static BitSet dontNeedEncoding;

static {
    //創(chuàng)建一個(gè)容量為256的位集合
    dontNeedEncoding = new BitSet(256);
    int i;
    //循環(huán)a-z，將其字符值（不是字符串值）存入
    for (i = 'a'; i <= 'z'; i++) {
        dontNeedEncoding.set(i);
    }
    //同上
    for (i = 'A'; i <= 'Z'; i++) {
        dontNeedEncoding.set(i);
    }
    //同上
    for (i = '0'; i <= '9'; i++) {
        dontNeedEncoding.set(i);
    }
    //在這個(gè)地方，官方特意做了說(shuō)明
    //空格將被編碼為加號(hào)
    dontNeedEncoding.set(' '); /* encoding a space to a + is done
                                * in the encode() method */
    dontNeedEncoding.set('-');
    dontNeedEncoding.set('_');
    dontNeedEncoding.set('.');
    dontNeedEncoding.set('*');
    //默認(rèn)編碼格式
    dfltEncName = AccessController.doPrivileged(
        new GetPropertyAction("file.encoding")
    );
}

public static String encode(String s, String enc)
    throws UnsupportedEncodingException {
//標(biāo)記是否有字符被轉(zhuǎn)碼了，如果沒(méi)有，就直接返回原始字符串
//如果有，就返回out的結(jié)果
    boolean needToChange = false;
    //存放轉(zhuǎn)碼后的結(jié)果
    StringBuffer out = new StringBuffer(s.length());
    Charset charset;
    CharArrayWriter charArrayWriter = new CharArrayWriter();
//如果未指定編碼，拋出異常
    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);
    }
//遍歷要編碼的字符串
    for (int i = 0; i < s.length();) {
        //取出指定位置的字符
        int c = (int) s.charAt(i);
        if (dontNeedEncoding.get(c)) {
            //這里，判斷如果字符是空格，就轉(zhuǎn)為“+”
            if (c == ' ') {
                c = '+';
                needToChange = true;
            }
            out.append((char)c);
            i++;
        } else {
            // convert to external encoding before hex conversion
            do {
                charArrayWriter.write(c);
                /*
                    * 下面就是轉(zhuǎn)碼unicode相關(guān)代碼
                    */
                if (c >= 0xD800 && c <= 0xDBFF) {
                    ......
                }
                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++) {
                //添加百分號(hào)，比如a'a轉(zhuǎn)為a%27a
                out.append('%');
                ......
            }
            charArrayWriter.reset();
            needToChange = true;
        }
    }
 
    return (needToChange? out.toString() : s);
}

    /**
     * Returns true if the given character is allowed.
     *
     * @param c character to check
     * @param allow characters to allow
     * @return true if the character is allowed or false if it should be
     *  encoded
     */
    private static boolean isAllowed(char c, String allow) {
        return (c >= 'A' && c <= 'Z')
                || (c >= 'a' && c <= 'z')
                || (c >= '0' && c <= '9')
                || "_-!.~'()*".indexOf(c) != NOT_FOUND
                || (allow != null && allow.indexOf(c) != NOT_FOUND);
    }