Java Spring AWS: Comparison: Kernel-Space vs User-Space Copying

Thursday, 30 January 2025

Method	System Calls Used	User-Space Copying?	Zero-Copy?
Files.copy()	`read()` + `write()`	✅ Yes	❌ No
ByteBuffer (NIO)	`read()` + `write()`	✅ Yes	❌ No
Memory-Mapped File (mmap)	`mmap()` + `memcpy()` + `msync()`	🚫 No (but may cause page faults)	✅ Yes (after mapping)
transferTo() (sendfile)	`sendfile()`	🚫 No	✅ Yes
transferFrom()	`sendfile()`	🚫 No	✅ Yes
Buffered Streams (I/O)	`read()` + `write()`	✅ Yes	❌ No
Apache `FileUtils.copyFile()`	`read()/write()` OR `sendfile()`	✅ Yes (if using streams) / 🚫 No (if using `sendfile`)	✅ Possible
InputStream.transferTo()	`read()` + `write()`	✅ Yes	❌ No

Java Spring AWS