Everything You Need to Know About Dependency Injection in Programming

Dependency injection in programming is one of the more talked about terms online, but it’s also one of the most misunderstood. What exactly is dependency injection? How does it relate to programming? How can you implement it? If you’re not exactly sure what dependency injection means, don’t worry — you aren’t alone! Luckily, we’ve got all the answers to your most pressing questions about dependency injection below.

Introduction

Dependency injection is a programming pattern that allows us to remove hard-coded dependencies and make our code cleaner, more flexible, and easier to maintain. With dependency injection, we can easily change how an object gets its dependencies. Instead of having some objects hard-coded with their dependencies (i.e., database connection strings), those dependencies are passed into an object via another object that serves as a container for these ‘dotted’ variables. When using dependency injection within our application, we are able to swap out objects (or create new ones) much easier than if we had hard-coded all of our database connections throughout our application source code. For example, let’s say you have an email service class named EmailService. If you were to use dependency injection within your application, you would not hard-code your SMTP server address inside of EmailService.

How to use it

If you’re familiar with injection, you might be able to skip right over much of my explanation here. If not, then I hope that you’ll find it helpful as a quick-and-dirty primer. To get us started on dependency injection, let’s consider an example that doesn’t use it: say we have a simple program that sends an email when a user submits data. This requires three objects: an emailer object, a validator object and a controller object. The emailer is responsible for sending emails; it takes a string of text as input and returns true or false depending on whether or not there are any errors. The validator checks if the input is legitimate; if so, it returns true; otherwise, it returns false. Finally, our controller object is responsible for displaying information about our form to users and accepting their inputs — it has no knowledge of how these inputs will be used (i.e., by which other object). So far so good?

Why use it

The dependency injection design pattern is used to separate interactions between classes from dependencies with other classes. This helps facilitate independent development and refactoring, promote code reuse, increase testability and ultimately improve modularity for large scale applications. The name dependency injection (or DI) comes from saying that you inject your dependencies into a class instead of creating them inside its constructor. So what is DI? What are its pros and cons? Let’s explore!

When developing software, it’s common to use third-party libraries and frameworks that provide features our project requires such as database access or encryption. While these modules help us get our projects up faster, they often have their own set of dependencies that might not be compatible with our existing project — which can result in serious issues at runtime. There are also cases where we just don’t want certain functionality within our own project but still need it during runtime — an example would be having a database connection without needing SQL statements or stored procedures available within it. The biggest advantage of using DI is decoupling code so libraries or frameworks can be changed without touching an app’s core logic or files outside of your control can easily be swapped out for something else.

Pros and cons

When used incorrectly, dependency injection can significantly increase coupling. Coupling is a fancy way of saying how much interdependence two different software components have. If software components are highly coupled together, it makes it more difficult to change one component because any changes made will also affect other dependent components. Software with high coupling can be very brittle; when certain parts break, they can take out lots of other parts with them. On the flip side, an easily modifiable system (low coupling) makes it easy for developers to add new features and make optimizations without worrying about breaking something else in their app. The key here is that it’s important to know what you’re doing. A small amount of decoupling may help your code run faster or use less memory, but if you go too far you could end up making things harder on yourself than they need to be. As always, there’s no substitute for knowing your tools well enough that you can confidently use them — and knowing when not to use them at all.

Summary

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