What exactly is Spring Framework for

Question

I hear a lot about Spring  people are saying all over the web that Spring is a good framework for web development  What exactly is Spring Framework for

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What you d probably want in a web application with Spring -    Spring MVC  which with 2 5  allows you to use POJOs as Controller classes  meaning you don t have to extend from any particular framework  as in Struts or Spring pre-2 5   Controller classes are also dead simple to test thanks in part to dependency injection Spring integration with Hibernate  which does a good job of simplifying work with that ORM solution  for most cases  Using Spring for a web app enables you to use your Domain Objects at all levels of the application - the same classes that are mapped using Hibernate are the classes you use as  form beans   By nature  this will lead to a more robust  domain model  in part because it s going to cut down on the number of classes  Spring form tags make it easier to create forms without much hassle     In addition  Spring is HUGE - so there are a lot of other things you might be interested in using in a web app such as Spring AOP or Spring Security  But the four things listed above describe the common components of Spring that are used in a web app

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The advantage is Dependency Injection  DI   It means outsourcing the task of object creation Let me explain with an example    public interface Lunch      public void eat       public class Buffet implements Lunch      public void eat                 Eat as much as you can          public class Plated implements Lunch      public void eat                 Eat a limited portion          Now in my code I have a class LunchDecide as follows    public class LunchDecide       private Lunch todaysLunch      public LunchDecide            this todaysLunch   new Buffet       choose Buffet - gt  eat as much as you want           this todaysLunch   new Plated       choose Plated - gt  eat a limited portion            In the above class  depending on our mood  we pick Buffet   or Plated    However this system is tightly coupled  Every time we need a different type of Object  we need to change the code  In this case  commenting out a line   Imagine there are 50 different classes used by 50 different people  It would be a hell of a mess  In this case  we need to Decouple the system  Let s rewrite the LunchDecide class    public class LunchDecide       private Lunch todaysLunch      public LunchDecide Lunch todaysLunch           this todaysLunch   todaysLunch                   Notice that instead of creating an object using new keyword we passed the reference to an object of Lunch Type as a parameter to our constructor  Here  object creation is outsourced  This code can be wired either using Xml config file  legacy  or Java Annotations  modern   Either way  the decision on which Type of object would be created would be done there during runtime  An object would be injected by Xml into our code - Our Code is dependent on Xml for that job  Hence  Dependency Injection  DI    DI not only helps in making our system loosely coupled  it simplifies writing of Unit tests since it allows dependencies to be mocked  Last but not the least  DI streamlines Aspect Oriented Programming  AOP  which leads to further decoupling and increase of modularity   Also note that above DI is Constructor Injection  DI can be done by Setter Injection as well - same plain old setter method from encapsulation

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In the past I thought about Spring framework from purely technical standpoint    Given some experience of team work and developing enterprise Webapps - I would say that Spring is for faster development of applications  web applications  by decoupling its individual elements  beans   Faster development makes it so popular  Spring allows shifting responsibility of building  wiring up  the application onto the Spring framework  The Spring framework s dependency injection is responsible for connecting  wiring up individual beans into a working application   This way developers can be focused more on development of individual components  beans  as soon as interfaces between beans are defined   Testing of such application is easy - the primary focus is given to individual beans  They can be easily decoupled and mocked  so unit-testing is fast and efficient   Spring framework defines multiple specialized beans such as  Controller   Restcontroller    Repository   Component to serve web purposes  Spring together with Maven provide a structure that is intuitive to developers  Team work is easy and fast as there is individual elements are kept apart and can be reused

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Spring was dependency injection in the begining  then add king of wrappers for almost everything  wrapper over JPA implementations etc    Long story     most parts of Spring preffer XML solutions  XML scripting engine     brrrr   so for DI I use Guice  Good library  but with growing depnedenciec  for example Spring JDBC  maybe one Java jdbc solution with real names parameters  take from maven 4-5 next   Using Spring MVC  part of  big spring   for web development     it is  request based  framework  there is holy war  request vs component      up to You

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Spring started off as a fairly simple dependency injection system  Now it is huge and has everything in it  except for the proverbial kitchen sink    But fear not  it is quite modular so you can use just the pieces you want   To see where it all began try   http   www amazon com Expert-One-Design-Development-Programmer dp 0764543857 ref sr 1 1 ie UTF8 amp s books amp qid 1246374863 amp sr 1-1   It might be old but it is an excellent book   For another good book this time exclusively devoted to Spring see   http   www amazon com Professional-Java-Development-Spring-Framework dp 0764574833 ref sr 1 2 ie UTF8 amp s books amp qid 1246374863 amp sr 1-2  It also references older versions of Spring but is definitely worth looking at

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Spring is a lightweight and flexible framework compare to J2EE  Spring container act as a inversion of control   Spring uses AOP i e  proxies and Singleton  Factory and Template Method Design patterns  Tiered architectures  Separation of concerns and Reusable layers and Easy maintenance

