Dependency injection in Spring Boot
Dependency injection
What is Dependency injection?
- Dependency injection is a form of the IoC container
- It is just passing dependencies to other objects or frameworks.
- Dependency injection allows you to remove hardcoded dependencies and make applications loosely coupled, extendable and maintainable.
- In Spring it uses two types of dependency injection methods, constructor and setter injection. In addition, we can use field injection as well.
Constructor injection
private final Flavor flavor;
Cake(Flavor flavor) {
Objects.requireNonNull(flavor);
this.flavor = flavor;
}
- If there is a single construction, @Autowired annotation is optional after Spring 4.3. But if there is more than one constructor, we need to specify the constructor by mentioning it using @Autowired annotation.
- Autowired field should be final
- This method is immutable and reliable
Setter injection
private Topping toppings;
@Autowired
void setTopping(Topping toppings) {
this.toppings = toppings;
}
- More flexible
- Objects are mutable
- Null checks are required
- Less secure than constructor injection, because dependencies can be overridden
Field injection
@Autowired
private Topping toppings;
- Very easy to use, but not recommended.
- If you use this, the unit tests can be failed.
When to use Setter or Constructor injections
- Constructor injection is better than Setter injection.
- Use Setter injection when a number of dependencies are more or you need readability.
- Use Constructor Injection when Object must be created with all of its dependencies.
- We can change a value easily when using the setter injection. It is more flexible than constructor injection.
- Setter injection will override the constructor injection. If we use both setter and constructor injection, the IOC container will use setter injection.
Why does constructor injection better than others?
- All required dependencies are available at the initialization time.
- This is very important in writing unit tests. Because it forces to load all objects for all dependencies.
Advantages of dependency injection
- Easy unit testing
- Configurable components
- Separations of concerns
- More control
