what is the difference between servlet and JSP? Please describe <% expr %> and <%! expr %> tags of JSP, when and where they get executed?

Answer 1

A Servlet is a Java class which conforms to the Java Servlet API, a protocol by which a Java class may respond to http requests.

A JSP or java server page is essentially a servlet that allows one to easily mix page logic with markup (html, xml, etc.) and write. JSPs get compiled into servlets which are then executed for http requests

The <%!..%> is used to add declarations in jsp pages

The <% %> code goes into the service method of the compiled JSP (which is a servlet) and is executed there.

More on Declarations and SCriptlets

Answer 2

First part is as rightly answered by naikus. The other part of answer is as follows :

<% ... %> is used to embed some java code within the main service() method of the JSP. It is executed during the rendering of the page.

<%! ... %> is used to define code outside of the flow of the page, and therefore outside the main service() method. Typically, this was used to define utility methods that would be called from within a <% ... %> block.