Servlets came into picture as a real good alternative to CGI. Compared to CGI, servlets have a lot of advantages. Have a look at it
This is also good
Servlets can be used as a presentation layer technology. Here we will write HTML tags in java.
Ofcourse, for the ease of use and code maintainability, and a lot more advantages, we can use jsp or any of the similar kind of presentation layer technologies.
(Do a google search for servlets vs jsp.)
As far as I know, we should not implement business logic in our presentation logic (servlets, jsp, xhtml etc). It should be handled in our business logic layer. For e.g. using EJB.
"Based on the situation we can decide.For example we are using Database related things(Opening a connection,executing statements etc.i.e, called as business logic)it is better to use Servelts(Re usability is possible with servlets but not with jsp's)
Can you provide the link where you have seen this?
Using the right tool for the job, and following accepted conventions and best practices is smart.
Servlets and JSP are intended, and best suited, to be used as part of the Presentation Layer of web applications. As such neither should be used for business logic, which should be ensconced in its own UI-agnostic layer.
Perhaps this article can help.