To add an AsyncListener to an AsyncContext, use the addAsyncListener() method.

In asynchronous servlets, the complete() method of AsyncContext is used to send the response to the client.

To retrieve an AsyncContext object in a servlet, you use the getAsyncContext() method of the HttpServletRequest.

The HTTP status code 200 OK is typically sent by an HTTP servlet to indicate a successful response. This status code signals that the request has been successfully processed, and the server is returning the requested resource.

To handle multiple file uploads, a servlet may use the JavaBeans Activation Framework (JAF) API to work with various types of data, including files.

For secure file uploads, it's essential to validate both the file type and content to ensure the uploaded file is safe and meets the required criteria.

The response header Content-Disposition is crucial for indicating a file download rather than display.

To read uploaded files in a servlet, the doPost() method is commonly used.

For a file upload, the request type must be POST.

The Content-Disposition header plays a crucial role in file downloading by prompting the browser to download the file instead of displaying it. It specifies the disposition of the content, ensuring a proper download experience for users.

To handle large file uploads efficiently, a servlet can use streaming APIs to process the file in smaller chunks, avoiding memory overflow. Increasing heap memory may not be a scalable solution, and splitting the file into chunks is a better approach.

When implementing file upload in servlets, it's crucial to consider security concerns such as validating file types, checking file size, and securing the file storage location to prevent vulnerabilities.