谈谈html转义字符

偶尔会在数据中看到诸如' 这样的字符，特征如下

• 以&#开头，中间是一串数字，以；结尾
• 以&开头，中间一串字符，以；结尾

比如最常见的 或者等价的 

浏览器遇到这些转义符，会转义回来，但如何通过代码识别？ org.apache.commons.lang.StringEscapeUtils.unescapeHtml提供了很好的说明

1. 遇到上面的第一种情况，中间是数字的，直接将数字（unicode）转为char
2. 遇到第二情况，中间是字符，只能查映射表了,从映射表中找到字符对应的数字再转换为char 看看代码就一目了然了

• 看看HTML40如何定义的

static {
    HTML40 = new Entities();
    fillWithHtml40Entities(HTML40);
}

static void fillWithHtml40Entities(Entities entities) {
    entities.addEntities(BASIC_ARRAY);
    entities.addEntities(ISO8859_1_ARRAY);
    entities.addEntities(HTML40_ARRAY);
}

• 再看看BASIC_ARRAY、ISO8859_1_ARRAY、HTML40_ARRAY 分别是什么

BASIC_ARRAY

private static final String[][] BASIC_ARRAY = {{"quot", "34"}, // " - double-quote
    {"amp", "38"}, // & - ampersand
    {"lt", "60"}, // < - less-than
    {"gt", "62"}, // > - greater-than
};

