Android URL encode 空格处理
在应用程序开发过程中或多或少的需要通过HTTP、HTTPS协议与服务器交互。这其中一个常见的情况就是要求对URL进行encode处理，以保证中文、特殊字符的正确传递。不过这个小小的url encode方法有些时候也会酿成"血案"。
这里dontNeedEncoding的初始化是在静态代码块中完成的，如下：
看到这里，自己也就明白了，URLEncoder.encode(String s, String enc) 会将空格编码为“+”，原文：encoding a space to a + is done in the encode() method。当然，这是符合URL规范的，见RFC-1738。
今天在Android开发中做一个商户搜索功能时碰到一个问题，用户有些时候可以需要通过多个关键字进行搜索，多个关键字之间用空格分隔，客户端通过http get去请求服务器数据，有经验的开发人员都知道此时需要对URL进行编码，很自然的通过URLEncoder.encode(input, "UTF-8")方法进行URL编码，但是Server却不能正确识别。平时开发一直都是这么用啊，怎么就不行呢？困惑的同时也一步一步的debug代码，最后发现URL中的空格符（ASCII码是0x20），在经过java.net.URLEncoder类encode以后，会变成+号，而不是%20，从而造成了server不能正确识别。
然后自己去看了一眼JDK中 URLEncoder.encode(String s, String enc) 源码，这里给大家贴一下：
public static String encode(String s, String enc)
throws UnsupportedEncodingException {
boolean needToChange = false;
StringBuffer out = new StringBuffer(s.length());
Charset charset;
CharArrayWriter charArrayWriter = new CharArrayWriter();
if (enc == null)
throw new NullPointerException("charsetName");
try {
charset = Charset.forName(enc);
} catch (IllegalCharsetNameException e) {
throw new UnsupportedEncodingException(enc);
} catch (UnsupportedCharsetException e) {
throw new UnsupportedEncodingException(enc);
}
for (int i = 0; i < s.length();) {
int c = (int) s.charAt(i);
//System.out.println("Examining character: " + c);
if (dontNeedEncoding.get(c)) {
if (c == ' ') {
c = '+';
needToChange = true;
}
//System.out.println("Storing: " + c);
out.append((char)c);
i++;
} else {
// convert to external encoding before hex conversion
do {
charArrayWriter.write(c);
/*
* If this character represents the start of a Unicode
* surrogate pair, then pass in two characters. It's not
* clear what should be done if a bytes reserved in the
* surrogate pairs range occurs outside of a legal
* surrogate pair. For now, just treat it as if it were
* any other character.
*/
if (c >= 0xD800 && c <= 0xDBFF) {
/*
System.out.println(Integer.toHexString(c)
+ " is high surrogate");
*/
if ( (i+1) < s.length()) {
int d = (int) s.charAt(i+1);
/*
System.out.println("\tExamining "
+ Integer.toHexString(d));
*/
if (d >= 0xDC00 && d <= 0xDFFF) {
/*
System.out.println("\t"
+ Integer.toHexString(d)
+ " is low surrogate");
*/
charArrayWriter.write(d);
i++;
}
}
}
i++;
} while (i < s.length() && !dontNeedEncoding.get((c = (int) s.charAt(i))));
charArrayWriter.flush();
String str = new String(charArrayWriter.toCharArray());
byte[] ba = str.getBytes(charset);
for (int j = 0; j < ba.length; j++) {
out.append('%');
char ch = Character.forDigit((ba[j] >> 4) & 0xF, 16);
// converting to use uppercase letter as part of
// the hex value if ch is a letter.
if (Character.isLetter(ch)) {
ch -= caseDiff;
}
out.append(ch);
ch = Character.forDigit(ba[j] & 0xF, 16);
if (Character.isLetter(ch)) {
ch -= caseDiff;
}
out.append(ch);
}
charArrayWriter.reset();
needToChange = true;
}
}
return (needToChange? out.toString() : s);
}这里dontNeedEncoding的初始化是在静态代码块中完成的，如下：
static BitSet dontNeedEncoding;
static final int caseDiff = ('a' - 'A');
static String dfltEncName = null;
static {
/* The list of characters that are not encoded has been
* determined as follows:
*
* RFC 2396 states:
* -----
* Data characters that are allowed in a URI but do not have a
* reserved purpose are called unreserved. These include upper
* and lower case letters, decimal digits, and a limited set of
* punctuation marks and symbols.
*
* unreserved = alphanum | mark
*
* mark = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"
*
* Unreserved characters can be escaped without changing the
* semantics of the URI, but this should not be done unless the
* URI is being used in a context that does not allow the
* unescaped character to appear.
* -----
*
* It appears that both Netscape and Internet Explorer escape
* all special characters from this list with the exception
* of "-", "_", ".", "*". While it is not clear why they are
* escaping the other characters, perhaps it is safest to
* assume that there might be contexts in which the others
* are unsafe if not escaped. Therefore, we will use the same
* list. It is also noteworthy that this is consistent with
* O'Reilly's "HTML: The Definitive Guide" (page 164).
*
* As a last note, Intenet Explorer does not encode the "@"
* character which is clearly not unreserved according to the
* RFC. We are being consistent with the RFC in this matter,
* as is Netscape.
*
*/
dontNeedEncoding = new BitSet(256);
int i;
for (i = 'a'; i <= 'z'; i++) {
dontNeedEncoding.set(i);
}
for (i = 'A'; i <= 'Z'; i++) {
dontNeedEncoding.set(i);
}
for (i = '0'; i <= '9'; i++) {
dontNeedEncoding.set(i);
}
dontNeedEncoding.set(' '); /* encoding a space to a + is done
* in the encode() method */
dontNeedEncoding.set('-');
dontNeedEncoding.set('_');
dontNeedEncoding.set('.');
dontNeedEncoding.set('*');
dfltEncName = AccessController.doPrivileged(
new GetPropertyAction("file.encoding")
);
}看到这里，自己也就明白了，URLEncoder.encode(String s, String enc) 会将空格编码为“+”，原文：encoding a space to a + is done in the encode() method。当然，这是符合URL规范的，见RFC-1738。
那么要想将空格符编码为%20，怎么办呢？Android SDK为我们提供了 android.net.Uri类完成此功能，该类使用RFC-2396标准。语法如下：
| static String | encode(String s, String allow)
Encodes characters in the given string as '%'-escaped octets using the UTF-8 scheme.
|
| static String | encode(String s)
Encodes characters in the given string as '%'-escaped octets using the UTF-8 scheme.
|
示例代码如下：
//1、JDK中URLEncoder encode实现
String input = "肯德基 柳芳";
try {
String urlEncode = URLEncoder.encode(input, "UTF-8");
System.out.println("urlEncode="+urlEncode);
} catch (UnsupportedEncodingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//2、Android中Uri encode实现
String input = "肯德基 柳芳";
String urlEncode = Uri.encode(input);
System.out.println("urlEncode="+urlEncode);输出结果：
urlEncode=%E8%82%AF%E5%BE%B7%E5%9F%BA+%E6%9F%B3%E8%8A%B3
urlEncode=%E8%82%AF%E5%BE%B7%E5%9F%BA%20%E6%9F%B3%E8%8A%B3
关于Url、Uri的区别可以参考IBM developerworks上的一篇文章：分清 URI、URL 和 URN和百度百科上的介绍