Dependency Injection

What is Dependency Injection?

Each Object-oriented application consists of many classes that work together in order to solve a problem. However, when writing a complex application, application classes should be as independent as possible. DI helps in gluing these classes together and at the same time keeping them independent. Dependency injection is a technique whereby one object (or static method) supplies the dependencies of another object.

Pros

increase the possibility to reuse classes
test them independently of other classes while unit testing

Code with no DI vs code with DI

				
					public class Computer {

    private CPU cpu;
    
    public Computer() {
        this.cpu = new CPU();
    }
}

Dependency between Computer and CPU.

				
					public class Computer {

    private CPU cpu;
    
    public Computer(CPU cpu) {
        this.cpu = cpu;
    }
    
}

Dependency between Computer and CPU. Dependency is injected through class constructor (Computer should not worry about CPU implementation).

Types of DI

Constructor-based dependency injection – used for mandatory dependencies

				
					public Computer(CPU cpu) {
    this.cpu = cpu;
}

Setter-based dependency injection – used for optional dependencies

				
					public Computer(CPU cpu) {
    this.cpu = cpu;
}

Property-based dependency injection – considered as bad practice!

				
					@Autowired
private CPU cpu;

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