Server-Side Includes (SSI) is a technology in the Java Servlet API that allows dynamic content to be included in a static HTML page. With SSI, you can insert the contents of one file into another file at the time of the request, before the response is sent back to the client. This allows you to create dynamic web pages that contain elements such as headers, footers, navigation menus, or advertising banners that are shared across multiple pages.

To use SSI in a Java web application, you need to enable SSI in the web server configuration, and then use the SSI directives in your HTML pages. The SSI directives are simple HTML comments that instruct the server to include or execute dynamic content at a specific point in the page.

Here are some common SSI directives:

1. <!--#include file="filename" --> - This directive includes the contents of the specified file in the current page.

2. <!--#echo var="variable_name" --> - This directive echoes the value of the specified server variable in the current page. Server variables can include information such as the date and time, the user agent, or the request URI.

3. <!--#set var="variable_name" value="variable_value" --> - This directive sets the value of the specified server variable to the specified value.

4. <!--#if expr="expression" --> - This directive executes the enclosed content if the specified expression is true.

5. <!--#else --> - This directive provides an alternative content to be displayed if the preceding if directive is not true.

6. <!--#endif --> - This directive ends the enclosed content of an if directive.

Server-Side Includes can be a powerful tool for creating dynamic web pages in a simple way, without the need for complex programming. However, it is important to use SSI with caution, as it can introduce security risks such as cross-site scripting (XSS) or code injection if not properly sanitized.