Dependency injection in Spring Boot

Dependency injection in Spring Boot

   June 26, 2021       Ravi Yasas

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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