ISO8859_1_ARRAY

static final String[][] ISO8859_1_ARRAY = {{"nbsp", "160"}, // non-breaking space
    {"iexcl", "161"}, // inverted exclamation mark
    {"cent", "162"}, // cent sign
    {"pound", "163"}, // pound sign
    {"curren", "164"}, // currency sign
    {"yen", "165"}, // yen sign = yuan sign
    {"brvbar", "166"}, // broken bar = broken vertical bar
    {"sect", "167"}, // section sign
    {"uml", "168"}, // diaeresis = spacing diaeresis
    {"copy", "169"}, // � - copyright sign
    {"ordf", "170"}, // feminine ordinal indicator
    {"laquo", "171"}, // left-pointing double angle quotation mark = left pointing guillemet
    {"not", "172"}, // not sign
    {"shy", "173"}, // soft hyphen = discretionary hyphen
    {"reg", "174"}, // � - registered trademark sign
    {"macr", "175"}, // macron = spacing macron = overline = APL overbar
    {"deg", "176"}, // degree sign
    {"plusmn", "177"}, // plus-minus sign = plus-or-minus sign
    {"sup2", "178"}, // superscript two = superscript digit two = squared
    {"sup3", "179"}, // superscript three = superscript digit three = cubed
    {"acute", "180"}, // acute accent = spacing acute
    {"micro", "181"}, // micro sign
    {"para", "182"}, // pilcrow sign = paragraph sign
    {"middot", "183"}, // middle dot = Georgian comma = Greek middle dot
    {"cedil", "184"}, // cedilla = spacing cedilla
    {"sup1", "185"}, // superscript one = superscript digit one
    {"ordm", "186"}, // masculine ordinal indicator
    {"raquo", "187"}, // right-pointing double angle quotation mark = right pointing guillemet
    {"frac14", "188"}, // vulgar fraction one quarter = fraction one quarter
    {"frac12", "189"}, // vulgar fraction one half = fraction one half
    {"frac34", "190"}, // vulgar fraction three quarters = fraction three quarters
    {"iquest", "191"}, // inverted question mark = turned question mark
    {"Agrave", "192"}, // � - uppercase A, grave accent
    {"Aacute", "193"}, // � - uppercase A, acute accent
    {"Acirc", "194"}, // � - uppercase A, circumflex accent
    {"Atilde", "195"}, // � - uppercase A, tilde
    {"Auml", "196"}, // � - uppercase A, umlaut
    {"Aring", "197"}, // � - uppercase A, ring
    {"AElig", "198"}, // � - uppercase AE
    {"Ccedil", "199"}, // � - uppercase C, cedilla
    {"Egrave", "200"}, // � - uppercase E, grave accent
    {"Eacute", "201"}, // � - uppercase E, acute accent
    {"Ecirc", "202"}, // � - uppercase E, circumflex accent
    {"Euml", "203"}, // � - uppercase E, umlaut
    {"Igrave", "204"}, // � - uppercase I, grave accent
    {"Iacute", "205"}, // � - uppercase I, acute accent
    {"Icirc", "206"}, // � - uppercase I, circumflex accent
    {"Iuml", "207"}, // � - uppercase I, umlaut
    {"ETH", "208"}, // � - uppercase Eth, Icelandic
    {"Ntilde", "209"}, // � - uppercase N, tilde
    {"Ograve", "210"}, // � - uppercase O, grave accent
    {"Oacute", "211"}, // � - uppercase O, acute accent
    {"Ocirc", "212"}, // � - uppercase O, circumflex accent
    {"Otilde", "213"}, // � - uppercase O, tilde
    {"Ouml", "214"}, // � - uppercase O, umlaut
    {"times", "215"}, // multiplication sign
    {"Oslash", "216"}, // � - uppercase O, slash
    {"Ugrave", "217"}, // � - uppercase U, grave accent
    {"Uacute", "218"}, // � - uppercase U, acute accent
    {"Ucirc", "219"}, // � - uppercase U, circumflex accent
    {"Uuml", "220"}, // � - uppercase U, umlaut
    {"Yacute", "221"}, // � - uppercase Y, acute accent
    {"THORN", "222"}, // � - uppercase THORN, Icelandic
    {"szlig", "223"}, // � - lowercase sharps, German
    {"agrave", "224"}, // � - lowercase a, grave accent
    {"aacute", "225"}, // � - lowercase a, acute accent
    {"acirc", "226"}, // � - lowercase a, circumflex accent
    {"atilde", "227"}, // � - lowercase a, tilde
    {"auml", "228"}, // � - lowercase a, umlaut
    {"aring", "229"}, // � - lowercase a, ring
    {"aelig", "230"}, // � - lowercase ae
    {"ccedil", "231"}, // � - lowercase c, cedilla
    {"egrave", "232"}, // � - lowercase e, grave accent
    {"eacute", "233"}, // � - lowercase e, acute accent
    {"ecirc", "234"}, // � - lowercase e, circumflex accent
    {"euml", "235"}, // � - lowercase e, umlaut
    {"igrave", "236"}, // � - lowercase i, grave accent
    {"iacute", "237"}, // � - lowercase i, acute accent
    {"icirc", "238"}, // � - lowercase i, circumflex accent
    {"iuml", "239"}, // � - lowercase i, umlaut
    {"eth", "240"}, // � - lowercase eth, Icelandic
    {"ntilde", "241"}, // � - lowercase n, tilde
    {"ograve", "242"}, // � - lowercase o, grave accent
    {"oacute", "243"}, // � - lowercase o, acute accent
    {"ocirc", "244"}, // � - lowercase o, circumflex accent
    {"otilde", "245"}, // � - lowercase o, tilde
    {"ouml", "246"}, // � - lowercase o, umlaut
    {"divide", "247"}, // division sign
    {"oslash", "248"}, // � - lowercase o, slash
    {"ugrave", "249"}, // � - lowercase u, grave accent
    {"uacute", "250"}, // � - lowercase u, acute accent
    {"ucirc", "251"}, // � - lowercase u, circumflex accent
    {"uuml", "252"}, // � - lowercase u, umlaut
    {"yacute", "253"}, // � - lowercase y, acute accent
    {"thorn", "254"}, // � - lowercase thorn, Icelandic
    {"yuml", "255"}, // � - lowercase y, umlaut
};

