What is inject in java

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. If this article was helpful, tweet it. Learn to code for free. Get started. Forum Donate. A dependency is an object that can be used a service. Showing dependencies between classes In Java, before we can use methods of other classes, we first need to create the object of that class i. What if code could speak? Why should I use dependency injection? It makes our Car class independent from creating the objects of Wheels, Battery, etc.

There are basically three types of dependency injection: constructor injection: the dependencies are provided through a class constructor. Clients must implement an interface that exposes a setter method that accepts the dependency.

Inversion of control —the concept behind DI This states that a class should not configure its dependencies statically but should be configured by some other class from outside. Identifies injectable constructors, methods, and fields. May apply to static as well as instance members. An injectable member may have any access modifier private, package-private, protected, public. Constructors are injected first, followed by fields, and then methods.

Fields and methods in superclasses are injected before those in subclasses. Ordering of injection among fields and among methods in the same class is not specified. Injectable constructors are annotated with Inject and accept zero or more dependencies as arguments.

Inject can apply to at most one constructor per class. So, this method will be called during application startup. You probably already recognized the Inject annotation in the previous code snippet. So, you can use the CoffeeApp app attribute in the startCoffeeMachine method to brew a cup of filter coffee.

But I think this is one of the rare occasions in which it is acceptable to inject the service implementation directly. The only task of the CoffeeAppStarter class is to start the coffee machine by calling the prepareCoffee method on the injected CoffeeApp object. In the future, this application will need to control different kinds of coffee machines. I want to make it as easy as possible to replace them. As you can see in the following code snippet, the CoffeeApp class only depends on the CoffeeMachine interface.

It has no dependency on any interface implementation. In the previous articles, the CoffeeAppStarter class had to instantiate a specific implementation of the CoffeeMachine interface. It provided that object as a constructor parameter while instantiating a CoffeeApp object. Constructor injection now enables you to replace the compile time dependency to a specific implementation class with a runtime dependency to any implementation class.

That makes it very easy to replace the CoffeeMachine implementation. You only need to add a different implementation of the CoffeeMachine interface to your classpath when you start the application. That decouples the higher-level class from its dependencies so that you can change the code of a lower-level class without changing the code that uses it.

The only code that uses a dependency directly is the one that instantiates an object of a specific class that implements the interface.

The dependency injection technique enables you to improve this even further. It provides a way to separate the creation of an object from its usage. By doing that, you can replace a dependency without changing any code and it also reduces the boilerplate code in your business logic. With APM, server health metrics, and error log integration, improve your application performance with Stackify Retrace.

Try your free two week trial today. Click here to read more about the acquisition. Try Our Free Code Profiler. Try Our Code Profiler. By Role. By Technology. By Language. Documentation Support Ideas Portal Menu. Start Free Trial. Tip: Find application errors and performance problems instantly with Stackify Retrace. Troubleshooting and optimizing your code is easy with integrated errors, logs and code level performance insights.