Dependency Injection

 

• a technique for assembling applications from a set of concrete classes that implement generic interfaces

• create loosely coupled systems as the code you write only ever references the generic interfaces that hide the concrete classes

   

   

Dependency Injection origin in a famous blog post by Martin Fowler

see http://martinfowler.com/articles/injection.html

 

A MovieLister class = lists movies with certain characteristics after being provided a database of movies by an instance of MovieFinder.

MovieFinder = interface

TabDelimitedMovieFinder = is a concrete class that can read in a movie database that is stored in a tab-delimited text file

Goal: Loosely coupled system = to avoid having our code depend on concrete classes

 

DO NOT want depencencies on concrete code or hard coded values.

Problems with this code:

• a reference to a concrete class that implements MovieFinder

• a reference to a hard-coded string

• The name of the movie database cannot change without causing MovieLister to be changed and recompiled

• The format of the database cannot change without causing MovieLister to be changed to reference the name of the new concrete MovieFinder implementation

Solution:

public class MovieLister {

         private MovieFinder finder;

          public MovieLister(MoveFinder finder) {
             this.finder = finder;
           }

…

}

 

Some examples of Dependency Injection

 

The IDEA

classes in an application indicate their dependencies in ABSTRACT ways

• MovieLister NEEDS-A MovieFinder

• Main NEEDS-A MovieLister

• A third party injects (or inserts) a class that will meet that dependency at run-time --> The “third party” is known as a “Inversion of Control container” or a “dependency injection framework”

 

The CONCEPT -- from a non-framework perspective "Dependency injection is basically providing the objects that an object needs (its dependencies) instead of having it construct them itself"

 

The CONCEPT - from a more framework perspective "dependency injection is a software design pattern that implements inversion of control for resolving dependencies. A dependency is an object that can be used (a service). An injection is the passing of a dependency to a dependentobject (a client) that would use it."

 

There are many frameworks that implement the Dependency Injection Framework.

• Spring, JSF for Server side Java

• Tapestry

 

How many current Dependency Injection Frameworks work = classes, interfaces + meta data files (often xml) + Inversion of Control Container

{ a set of components (concrete classes + interfaces) } + {a set of configuration metadata} + Dependency Injection Framework --> small set of code that gets acces to the "Inversion of Control Container" that retrieves first object from that container representing the needed interface.