Service: Serving somebody. Taking inputs, processing it and giving the response.
First, we are aware of Servlets
For UI purposes JSP came into the picture.
When mixed Core Java / JSP, we developed WAR files to deploy independent applications.
Later deployed servlets / REST Services as independent components.
Micro Service: Independently deployable components.
We did the same in the past. This was simplified with latest technologies like Spring Boot, AKA, …etc