The basic concept of the dependency
injection (also known as Inversion of Control pattern) is that you do
not create your objects but describe how they should be created. You don't
directly connect your components and services together in code but describe
which services are needed by which components in a configuration file. A
container (in the case of the Spring framework, the IOC container) is then
responsible for hooking it all up.
i.e.Applying IoC, objects are given their dependencies at creation time by
some external entity that coordinates each object in the system. That is,
dependencies are injected into objects. So, IoC means an inversion of
responsibility with regard to how an object obtains references to collaborating
objects.
Example:
Lets we have two classes-Car and Engine.Car
has a object of Engine.
Normal way:
There are many ways to instantiate a
object. A simple and common way is with new operator.
so here Car class contain object of
Engine and we have it instantiated using new operator.
WITHOUT DI
With help of Dependency Injection:
Now we outsource instantiation and
supply job of instantiating to third party.Car needs object
of Engine to operate but it outsources that job to some third
party. The designated third party, decides the moment of instantiation and the
type to use to create the instance. The dependency between class Car and
class Engine is injected by a third party. Whole of this
agreement involves some configuration information too. This whole process is
called dependency injection.
WITH DI
BENEFITS OF DEPENDENCY INJECTION IN SPRING:
- Ensures configuration and uses of services are separate.
- Can switch implementations by just changing configuration.
- Enhances Testability as mock dependencies can be injected.
- Dependencies can be easily identified.
- No need to read code to see what dependencies your code talks to.
TYPES OF DEPENDENCY INJECTION:
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.
Constructor Injection :
Constructor-based DI is realized by invoking a constructor with a number
of arguments, each representing a collaborator.