将Spring Boot bean注入bean注入方法

Question

Can anyone tell me if there is a difference (in terms of spring bean injection and respecting singleton conditions or any other spring boot magic) in these two spring boot application classes?

@Bean
@Scope("singleton")
public UserService userService(Foo foo){
    return new UserService(foo);
}

@Bean
@Scope("singleton")
public Foo foo(){
    return new Foo();
}

AND calling not declaring Foo as a method parameter on userService() but rather injecting it via a direct method call to foo()

@Bean
@Scope("singleton")
public UserService userService(){
    return new UserService(foo());
}

@Bean
@Scope("singleton")
public Foo foo(){
    return new Foo();
}

Answer 1

No, there is no difference. One might think, you would get a new bean instance everytime you call foo() in that configuration class, but the way Spring works in that case is, it creates a proxy for that configuration class which intercepts all method calls. The proxy then checks, if there is already a bean of type Foo , if so it returns the existing instance, otherwise the method call is delegated to the implementation and a new bean is created.

Code style wise, however, i think in your first example the dependency to the Foo bean is more clearly marked than in the second example.