What is the difference between @Qualifier annotation and interface?

Question

Java

I'm reading the book Spring in action and the question arose why use qualifiers (in the book it shows on the example of string instruments), if you can simply implement the marker interface?
Interested in the pros and cons of the two methods, this is not described in the book.

Answer 1

`@Qualifier` allows you to make multiple implementations of the same interface.
A marker interface is not always an option. For example, if you need multiple database connections, then you need the `DataSource` interface for all of them. And to inherit from its implementations in order to add this very marker interface is not even always possible. For example, if the DataSource is implemented somewhere deep inside a connection pool or a JDBC driver.

Answer 2

No need to produce entities unnecessarily.
If we do everything through interfaces, then for three, say, implementations, we need 4 files - a base interface with methods, and three heirs that will act as a selector.
Through @Qualifier all this is not necessary.