Dependency Injection

What is Dependency

Quick Example:EMPLOYEE OBJECT WHEN CREATED,
              IT WILL AUTOMATICALLY CREATE ADDRESS OBJECT
   (if address is defines as dependency by Employee object)

Now in the above EMPLOYEE Class is dependent on ADDRESS Class.

You can not create the Address Object unless you create Employee Object

EMPLOYEE Class is dependent and ADDRESS Class is dependency

What is the purpose of DI?
With dependency injection, objects don’t define their dependencies themselves, the dependencies are injected to them as needed.The purpose of Dependency Injection is to reduce coupling in your application to make it more flexible and easier to test.

How does it benefit ? The objects don’t need to know where and how to get their dependencies, which results in loose coupling between objects, which makes them a lot easier to test.

When to use Dependency Injection
One of the most compelling reasons for DI is to allow easier unit testing without having to hit a database and worry about setting up ‘test’ data.Dependency Injection gives you the ability to test specific units of code in isolation.

Dependency injection is basically providing the objects that an object needs (its dependencies) instead of having it construct them itself. It’s a very useful technique for testing, since it allows dependencies to be mocked or stubbed out.

Dependencies can be injected into objects by many means (such as constructor injection or setter injection). One can even use specialized dependency injection frameworks (e.g Spring) to do that, but they certainly aren’t required.

Example
A Car depends on wheels, engine, fuel, battery, etc. to run. Traditionally we define the brand of such dependent objects along with the definition of the Car object

Without Dependency Injection (DI):

class Car
{
  private Wheel wh   = new ApolloWheel();
  private Battery bt = new ExideBattery();

  //The rest
}

Here, the Car object is responsible for creating the dependent objects.

What if we want to change the type of its dependent object – say Wheel – after the initial ApolloWheel() punctures? We need to recreate the Car object with its new dependency say SpareWheel(), but only the Car manufacturer can do that.

Then what does the Dependency Injection do us for…?

When using dependency injection, objects are given their dependencies at run time rather than compile time (car manufacturing time). So that we can now change the Wheel whenever we want. Here, the dependency (wheel) can be injected into Car at run time.

After using dependency injection:

class Car
{
  private Wheel wh   = [Inject an Instance of Wheel at runtime];
  private Battery bt = [Inject an Instance of Battery at runtime];

  Car(Wheel wh,Battery bt) 
  {
      this.wh = wh;
      this.bt = bt;
  }

  //Or we can have setters
  void setWheel(Wheel wh) 
  {
      this.wh = wh;
  }
}

Lets take the below example

public class Foo
{
    private Bar _bar;

    public Foo(Bar bar)
    {
        _bar = bar;
    }

    public bool IsPropertyOfBarValid()
    {
        return _bar.SomeProperty == PropertyEnum.ValidProperty;
    }
}

without dependency injection the Bar object is dependent and tightly coupled with Foo class like below

public class Foo
{
    private Bar _bar = new Bar();
    .
    . 
}

But by using dependency injection like before code you can mock the Bar Object at runtime and call the IsPropertyOfBarValid() method over it.

IoC is a generic term meaning rather than having the application call the methods in a framework, the framework calls implementations provided by the application.

Say the Excel Jar files with utility methods used the application uses the methods in the Utility Class and the flow is controlled by the way the method gets called in order the get the things done.

whereas

In framework like spring the implementation is defined in XML files and by using annotation and the framework calls the methods as per defined in xml.

Without Ioc

  Application -> Methods -> Framework      

With Ioc

  Framework -> XML File ->  Method Call (or) Implementation      

Inversion of Control(IoC) Container:
Common characteristic of frameworks IOC manages java objects

1. From instantiation to destruction through its BeanFactory.
2. Java components that are instantiated by the IoC container are called beans, and the IoC container manages a bean’s scope, lifecycle events, and any AOP features for which it has been configured and coded.

Flow of control is “inverted” by dependency injection because you have effectively delegated dependencies to some external system

The Inversion of Control (IoC) and Dependency Injection (DI) patterns are all about removing dependencies from your code.

For example, say your application has a text editor component and you want to provide spell checking. Your standard code would look something like this:

public class TextEditor
{
    private SpellChecker checker;
    public TextEditor()
    {
        this.checker = new SpellChecker();
    }
}

What we’ve done here is create a dependency between the TextEditor and the SpellChecker. In an IoC scenario we would instead do something like this:

public class TextEditor
{
    private ISpellChecker checker;
    public TextEditor(ISpellChecker checker)
    {
        this.checker = checker;
    }
}

Now, the client creating the TextEditor class has the control over which SpellChecker implementation to use. We’re injecting the TextEditor with the dependency.

Without IoC: you ask for “apple”, and you are always served apple when you ask more.

With IoC: You can ask for “fruit”. You can get different fruits each time you get served. for example, apple, orange, or water melon.

Inversion of Control, (or IoC), is about getting
freedom (You get married, you lost freedom and you are being controlled. You divorced, you have just implemented Inversion of Control. That’s what we called, “decoupled”. Good computer system discourages some very close relationship.)

flexibility (The kitchen in your office only serves clean tap water, that is your only choice when you want to drink. Your boss implemented Inversion of Control by setting up a new coffee machine. Now you get the flexibility of choosing either tap water or coffee.)

less dependency (Your partner has a job, you don’t have a job, you financially depend on your partner, so you are controlled. You find a job, you have implemented Inversion of Control. Good computer system encourages in-dependency.)