A way of armouring, i.e. sending awkward characters. Browsers use url-encoding on HTTP GET and PUT requests to
the server. They embed data in the URLs. Url-encoding is also used by the url-encoded
and x-www-form-urlencoded mime types.
You see url-encoding every time you do a Google search e.g.
http://www.google.com/search?client=opera&rls=en&q=%22rabbits%22%2BEaster+eggs
&sourceid=opera&ie=utf-8&oe=utf-8
The request url-encodes my query:
"rabbits"+Easter eggs
There are two flavours of urlencoding, one used in URLs, and one used in forms.
URL Encoding
Ironically, despite the name, you are not supposed to java.net. URLEncoder. encode/decode to handle encoding URLs or GET parameters.
It will work most of the time however. Unfortunately, the URL class provides no
escaping features. You must use the URI class and convert the URL with toURL(). The encoding algorithm is described in
RFC 3986.
To decode a String, you just feed it to the single-argument URI constructor, then extract the various fields with methods like
URI.getPath().
Properly speaking, you should not see bare & in URLs; they should be pre-encoded
as &. I wrote a utility called Amper
that processes *.html files to make this correction.
Form Encoding
Form url-encoding/decoding is handled by java.net.URLEncoder
. encode/decode. This is only intended for String data
with a few awkward characters in it, not heavy-duty binary. Encodings you will likely use in conjunction with
URLEncoder include ISO-8859-1 (Latin-1), UTF-8 and windows-1250.
When you use URLEncode.encode
you must specify an 8-bit encoding such as UTF-8 or ISO-8859-1. The algorithm first converts to 8-bit characters then encodes. Thus the encoded
string depends on the encoding you choose. The encoding is not embedded in the output. You just have to know what
it is when an incoming encoding url-encoded string arrives.
java.net.URLEncoder uses the following set of characters to convert 8-bit data into
printable characters :a to z, A to Z, 0 to 9, -, ., *,
and _. It works like this:
- The alphanumeric characters a to z, A to Z, 0 to 9 remain the same.
- The special characters ., -, *, and _ remain the same.
- The space character is converted into a plus sign +.
- All other 16-bit characters are unsafe and are first converted into one or more bytes using some encoding scheme. Then each byte is represented by the 3-character string
%FF, where FF is the two-digit hexadecimal representation of the byte, e. g. $
→ %24, % → %25, & → %26, / → %2F, : → %3A, = ⇒ $3D, ? ⇒ %3F. You must
URLEncode only once. If you URLEncode something already URLEncoded you will get gibberish.
In the best case, your message is the same size as the original. In a pathological case, your message can balloon
up to three times the original size.
Learning More
Sun’s Javadoc on
URLEncoder class : available:
Sun’s Javadoc on
URLDecoder class : available:
Sun’s Javadoc on
URL class : available:
Sun’s Javadoc on
URI class : available:
Sun’s Javadoc on
URI.toString : available:
Sun’s Javadoc on
URI.toASCIIString : available: