what is spring and what is spring for

1 what is spring

spring是一个轻量级的容器。

它使用依赖注入技术来构建耦合性很低的系统。

2 what is spring for

用于系统的依赖解耦合。在一个系统中，A类依赖于B类，那么我在使用A类的时候，需要先用B来创建一个对象，然后再创建A类的对象。如果现在我换了一个C类，A类依赖于C类，那么我得修改这段使用代码，这是违背OCP的closed原则的。我扩展的时候，不要去修改已有的代码。这个问题该怎么解决呢？spring提供了一个很好的方法。这种依赖关系只需要在xml文件中体现就可以了，不需要显式的创建这个对象，这个对象的创建交给spring，所以称它为一个容器。

3 例子，引用于"http://stackoverflow.com/questions/1061717/what-exactly-is-spring-framework-for"

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 doesn't have 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 last line by:

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 a XML file, 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 an XML like this:

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

<bean class="SomeView">

    <property name="userLister" ref="userLister" />

</bean>

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".