Dependency Injection - An Introduction

Dependency

Dependency is when a class needs instances of another class to function correctly.
Following is an example of dependency:
    class X
    {
        ...
        Y obj = new Y("xyz", "abc") ;
        ...
    }

Dependency Problems

• Dependencies are hardcoded.
• Unit testing becomes difficult as well, where we need to mock the depencies to perform isolated test cases.
• With dependencies like above, if different implementation of Y (with same interface) is needed. Actual code needs to be changed.
• Also the code here is not based on interface so its even further less flexible.
• If dependency class has further dependencies, those will be needed as well.

Dependency Injection

• Injecting Dependency from (pushed from) outside into a class at runtime.
• Dependent object instead of instantiating dependencies using "new" operator from inside of the class should take dependent object via constructor parameters or setters.
• It decouples class's construction from the construction of its dependencies.
• Code should depend upon abstractions(interfaces) rathen than implementation(class references).
• Dependency Injection is for Loose Coupling.
• An object's dependencies should be on interfaces and not on "concrete" objects.
• Dependencies should be less.
• Loose coupling promotes greater reusability, easier maintainability.

    class X
    {
        InterfaceY objY = null;
        ...
        X (InterfaceY dependencyObj)
        {
            objY = dependencyObj ;
         }
        ...
    }

• Software design pattern that allows the removal of hard-coded dependencies and makes it possible to change them, whether at run-time or compile-time
• Its a configuration style of prorgamming, object is configured from outside (may be a configuration file) rather than inside.

Primary ways to implement Dependency Injection:

• Constructor Injection
• These are considered better than others since at the time of the creation of the class you know what is needed to perform its job.
• Easier to maintain.
• Setter Injection
• Method Injection

How to manage and instantiates dependencies?

• Each of our classes require dependencies, we need to figure what dependency each class needs and how to instantiate the dependencies.
• The answer is Dependency Injection container.
• Dependency Injection container contains map of dedencies needed by a class with the logic to create instances of those dependencies if needed.

Related Topics:
Dependency Inversion Principle
Inversion control Principle
Inversion of Control Container
Dependency Lookup
DI Container Implementations
Butterfly container
factory vs container
factory pattern vs DI Container