Dependency Injection (DI) is a popular design pattern used in Java applications to decrease coupling between classes and improve code maintainability. It allows for the inversion of control, where one class (the client) depends on another class (the dependency) and delegates the responsibility of providing the dependency to an external entity.

In Java, there are mainly three types of Dependency Injection:

1. Constructor Injection

Constructor Injection is the most common type of DI in Java, where the dependencies are provided through the constructor of a class. The client class declares a constructor that accepts the necessary dependencies as parameters. The dependencies are then passed to the constructor when creating an instance of the class.

public class ClientClass {
    private Dependency dependency;

    public ClientClass(Dependency dependency) {
        this.dependency = dependency;
    }

    // ...
}

The client class can now use the dependency instance within its methods, knowing that it has been injected through the constructor.

2. Setter Injection

Setter Injection, also known as Method Injection, involves providing dependencies through setter methods in the client class. The client class declares setter methods for each dependency it requires, which are then called by an external entity to inject the dependencies.

public class ClientClass {
    private Dependency dependency;

    public ClientClass() {
    }

    public void setDependency(Dependency dependency) {
        this.dependency = dependency;
    }

    // ...
}

The external entity calls the setter method on the client class instance to set the dependency before using its methods.

3. Field Injection

Field Injection involves directly injecting the dependency into the client class’s fields using annotations. With Field Injection, the client class declares the dependency as a field and annotates it with the @Inject annotation.

public class ClientClass {
    @Inject
    private Dependency dependency;

    // ...
}

The injection is performed by a separate framework or container that scans the class and injects the dependencies before the class is used.

Conclusion

Dependency Injection is an essential technique for writing clean and maintainable Java code. By following one of these types (constructor, setter, or field) of DI, you can achieve loose coupling and improve testability and flexibility in your applications.

#java #dependencyinjection