Dependency Injection in Spring

Dependency Injection

Inversion of Control (IoC) is a general concept, and it can be expressed in many different ways and Dependency Injection is merely one concrete example of Inversion of Control.

This concept says that you do not create your objects but describe how they should be created. You don't directly connect your components and services together in code but describe which services are needed by which components in a configuration file. A container (the IOC container) is then responsible for hooking it all up.

What are the different types of IoC (dependency injection)?

    ·   Constructor-based dependency injection: Constructor-based DI is accomplished when the container invokes a class constructor with a number of arguments, each representing a dependency on other class.

    ·    Setter-based dependency injection: Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Which DI would you suggest Constructor-based or setter-based DI?

Since you can mix both, Constructor- and Setter-based DI, it is a good rule of thumb to use constructor arguments for mandatory dependencies and setters for optional dependencies. Note that the use of a @Required annotation on a setter can be used to make setters required dependencies.