HTML40_ARRAY

static final String[][] HTML40_ARRAY = {
// <!-- Latin Extended-B -->
    {"fnof", "402"}, // latin small f with hook = function= florin, U+0192 ISOtech -->
    // <!-- Greek -->
    {"Alpha", "913"}, // greek capital letter alpha, U+0391 -->
    {"Beta", "914"}, // greek capital letter beta, U+0392 -->
    {"Gamma", "915"}, // greek capital letter gamma,U+0393 ISOgrk3 -->
    {"Delta", "916"}, // greek capital letter delta,U+0394 ISOgrk3 -->
    {"Epsilon", "917"}, // greek capital letter epsilon, U+0395 -->
    {"Zeta", "918"}, // greek capital letter zeta, U+0396 -->
    {"Eta", "919"}, // greek capital letter eta, U+0397 -->
    {"Theta", "920"}, // greek capital letter theta,U+0398 ISOgrk3 -->
    {"Iota", "921"}, // greek capital letter iota, U+0399 -->
    {"Kappa", "922"}, // greek capital letter kappa, U+039A -->
    {"Lambda", "923"}, // greek capital letter lambda,U+039B ISOgrk3 -->
    {"Mu", "924"}, // greek capital letter mu, U+039C -->
    {"Nu", "925"}, // greek capital letter nu, U+039D -->
    {"Xi", "926"}, // greek capital letter xi, U+039E ISOgrk3 -->
    {"Omicron", "927"}, // greek capital letter omicron, U+039F -->
    {"Pi", "928"}, // greek capital letter pi, U+03A0 ISOgrk3 -->
    {"Rho", "929"}, // greek capital letter rho, U+03A1 -->
    // <!-- there is no Sigmaf, and no U+03A2 character either -->
    {"Sigma", "931"}, // greek capital letter sigma,U+03A3 ISOgrk3 -->
    {"Tau", "932"}, // greek capital letter tau, U+03A4 -->
    {"Upsilon", "933"}, // greek capital letter upsilon,U+03A5 ISOgrk3 -->
    {"Phi", "934"}, // greek capital letter phi,U+03A6 ISOgrk3 -->
    {"Chi", "935"}, // greek capital letter chi, U+03A7 -->
    {"Psi", "936"}, // greek capital letter psi,U+03A8 ISOgrk3 -->
    {"Omega", "937"}, // greek capital letter omega,U+03A9 ISOgrk3 -->
    {"alpha", "945"}, // greek small letter alpha,U+03B1 ISOgrk3 -->
    {"beta", "946"}, // greek small letter beta, U+03B2 ISOgrk3 -->
    {"gamma", "947"}, // greek small letter gamma,U+03B3 ISOgrk3 -->
    {"delta", "948"}, // greek small letter delta,U+03B4 ISOgrk3 -->
    {"epsilon", "949"}, // greek small letter epsilon,U+03B5 ISOgrk3 -->
    {"zeta", "950"}, // greek small letter zeta, U+03B6 ISOgrk3 -->
    {"eta", "951"}, // greek small letter eta, U+03B7 ISOgrk3 -->
    {"theta", "952"}, // greek small letter theta,U+03B8 ISOgrk3 -->
    {"iota", "953"}, // greek small letter iota, U+03B9 ISOgrk3 -->
    {"kappa", "954"}, // greek small letter kappa,U+03BA ISOgrk3 -->
    {"lambda", "955"}, // greek small letter lambda,U+03BB ISOgrk3 -->
    {"mu", "956"}, // greek small letter mu, U+03BC ISOgrk3 -->
    {"nu", "957"}, // greek small letter nu, U+03BD ISOgrk3 -->
    {"xi", "958"}, // greek small letter xi, U+03BE ISOgrk3 -->
    {"omicron", "959"}, // greek small letter omicron, U+03BF NEW -->
    {"pi", "960"}, // greek small letter pi, U+03C0 ISOgrk3 -->
    {"rho", "961"}, // greek small letter rho, U+03C1 ISOgrk3 -->
    {"sigmaf", "962"}, // greek small letter final sigma,U+03C2 ISOgrk3 -->
    {"sigma", "963"}, // greek small letter sigma,U+03C3 ISOgrk3 -->
    {"tau", "964"}, // greek small letter tau, U+03C4 ISOgrk3 -->
    {"upsilon", "965"}, // greek small letter upsilon,U+03C5 ISOgrk3 -->
    {"phi", "966"}, // greek small letter phi, U+03C6 ISOgrk3 -->
    {"chi", "967"}, // greek small letter chi, U+03C7 ISOgrk3 -->
    {"psi", "968"}, // greek small letter psi, U+03C8 ISOgrk3 -->
    {"omega", "969"}, // greek small letter omega,U+03C9 ISOgrk3 -->
    {"thetasym", "977"}, // greek small letter theta symbol,U+03D1 NEW -->
    {"upsih", "978"}, // greek upsilon with hook symbol,U+03D2 NEW -->
    {"piv", "982"}, // greek pi symbol, U+03D6 ISOgrk3 -->
    // <!-- General Punctuation -->
    {"bull", "8226"}, // bullet = black small circle,U+2022 ISOpub -->
    // <!-- bullet is NOT the same as bullet operator, U+2219 -->
    {"hellip", "8230"}, // horizontal ellipsis = three dot leader,U+2026 ISOpub -->
    {"prime", "8242"}, // prime = minutes = feet, U+2032 ISOtech -->
    {"Prime", "8243"}, // double prime = seconds = inches,U+2033 ISOtech -->
    {"oline", "8254"}, // overline = spacing overscore,U+203E NEW -->
    {"frasl", "8260"}, // fraction slash, U+2044 NEW -->
    // <!-- Letterlike Symbols -->
    {"weierp", "8472"}, // script capital P = power set= Weierstrass p, U+2118 ISOamso -->
    {"image", "8465"}, // blackletter capital I = imaginary part,U+2111 ISOamso -->
    {"real", "8476"}, // blackletter capital R = real part symbol,U+211C ISOamso -->
    {"trade", "8482"}, // trade mark sign, U+2122 ISOnum -->
    {"alefsym", "8501"}, // alef symbol = first transfinite cardinal,U+2135 NEW -->
    // <!-- alef symbol is NOT the same as hebrew letter alef,U+05D0 although the
    // same glyph could be used to depict both characters -->
    // <!-- Arrows -->
    {"larr", "8592"}, // leftwards arrow, U+2190 ISOnum -->
    {"uarr", "8593"}, // upwards arrow, U+2191 ISOnum-->
    {"rarr", "8594"}, // rightwards arrow, U+2192 ISOnum -->
    {"darr", "8595"}, // downwards arrow, U+2193 ISOnum -->
    {"harr", "8596"}, // left right arrow, U+2194 ISOamsa -->
