What Is the File Extension Do?by Chris Davis
The file extension DO (.do) is used for Java servlets in computer and Web applications. They are most commonly seen in the address bar of a Web browser, but they are also used internally by applications and thus are not seen at all.
Early on, performing most actions on a website could not be done without a page refresh. For example, a website that allowed the user to enter two numbers and click an "Add" button would have to reload the page to display the result. This was slow and frustrating compared to computer applications, which did not have this limitation.
Asynchronous Web Applications
To bridge this gap, technology for asynchronous Web applications was developed. Asynchronous applications use programming languages to allow only the relevant part of a Web page to be refreshed when submitting a query.
Java is a very popular programming language that was developed by Sun Microsystems, now owned by Oracle Corporation. It is used both for computer applications and Web applications.
When a Java program is embedded in another application, such as in a Web page displayed by a Web browser, it is known as a Java applet.
A servlet is a Java class that uses a specific protocol allowing it to respond to HTTP requests. Servlets have the file extension DO. They are often used only as a part of a larger Java applet or application and are not directly seen by users, but some Web or computer applications may use a servlet as a standalone.