A servlet must implement the Async interface to support asynchronous processing. This interface provides methods for starting and completing asynchronous tasks.

The primary use of RequestDispatcher in servlets is for forwarding requests to another resource (servlet, JSP, or HTML) on the server.

The dispatch() method of AsyncContext is used to dispatch the request to another servlet during asynchronous processing.

The startAsync() method is called to initiate asynchronous processing in a servlet.

Asynchronous processing can extend the servlet lifecycle, as it allows tasks to continue beyond the usual request-response cycle, providing more flexibility in handling long-running operations.

The AsyncListener plays a role in monitoring asynchronous operations, allowing developers to respond to events such as the start and completion of asynchronous tasks or handling errors in asynchronous processing.

Asynchronous processing reduces thread blocking, allowing the server to handle more requests without waiting for each to complete, thus improving resource utilization.

The isAsyncSupported() method is used by a servlet to determine if an incoming request supports asynchronous processing.

The complete() method is used to complete the asynchronous process and send a response in asynchronous servlets.

For file download, the Content-Type in the servlet response should be set to application/octet-stream to indicate that the response contains binary data without any specific format.

The primary interface used for file upload in servlets is ServletFileUpload. It provides a simple way to parse and process HTTP requests with content type multipart/form-data.

The doPost() method is typically used for file uploads in a servlet as it is designed to handle HTTP POST requests, commonly used for submitting forms and uploading files.