Dependency Injection, Field, Setter, or Constructor Injection

I was doing some investigation on the different approaches of Dependency Injection in the Spring Framework. Following is a summary of content that I gathered from various websites.

Inversion of Control (IOC)

IoC enables a framework code,  to take control of the flow of a program and make calls to our custom code.

Dependency Injection

Dependency injection is a pattern through which to implement IoC.

The act of connecting objects with other objects, or rather “injecting” objects into other objects, is done by a the framework code  rather than by the objects themselves

Field Injection

Using field injection makes it hard to test , because we do not have a way to inject our dependency into the class we are testing, and we would have to rely on the IOC container to inject this dependency for us.

One of the purposes of Dependency Injection is to decouple code, which in turn should make the code easier to test.

However, it is worthwhile noting that field injection makes the code more elegant than setter or constructor injection.

Setter Injection

In this approach, we can not guarantee that our class would have injected the dependency. However note that, It is good to use setter injection when the dependencies are optional.

This blog Repeat After Me: Setter Injection is a Symptom of Design Problems explains further the disadvantages of using setter injection. 

Another blog explaining why setter injection is not a good idea is Setter injection sucks.

I came across this blog: Constructor vs Setter Injection, happen to explain why setter injection is preferred over constructor injection.

Constructor Injection

Constructor Injection guarantees that the dependency is injected during class creation.

This blog: One correct way to do dependency injection explains why constructor injection is a better option over setter injection.