SSI (Server Side Includes) are directives that are placed in HTML pages, and evaluated on the server while the pages are being served. They let you add dynamically generated content to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology.

If documents containing server-side include directives are given the extension .shtml, Apache parse them and assign the resulting document the mime type of text/html.

At the earlier days of Servlets, Java Web Server used a server-side-include servlet to implement .shtml, via a URI mapping. Now JSP is
overwhelming, and few people use .shtml in a Java site. Also, few people use servlet chaining.