    {"crarr", "8629"}, // downwards arrow with corner leftwards= carriage return, U+21B5 NEW -->
    {"lArr", "8656"}, // leftwards double arrow, U+21D0 ISOtech -->
    // <!-- ISO 10646 does not say that lArr is the same as the 'is implied by'
    // arrow but also does not have any other character for that function.
    // So ? lArr canbe used for 'is implied by' as ISOtech suggests -->
    {"uArr", "8657"}, // upwards double arrow, U+21D1 ISOamsa -->
    {"rArr", "8658"}, // rightwards double arrow,U+21D2 ISOtech -->
    // <!-- ISO 10646 does not say this is the 'implies' character but does not
    // have another character with this function so ?rArr can be used for
    // 'implies' as ISOtech suggests -->
    {"dArr", "8659"}, // downwards double arrow, U+21D3 ISOamsa -->
    {"hArr", "8660"}, // left right double arrow,U+21D4 ISOamsa -->
    // <!-- Mathematical Operators -->
    {"forall", "8704"}, // for all, U+2200 ISOtech -->
    {"part", "8706"}, // partial differential, U+2202 ISOtech -->
    {"exist", "8707"}, // there exists, U+2203 ISOtech -->
    {"empty", "8709"}, // empty set = null set = diameter,U+2205 ISOamso -->
    {"nabla", "8711"}, // nabla = backward difference,U+2207 ISOtech -->
    {"isin", "8712"}, // element of, U+2208 ISOtech -->
    {"notin", "8713"}, // not an element of, U+2209 ISOtech -->
    {"ni", "8715"}, // contains as member, U+220B ISOtech -->
    // <!-- should there be a more memorable name than 'ni'? -->
    {"prod", "8719"}, // n-ary product = product sign,U+220F ISOamsb -->
    // <!-- prod is NOT the same character as U+03A0 'greek capital letter pi'
    // though the same glyph might be used for both -->
    {"sum", "8721"}, // n-ary summation, U+2211 ISOamsb -->
    // <!-- sum is NOT the same character as U+03A3 'greek capital letter sigma'
    // though the same glyph might be used for both -->
    {"minus", "8722"}, // minus sign, U+2212 ISOtech -->
    {"lowast", "8727"}, // asterisk operator, U+2217 ISOtech -->
    {"radic", "8730"}, // square root = radical sign,U+221A ISOtech -->
    {"prop", "8733"}, // proportional to, U+221D ISOtech -->
    {"infin", "8734"}, // infinity, U+221E ISOtech -->
    {"ang", "8736"}, // angle, U+2220 ISOamso -->
    {"and", "8743"}, // logical and = wedge, U+2227 ISOtech -->
    {"or", "8744"}, // logical or = vee, U+2228 ISOtech -->
    {"cap", "8745"}, // intersection = cap, U+2229 ISOtech -->
    {"cup", "8746"}, // union = cup, U+222A ISOtech -->
    {"int", "8747"}, // integral, U+222B ISOtech -->
    {"there4", "8756"}, // therefore, U+2234 ISOtech -->
    {"sim", "8764"}, // tilde operator = varies with = similar to,U+223C ISOtech -->
    // <!-- tilde operator is NOT the same character as the tilde, U+007E,although
    // the same glyph might be used to represent both -->
    {"cong", "8773"}, // approximately equal to, U+2245 ISOtech -->
    {"asymp", "8776"}, // almost equal to = asymptotic to,U+2248 ISOamsr -->
    {"ne", "8800"}, // not equal to, U+2260 ISOtech -->
    {"equiv", "8801"}, // identical to, U+2261 ISOtech -->
    {"le", "8804"}, // less-than or equal to, U+2264 ISOtech -->
    {"ge", "8805"}, // greater-than or equal to,U+2265 ISOtech -->
    {"sub", "8834"}, // subset of, U+2282 ISOtech -->
    {"sup", "8835"}, // superset of, U+2283 ISOtech -->
    // <!-- note that nsup, 'not a superset of, U+2283' is not covered by the
    // Symbol font encoding and is not included. Should it be, for symmetry?
    // It is in ISOamsn --> <!ENTITY nsub", "8836"},
    // not a subset of, U+2284 ISOamsn -->
    {"sube", "8838"}, // subset of or equal to, U+2286 ISOtech -->
    {"supe", "8839"}, // superset of or equal to,U+2287 ISOtech -->
    {"oplus", "8853"}, // circled plus = direct sum,U+2295 ISOamsb -->
    {"otimes", "8855"}, // circled times = vector product,U+2297 ISOamsb -->
    {"perp", "8869"}, // up tack = orthogonal to = perpendicular,U+22A5 ISOtech -->
    {"sdot", "8901"}, // dot operator, U+22C5 ISOamsb -->
    // <!-- dot operator is NOT the same character as U+00B7 middle dot -->
    // <!-- Miscellaneous Technical -->
    {"lceil", "8968"}, // left ceiling = apl upstile,U+2308 ISOamsc -->
    {"rceil", "8969"}, // right ceiling, U+2309 ISOamsc -->
    {"lfloor", "8970"}, // left floor = apl downstile,U+230A ISOamsc -->
    {"rfloor", "8971"}, // right floor, U+230B ISOamsc -->
    {"lang", "9001"}, // left-pointing angle bracket = bra,U+2329 ISOtech -->
    // <!-- lang is NOT the same character as U+003C 'less than' or U+2039 'single left-pointing angle quotation
    // mark' -->
    {"rang", "9002"}, // right-pointing angle bracket = ket,U+232A ISOtech -->
    // <!-- rang is NOT the same character as U+003E 'greater than' or U+203A
    // 'single right-pointing angle quotation mark' -->
    // <!-- Geometric Shapes -->
    {"loz", "9674"}, // lozenge, U+25CA ISOpub -->
    // <!-- Miscellaneous Symbols -->
    {"spades", "9824"}, // black spade suit, U+2660 ISOpub -->
    // <!-- black here seems to mean filled as opposed to hollow -->
    {"clubs", "9827"}, // black club suit = shamrock,U+2663 ISOpub -->
    {"hearts", "9829"}, // black heart suit = valentine,U+2665 ISOpub -->
    {"diams", "9830"}, // black diamond suit, U+2666 ISOpub -->

