Content types also known as MIME type or media types are a two part identifier for file formats. The HTTP header Content-Type is responsible for telling the HTTP client or server what type of data is being sent.
Two main headers are involved when it comes to content types
- Accept - When a HTTP client requests data from a server it can send a comma separated list of media types. For example the headers value could be text/html, text/plain. This hints to the server what the client is looking for. Basically the client is asking the server to respond with text/html and if it cannot handle that try responding with text/plain. Whether or not the server can handle or will honor this is up to the server.
- Content-Type - The content type header tells the client or server what format the data is being transferred in. If the client asked for text/html and the server handled it properly the data should come back with the Content-Type: text/html header. This header is how your browser knows when to render the html vs just displaying raw text. This is also used for images / files.
$ curl -v http://www.google.com * Rebuilt URL to: http://www.google.com/ * Trying 126.96.36.199... * Connected to www.google.com (188.8.131.52) port 80 (#0) > GET / HTTP/1.1 > Host: www.google.com > User-Agent: curl/7.49.1 > Accept: */* > < HTTP/1.1 200 OK < Date: Tue, 10 Jan 2017 02:42:41 GMT < Expires: -1 < Cache-Control: private, max-age=0 < Content-Type: text/html; charset=ISO-8859-1
Notice the Accept: */*, This means accept any format. The Content-Type: text/html header tells the client the server responded with HTML.
- text/plain - Simple Raw text
- application/octet-stream - A binary file format often used to download files. If a browser gets this media type it automatically downloads the file instead of trying to display / render it.
- application/json - The JSON data format.
- application/pdf - PDF file.
- image/[png,jpeg,gif] - Standard image formats.
These are only some of the formats. As you can see the difference between rendering html vs showing raw text vs downloading a file is all controlled with a simple header. Handling content types with Undertow