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What is Spring for  I will answer that question shortly  but first  let s take another look at the example by victor hugo  It s not a great example because it doesn t justify the need for a new framework   public class BaseView     protected UserLister userLister     public BaseView         userLister   new UserListerDB       only line of code that needs changing        public class SomeView extends BaseView     public SomeView         super           public void render         List lt User gt  users   userLister getUsers        view render users           Done  So now even if you have hundreds or thousands of views  you still just need to change the one line of code  as in the Spring XML approach  But changing a line of code still requires recompiling as opposed to editing XML you say  Well my fussy friend  use Ant and script away   So what is Spring for  It s for    Blind developers who follow the herd Employers who do not ever want to hire graduate programmers because they don t teach such frameworks at Uni Projects that started off with a bad design and need patchwork  as shown by victor hugo s example    Further reading  http   discuss joelonsoftware com  joel 3 219431 12

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I see two parts to this     What exactly is Spring for  -  see the accepted answer by victor hugo         Spring is  a  good framework for web development  -  people saying this are talking about Spring MVC  Spring MVC is one of the many parts of Spring  and is a web framework making use of the general features of Spring  like dependency injection  It s a pretty generic framework in that it is very configurable  you can use different db layers  Hibernate  iBatis  plain JDBC   different view layers  JSP  Velocity  Freemarker       Note that you can perfectly well use Spring in a web application without using Spring MVC  I would say most Java web applications do this  while using other web frameworks like Wicket  Struts  Seam

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Basically Spring is a framework for dependency-injection which is a pattern that allows building very decoupled systems  The problem For example  suppose you need to list the users of the system and thus declare an interface called UserLister  public interface UserLister       List lt User gt  getUsers       And maybe an implementation accessing a database to get all the users  public class UserListerDB implements UserLister       public List lt User gt  getUsers                DB access code here          In your view you ll need to access an instance  just an example  remember   public class SomeView       private UserLister userLister       public void render             List lt User gt  users   userLister getUsers            view render users            Note that the code above hasn t initialized the variable userLister  What should we do  If I explicitly instantiate the object like this  UserLister userLister   new UserListerDB        I d couple the view with my implementation of the class that access the DB  What if I want to switch from the DB implementation to another that gets the user list from a comma-separated file  remember  it s an example   In that case  I would go to my code again and change the above line to  UserLister userLister   new UserListerCommaSeparatedFile     This has no problem with a small program like this but    What happens in a program that has hundreds of views and a similar number of business classes  The maintenance becomes a nightmare  Spring  Dependency Injection  approach What Spring does is to wire the classes up by using an XML file or annotations  this way all the objects are instantiated and initialized by Spring and injected in the right places  Servlets  Web Frameworks  Business classes  DAOs  etc  etc  etc      Going back to the example in Spring we just need to have a setter for the userLister field and have either an XML file like this   lt bean id  quot userLister quot  class  quot UserListerDB quot    gt    lt bean class  quot SomeView quot  gt       lt property name  quot userLister quot  ref  quot userLister quot    gt   lt  bean gt   or more simply annotate the filed in our view class with  Inject   Inject private UserLister userLister   This way when the view is created it magically will have a UserLister ready to work  List lt User gt  users   userLister getUsers        This will actually work                                               without adding any line of code  It is great  Isn t it   What if you want to use another implementation of your UserLister interface  Just change the XML  What if don t have a UserLister implementation ready  Program a temporal mock implementation of UserLister and ease the development of the view  What if I don t want to use Spring anymore  Just don t use it  Your application isn t coupled to it  Inversion of Control states   quot The application controls the framework  not the framework controls the application quot    There are some other options for Dependency Injection around there  what in my opinion has made Spring so famous besides its simplicity  elegance and stability is that the guys of SpringSource have programmed many many POJOs that help to integrate Spring with many other common frameworks without being intrusive in your application  Also  Spring has several good subprojects like Spring MVC  Spring WebFlow  Spring Security and again a loooong list of etceteras  Hope this helps  Anyway  I encourage you to read Martin Fowler s article about Dependency Injection and Inversion of Control because he does it better than me  After understanding the basics take a look at Spring Documentation  in my opinion  it is used to be the best Spring book ever

