Dependency Injection: when to use explanation

Question

I've read the reasons why to use (not to use) DI (see reasons below), and have a few questions.

Dependency injection is effective in these situations:

You need to inject configuration data into one or more components.

You need to inject the same dependency into multiple components.

You need to inject different implementations of the same dependency.

You need to inject the same implementation in different configurations.

You need some of the services provided by the container.

Dependency injection is not effective if:

You will never need a different implementation.

You will never need a different configuration.

Questions:

I afraid that don't understand all of them, so I hope you'll correct me if I'm wrong.

Don't understand. Is that about .properties files or I can apply the same xml configuration to multiple beans?
It's clear for me.
It's clear for me.
Don't understand. Example please.
It's clear for me.

P.S. And how often there are situations when you never need different implementations/configurations as pointed above?

Community · Accepted Answer · 2020-06-20 09:12:55Z

4

Dependency injection is not effective if:

You will never need a different implementation.

You will never need a different configuration.

This isn't true: dependency injection is useful even when you are wiring statically configured objects.

In fact, your whole argumentation moves along wrong lines. The main reasons in favor of dependency injection are the separated and centralized concerns of object:

naming (each object in the DI container is given a unique name),
lookup (each object can be easily looked up by that name from anywhere else),
and wiring (providing one object with a reference to another).

In addition, a key advantage is the declarative management of object lifecycle and scoping.

Further out, Spring IoC container adds many more advantages, such as:

annotation-based wiring;
automated exception handling and reporting;
annotation-based declarative transactions;
annotation-based mapping of HTTP requests to controller methods;

... and much more.

In a nutshell, doing an application based on a DI container means you can focus on your actual business logic and have most other concerns (which repeat in each application) delegated to the container.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 23, 2014 at 11:46

Marko Topolnik

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1 Comment

VB_ Over a year ago

thanks for your reply. I just quote rhis ref: tonymarston.net/php-mysql/dependency-injection-is-evil.html. Please explain more widely about naming and lookup statements pointed above.

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