I have a JAX-RS REST-Service that produces a CSV file and streams it back to the browser. Everything is set to UTF-8, so also the file I download via the browser is a valid UTF-8 File (without a BOM) that shows me valid, readable UTF-8 umlauts, etc. in Notepad++, Sublime, etc..
Opening such a file in Excel though leads to unreadable umlauts, etc. since Excel apparently tries to open it with another charset (CP-1252, I guess, but that doesn't really matter).
Saving the file with a BOM via Notepad++ and re-opening it in Excel works nicely. Seems like the detection of a BOM is the only way that Excel uses to detect UTF-8. Anyways - I thought that adding a BOM could help...
Did that. Same result. After a while, I figured out that the BOM gets removed under some circumstances: If I added any character right before the BOM, I could see the BOM in my Hex-Editor. After removal of that character, the BOM wouldn't be there anymore.
When I went on and downloaded the file via cURL I was really surprised. The BOM was there! Up until that I thought it might have to do with my application, Content-Types, Encodigs, HTTP Headers, etc. - but all of them seem to be fine.
Now, after hours of trying out different things, any ideas on how I can tell the browser to keep the BOM? Is there any HTTP Header I could set? Since Chrome, Internet Explorer, Edge, Firefox all remove the BOM, this sounds a little bit like a browser convention to me...
Many thanks for your highly appreciated help!
EDIT: Thanks to sideshowbarker answer, I found a workaround by prepending two BOMs to the content, so there will be a BOM remaining after the first BOM gets removed by the browser.
I think this might be because the relevant specs require the BOM to be stripped out, and that’s what browsers do. That is, browsers conform to the requirements of the UTF-8 decode algorithm in the Encoding spec, which is this:
To UTF-8 decode a byte stream stream, run these steps:
Let buffer be an empty byte sequence.
Read three bytes from stream into buffer.
If buffer does not match 0xEF 0xBB 0xBF, prepend buffer to stream.
Let output be a code point stream.
Run UTF-8’s decoder with stream and output.
Return output.
Step 3 is what causes the BOM to be stripped.
Given the Encoding spec requires that, I think there’s no way to tell browsers to keep the BOM.
来源：https://stackoverflow.com/questions/42715966/preserve-utf-8-bom-in-browser-downloads