Java

There is only one type of JSP comment available by JSP specification.
JSP Comment Syntax:
<%-- comment --%>
This JSP comment tag tells the JSP container to ignore the comment part from compilation. That is, the commented part of source code is not considered for the content parsed for ‘response’.
Example:
<%-- This JSP comment part will not be included in the response object --%>

Is this a JSP Comment?

<!-- comment --> is not a JSP comment. This is HTML comment. The JSP container treats this HTML comment tag as equal as any other HTML tags. When view source is done, the content given between this tag is visible. This is not anyway related to the JSP container, it is the expected behaviour of this HTML tag.

Example

<!-- This HTML comment part is included in the response object and can be seen in view source -->
// or /* comment */ used inside the <% %> scriplet tag is also not a JSP comment. This is just a java comment and JSP container doesn’t reserve any special treatment for this comments usage.
Therefore, there is nothing called as hidden comment or output comment in JSP by specification.