Java Reference
In-Depth Information
Guessing MIME Media Types
If this were the best of all possible worlds, every protocol and every server would use
standard MIME types to correctly specify the type of file being transferred. Unfortu‐
nately, that's not the case. Not only do we have to deal with older protocols such as FTP
that predate MIME, but many HTTP servers that should use MIME don't provide MIME
headers at all or lie and provide headers that are incorrect (usually because the server
has been misconfigured). The
URLConnection
class provides two static methods to help
programs figure out the MIME type of some data; you can use these if the content type
just isn't available or if you have reason to believe that the content type you're given isn't
correct. The first of these is
URLConnection.guessContentTypeFromName()
:
public
static
String
guessContentTypeFromName
(
String
name
)
This method tries to guess the content type of an object based upon the extension in
the filename portion of the object's URL. It returns its best guess about the content type
as a
String
. This guess is likely to be correct; people follow some fairly regular con‐
ventions when thinking up filenames.
The guesses are determined by the
content-types.properties
file, normally located in the
jre/lib
directory. On Unix, Java may also look at the
mailcap
file to help it guess.
This method is not infallible by any means. For instance, it omits various XML appli‐
cations such as RDF (
.rdf
), XSL (
.xsl
), and so on that should have the MIME type
application/xml
. It also doesn't provide a MIME type for CSS stylesheets (
.css
). How‐
ever, it's a good start.
The second MIME type guesser method is
URLConnection.guessContentTypeFrom
Stream()
:
public
static
String
guessContentTypeFromStream
(
InputStream
in
)
This method tries to guess the content type by looking at the first few bytes of data in
the stream. For this method to work, the
InputStream
must support marking so that
you can return to the beginning of the stream after the first bytes have been read. Java
inspects the first 16 bytes of the
InputStream
, although sometimes fewer bytes are
needed to make an identification. These guesses are often not as reliable as the guesses
made by
guessContentTypeFromName()
. For example, an XML document that begins
with a comment rather than an XML declaration would be mislabeled as an HTML file.
This method should be used only as a last resort.
HttpURLConnection
The
java.net.HttpURLConnection
class is an abstract subclass of
URLConnection
; it
provides some additional methods that are helpful when working specifically with
http
URLs. In particular, it contains methods to get and set the request method, decide
