Spring injects dependencies in constructor without @Autowired annotation

Question

I'm experimenting with examples from this official Spring tutorials and there is a dependency on this code:
https://github.com/spring-guides/gs-async-method/tree/master/complete

If you look at the code on AppRunner.java class, I have 2 questions:

1. When server is starting, if I put a breakpoint in this class's constructor, seems like in the constructor, the GitHubLookupService is provided by spring, using the @Service bean that was configured. BUT, there was no @Autowired annotation on the constructor, so how in the world this constructor get called with the right dependency? It was supposed to be null.

Is it an automatic assumption of Spring Boot?
Does Spring see "private field + constructor argument, and it assumes it should look for an appropriate bean?
Is it Spring Framework or Spring boot?
Am I missing something?

2. As I remember, it was mendatory to provide default constructor to beans / service etc. How come this class (AppRunner) doesn't have a default constructor? How does Spring knows that it should run the constructor with the argument? Is it because it is the only constructor?

Answer 1

Starting with Spring 4.3, if a class, which is configured as a Spring bean, has only one constructor, the @Autowired annotation can be omitted and Spring will use that constructor and inject all necessary dependencies.

Regarding the default constructor: You either need the default constructor, a constructor with the @Autowired annotation when you have multiple constructors, or only one constructor in your class with or without the @Autowired annotation.

Read the @Autowired chapter from the official Spring documentation for more information.