How does Spring Work?

The idea is that beans follow the Dependency Injection pattern. Beans have information on the classes dependent on them. Beans define their own attributes within bean descriptors and also define the beans they depend on in the same descriptors.

 The Bean does not need to locate and instantiate these classes using service locators or JNDI calls. The Assembler (Spring Framework in this case) takes care of finding these classes based on the information provided in the descriptor and makes them available to the calling class.

 The service locator pattern does almost the same job, so why do we need another pattern to do it?
o The class that needs the dependent classes needs to tell the Service Locator as to which classes are required by it and moreover the responsibility of finding these classes and invoking them falls on the calling class. This makes the classes tightly coupled with each other making them difficult to unit test them separately.

 In the case of the Dependency Injection pattern the responsibility is shifted to the Assembler to load these classes. The assembler can make changes to the dependent classes by simply modifying the descriptor file.