it with id7  another bean will be created from Sample class  same will be cached and returned each time you referred that with id7  This is unlikely with Singleton pattern  In Singlton pattern one object per class loader is created always  However in Spring  making the scope as Singleton does not restrict the container from creating many instances from that class  It just restricts new object creation for the same ID again  returning previously created object when an object is requested for the same id  Reference

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Let s take the simplest example  you have an application and you just use the default classloader  You have a class which  for whatever reason  you decide that it should not have more than one instance in the application   Think of a scenario where several people work on pieces of the application    If you are not using the Spring framework  the Singleton pattern ensures that there will not be more than one instance of a class in your application  That is because you cannot instantiate instances of the class by doing  new  because the constructor is private  The only way to get an instance of the class is to call some static method of the class  usually called  getInstance   which always returns the same instance   Saying that you are using the Spring framework in your application  just means that in addition to the regular ways of obtaining an instance of the class  new or static methods that return an instance of the class   you can also ask Spring to get you an instance of that class and Spring will ensure that whenever you ask it for an instance of that class it will always return the same instance  even if you didn t write the class using the Singleton pattern  In other words  even if the class has a public constructor  if you always ask Spring for an instance of that class  Spring will only call that constructor once during the life of your application     Normally if you are using Spring  you should only use Spring to create instances  and you can have a public constructor for the class  But if your constructor is not private you are not really preventing anyone from creating new instances of the class directly  by bypassing Spring   If you truly want a single instance of the class  even if you use Spring in your application and define the class in Spring to be a singleton  the only way to ensure that is also implement the class using the Singleton pattern  That ensures that there will be a single instance  whether people use Spring to get an instance or bypass Spring

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EX   per container per bean             lt bean id  myBean  class  com spring4hibernate4 TestBean  gt               lt constructor-arg name  i  value  1  gt  lt  constructor-arg gt               lt property name  name  value  1-name  gt  lt  property gt           lt  bean gt            lt bean id  testBean  class  com spring4hibernate4 TestBean  gt               lt constructor-arg name  i  value  10  gt  lt  constructor-arg gt               lt property name  name  value  10-name  gt  lt  property gt           lt  bean gt       lt  beans gt         public class Test             SuppressWarnings  resource           public static void main String   args                ApplicationContext ac   new ClassPathXmlApplicationContext  ws xml                TestBean teatBean    TestBean  ac getBean  testBean                TestBean myBean1    TestBean  ac getBean  myBean                System out println  a       teatBean test             teatBean getName                 teatBean setName  a TEST BEAN 1                System out println  uPdate       teatBean test            teatBean getName                 System out println  a1       myBean1 test           myBean1 getName                 myBean1 setName   a1 TEST BEAN 10                System out println  a1 update       teatBean test           myBean1 getName                      public class TestBean       public int test   0       public String getName             return name             public void setName String name            this name   name             private String name    default        public TestBean int i            test    i            JAVA SINGLETON   public class Singleton       private static Singleton singleton   new Singleton        private int i   0       private Singleton                public static Singleton returnSingleton              return singleton             public void increment             i               public int getInt             return i           public static void main String   args            System out println  Test             Singleton sin1   Singleton returnSingleton            sin1 increment            System out println sin1 getInt             Singleton sin2   Singleton returnSingleton            System out println  Test            sin1 increment            System out println sin1 getInt

[java] Singleton design pattern vs Singleton beans in Spring container

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