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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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