What exactly is Spring Framework for?

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 in a nutshell? Why should I used it over just plain Java.

Basically Spring is a framework for [Tag: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<User> getUsers();
}

And maybe an implementation accessing a database to get all the users:

public class UserListerDB implements UserLister {
    public List<User> 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<User> 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:

<bean id="userLister" class="UserListerDB" />

<bean class="SomeView">
    <property name="userLister" ref="userLister" />
</bean>

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<User> 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: "The application controls the framework, not the framework controls the application".

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.

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

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.

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<User> 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:

1. Blind developers who follow the herd
2. Employers who do not ever want to hire graduate programmers because they don't teach such frameworks at Uni
3. 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

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.

Spring is three things.

1. Spring handles Dependency Injection and I recommend you read Martin Fowler's excellent introduction on dependency injection.
2. 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.
3. 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.

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.

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).

I see two parts to this:

1. "What exactly is Spring for" -> see the accepted answer by victor hugo.
2. "[...] 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, ...

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 -> eat as much as you want
        //this.todaysLunch = new Plated(); // choose Plated -> 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.

The accepted answer doesn't involve the annotations usage since Spring introduced support for various annotations for configuration.

Spring annotations approach (Dependency Injection)

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<User> 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.

• 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.

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

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&s=books&qid=1246374863&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&s=books&qid=1246374863&sr=1-2

It also references older versions of Spring but is definitely worth looking at.

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

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.

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.