alibaba/easyexcel ? reverse-engineered prompt

Build me a simple Java Excel library that can read and write both xls and xlsx files without blowing up memory, even for very large spreadsheets. I want an easy to use API where I can map rows to plain Java objects, read files one row at a time with a callback, and write data out to a workbook with just a few lines of code.

Please include basic examples for reading, writing, and using it in a web app for upload and download. Make it feel lightweight and practical, with good defaults so a beginner can get started quickly. It should handle big files efficiently, work well with common Excel use cases, and come with clear demo code and docs so I can copy and run it right away.

If you need to check current behavior or API details, look up the latest docs online first.