You can do via Page directive.
For example:
<%@ page language="java" contentType="application/json; charset=UTF-8"
pageEncoding="UTF-8"%>
The MIME type and character encoding the JSP file uses for the response it sends to the client. You can use any MIME type or character set that are valid for the JSP container. The default MIME type is text/html, and the default character set is ISO-8859-1.