Spring Dependencies

Spring Dependencies

An application will have certain objects that work together to present what the end-user sees as a coherent application. We can define a number of bean definitions that are stand-alone, each to themselves, to a fully realized application where objects work (or collaborate) together to achieve some goal.

Injecting Dependencies

It becomes evident upon usage that code gets much cleaner when the DI principle is applied, and reaching a higher grade of decoupling is much easier when beans do not look up their dependencies, but are provided with them.

Setter Injection

Setter-based DI is realized by calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

public class SimpleMovieLister { // the SimpleMovieLister has a dependency on the MovieFinder     private MovieFinder movieFinder; // a setter method so that the Spring container can 'inject' a MovieFinder     public void setMovieFinder         (MovieFinder movieFinder) {       this.movieFinder = movieFinder;     } }

Constructor Injection

Constructor-based DI is realized by invoking a constructor with a number of arguments, each representing a collaborator. Additionally, calling a static factory method can be considered almost equivalent.

public class SimpleMovieLister {     // the SimpleMovieLister has a dependency on the MovieFinder     private MovieFinder movieFinder;     // a constructor so that the Spring container can 'inject' a MovieFinder     public SimpleMovieLister        (MovieFinder movieFinder) {         this.movieFinder = movieFinder;     }     }

Constructor or Setter DI?

·         Setter based DI makes objects of that class responsive to being re-configured (or re-injected) at some later time.

·         Constructor-injection supplying all of an object's dependencies means that that object is never returned to client (calling) code in a less than totally initialized state.