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Spring contains  as Skaffman rightly pointed out  a MVC framework  To explain in short here are my inputs  Spring supports segregation of service layer  web layer and business layer  but what it really does best is  injection  of objects  So to explain that with an example consider the example below   public interface FourWheel      public void drive       public class Sedan implements FourWheel      public void drive                drive gracefully         public class SUV implements FourWheel      public void drive                Rule the rough terrain          Now in your code you have a class called RoadTrip as follows  public class RoadTrip       private FourWheel myCarForTrip      Now whenever you want a instance of Trip  sometimes you may want a SUV to initialize FourWheel or sometimes you may want Sedan  It really depends what you want based on specific situation    To solve this problem you d want to have a Factory Pattern as creational pattern  Where a factory returns the right instance  So eventually you ll end up with lots of glue code just to instantiate objects correctly  Spring does the job of glue code best without that glue code  You declare mappings in XML and it initialized the objects automatically  It also does lot using singleton architecture for instances and that helps in optimized memory usage   This is also called Inversion Of Control  Other frameworks to do this are Google guice  Pico container etc    Apart from this  Spring has validation framework  extensive support for DAO layer in collaboration with JDBC  iBatis and Hibernate  and many more   Provides excellent Transactional control over database transactions    There is lot more to Spring that can be read up in good books like  Pro Spring    Following URLs may be of help too  http   static springframework org docs Spring-MVC-step-by-step  http   en wikipedia org wiki Spring Framework http   www theserverside com tt articles article tss l SpringFramework

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Spring is great for gluing instances of classes together   You know that your Hibernate classes are always going to need a datasource  Spring wires them together  and has an implementation of the datasource too      Your data access objects will always need Hibernate access  Spring wires the Hibernate classes into your DAOs for you   Additionally  Spring basically gives you solid configurations of a bunch of libraries  and in that  gives you guidance in what libs you should use    Spring is really a great tool    I wasn t talking about Spring MVC  just the base framework

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The accepted answer doesn t involve the annotations usage since Spring introduced support for various annotations for configuration  Spring  Dependency Injection  approach There the another way to wire the classes up alongside using a XML file  the annotations  Let s use the example from the accepted answer and register the bean directly on the class using one of the annotations  Component   Service   Repository or  Configuration   Component public class UserListerDB implements UserLister       public List lt User gt  getUsers                DB access code here           This way when the view is created it magically will have a UserLister ready to work   The above statement is valid with a little bonus of no need of any XML file usage and wiring with another annotation  Autowired that finds a relevant implementation and inject it in   Autowired private UserLister userLister   Use the  Bean annotation on a method used to get the bean implementation to inject

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Very short summarized  I will say that Spring is the  glue  in your application  It s used to integrate different frameworks and your own code

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Spring is three things    Spring handles Dependency Injection and I recommend you read Martin Fowler s excellent introduction on dependency injection  The second thing Spring does is wrap excellent Java libraries in a very elegant way to use in your applications  For a good example see how Spring wraps Task Executors and Quartz Scheduler   Thirdly Spring provides a bunch of implementations of web stuff like REST  an MVC web framework and more  They figure since you are using Spring for the first two  maybe you can just use it for everything your web app needs     The problem is that Spring DI is really well thought out  the wrappers around other things are really well thought out in that the other things thought everything out and Spring just nicely wraps it  The Spring implementations of MVC and REST and all the other stuff is not as well done  YMMV  IMHO  but there are exceptions  Spring Security is da bomb   So I tend to use Spring for DI  and its cool wrappers but prefer other stuff for Web  I like Tapestry a lot   REST  Jersey is really robust   etc

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Spring is a good alternative to Enterprise JavaBeans  EJB  technology  It also has web framework and web services framework component

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Spring framework is definitely good for web development and to be more specific for restful api services   It is good for the above because of its dependency injection  and integration with other modules like spring security  spring aop  mvc framework  microservices   With in any application  security is most probably a requirement   If you aim to build a product that needs long maintenance  then you will need the utilize the Aop concept   If your application has to much traffic thus increasing the load  you need to use the microservices concept    Spring is giving all these features in one platform  Support with many modules   Most importantly  spring is open source and an extensible framework have a hook everywhere to integrate custom code in life cycle    Spring Data is one project which provides integration with your project     So spring can fit into almost every requirement

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Old days  Spring was a dependency injection frame work only like  Guice  PicoContainer       but nowadays it is a total solution for building your Enterprise Application   The spring dependency injection  which is  of course  the heart of spring is still there  and you can review other good answers here   but there are more from spring     Spring now has lots of projects  each with some sub-projects  http   spring io projects   When someone speaks about spring  you must find out what spring project he is talking about  is it only spring core  which is known as spring framework  or it is another spring projects   Some spring projects which is worth too mention are    Spring Security    - http   projects spring io spring-security  Spring Webservices - http   projects spring io spring-ws  Spring Integration  - http   projects spring io spring-integration    If you need some more specify feature for your application  you may find it there too    Spring Batch  batch framework designed to enable the development of batch application Spring HATEOAS easy creation of REST API based on HATEOAS principal Spring Mobile and Spring Andriod for mobile application development Spring Shell  builds a full-featured shell   aka command line  application  Spring Cloud and Spring Cloud Data Flow for cloud applications   There are also some tiny projects there for example spring-social-facebook  http   projects spring io spring-social-facebook    You can use spring for web development as it has the Spring MVC module which is part of Spring Framework project  Or you can use spring with another web framework  like struts2

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