Spring injects dependencies in constructor without @Autowired annotation

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.

Think of it this way... Suppose you have the following component:

@Component 
public class FooService {
    public FooService(Bar bar) { /*whatever*/ }
}

When Spring is scanning this class, it wants to know how it should go about constructing an instance. It's using reflection so it can get a list of all of the constructors at runtime.

In this case, it is completely unambiguous how Spring must construct this instance. There's only one constructor so there is no decision to be made, and no ambiguity at all.

If you add @Autowired here, you are not narrowing anything down, and you are not giving Spring any extra information to help make its decision - its decision is already made because there is only one candidate.

And so, as a convenience, the Spring team decided @Autowired should be optional. Since its not helping the Spring framework to make a decision, its presence is just noise.

If your component has multiple constructors then you can use @Autowired on one of them to tell Spring "use this one, not that one".