What do you mean by dependency injection

This states that a class should not configure its dependencies statically but should be configured by some other class from outside. It is the fifth principle of S. D — the five basic principles of object-oriented programming and design by Uncle Bob — which states that a class should depend on abstraction and not upon concretions in simple terms, hard-coded.

According to the principles, a class should concentrate on fulfilling its responsibilities and not on creating objects that it requires to fulfill those responsibilities. You can implement dependency injection on your own Pure Vanilla or use third-party libraries or frameworks. To learn more about dependency injection, you can check out the below resources:. Using dependency injection in Java — Introduction — Tutorial — Vogella.

If you liked the article and want to read more amazing articles, then do follow me here Bhavya Karia and show your support as it motivates me to write more. Thanks to Sergey Ufocoder now this article has been converted into the Russian language.

My Russian friends and who all can read the Russian language do give it a read. Link to the article. Also, if you want to apply DI in JavaScript and are looking for a library then Jo Surikat suggests that you give a try to his library. Gordon Forsythe recommended this amazing library which you all might want to try out. Thanks for all the kind words that I have been receiving.

Do share the article so that more and more people can be benefited. Dependency injection supports these goals by decoupling the creation of the usage of an object. That enables you to replace dependencies without changing the class that uses them.

It also reduces the risk that you have to change a class just because one of its dependencies changed. The dependency injection technique is a popular alternative to the service locator pattern.

A lot of modern application frameworks implement it. These frameworks provide the technical parts of the technique so that you can focus on the implementation of your business logic. Popular examples are:. You can introduce interfaces to break the dependencies between higher and lower level classes.

If you do that, both classes depend on the interface and no longer on each other. I explained this approach in great details in my article about the dependency inversion principle. That principle improves the reusability of your code and limits the ripple effect if you need to change lower level classes.

But even if you implement it perfectly, you still keep a dependency on the lower level class. The interface only decouples the usage of the lower level class but not its instantiation. At some place in your code, you need to instantiate the implementation of the interface.

That prevents you from replacing the implementation of the interface with a different one. The goal of the dependency injection technique is to remove this dependency by separating the usage from the creation of the object. This reduces the amount of required boilerplate code and improves flexibility. But before we take a look at an example, I want to tell you more about the dependency injection technique. You already implement three of these four roles by following the dependency inversion principle.

The service and the client are the two classes between which the dependency inversion principle intends to remove the dependency by introducing an interface. You can skip the interface role and inject the service object directly into the client. But by doing that, you break with the dependency inversion principle and your client has an explicit dependency on the service class. In some situations, this might be ok. All frameworks that I listed at the beginning of this article provide ready-to-use implementations of it.

As you can see, dependency injection is a great fit for applications that follow the dependency inversion principle. You already implement most of the required roles, and the dependency injection technique enables you to remove the last dependency to the service implementation. I used the CoffeeApp example in my article about the dependency inversion principle. This small application enables you to control different coffee machines, and you can use it to brew a fresh cup of filter coffee.

It consists of a CoffeeApp class that calls the brewFilterCoffee method on the CoffeeMachine interface to brew a fresh cup of coffee. As you can see in the diagram, this application already follows the dependency inversion principle. It also provides three of the four roles required by the dependency inversion technique:. Handling data. User input.

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