What is the difference between @RequiredArgsConstructor(onConstructor = @__(@Inject)) and @RequiredArgsConstructor?

@RequiredArgsConstructor
class MyClass {
  private final DependencyA a;
  private final DependencyB b;
}

will generate

public MyClass(DependencyA a, DependencyB b) {
  this.a = a;
  this.b = b;
}

while

@RequiredArgsConstructor(onConstructor = @__(@Inject))
class MyClass {
  private final DependencyA a;
  private final DependencyB b;
}

will generate

@Inject
public MyClass(DependencyA a, DependencyB b) {
  this.a = a;
  this.b = b;
}

From JDK 8 onwards, the syntax @RequiredArgsConstructor(onConstructor_ = {@Inject}) is also accepted.

I know RequiredArgsConstructor injects all the final dependencies.

All required dependencies, which include final and @NonNull fields.

The answers given have explained clearly what is the difference as asked by the OP. But i also feel that knowing why you would need @RequiredArgsConstructor(onConstructor = @__(@Inject)) instead of @RequiredArgsConstructor? is also important. If you are interested, read on...

In short, when Spring construct your beans (the classes annotated with @Component or related @Controller, @Service, @Repository - they all have @Component + extra functionality), Spring will need to look at the class constructor, to construct it. If you have only 1 constructor in your class, fine, no confusion, you only need @RequiredArgsConstructor.

What if you have 2 or more constructors? Which one does Spring use to construct your bean? Enter Lombok's @RequiredArgsConstructor(onConstructor = @__(@Inject)) or the more popular @RequiredArgsConstructor(onConstructor = @__(@Autowired)). As the annotation's attribute says, it puts @Autowired on the constructor to tell Spring to use that constructor at construction time.

That's it!

P.S I recommend this article if you want to read more about it.

The second one will put the annotations you mention on the generated constructor.

For example, this: @RequiredArgsConstructor(onConstructor = @__(@Inject)) will generate a constructor annotated with @Inject