    // <!-- Latin Extended-A -->
    {"OElig", "338"}, // -- latin capital ligature OE,U+0152 ISOlat2 -->
    {"oelig", "339"}, // -- latin small ligature oe, U+0153 ISOlat2 -->
    // <!-- ligature is a misnomer, this is a separate character in some languages -->
    {"Scaron", "352"}, // -- latin capital letter S with caron,U+0160 ISOlat2 -->
    {"scaron", "353"}, // -- latin small letter s with caron,U+0161 ISOlat2 -->
    {"Yuml", "376"}, // -- latin capital letter Y with diaeresis,U+0178 ISOlat2 -->
    // <!-- Spacing Modifier Letters -->
    {"circ", "710"}, // -- modifier letter circumflex accent,U+02C6 ISOpub -->
    {"tilde", "732"}, // small tilde, U+02DC ISOdia -->
    // <!-- General Punctuation -->
    {"ensp", "8194"}, // en space, U+2002 ISOpub -->
    {"emsp", "8195"}, // em space, U+2003 ISOpub -->
    {"thinsp", "8201"}, // thin space, U+2009 ISOpub -->
    {"zwnj", "8204"}, // zero width non-joiner,U+200C NEW RFC 2070 -->
    {"zwj", "8205"}, // zero width joiner, U+200D NEW RFC 2070 -->
    {"lrm", "8206"}, // left-to-right mark, U+200E NEW RFC 2070 -->
    {"rlm", "8207"}, // right-to-left mark, U+200F NEW RFC 2070 -->
    {"ndash", "8211"}, // en dash, U+2013 ISOpub -->
    {"mdash", "8212"}, // em dash, U+2014 ISOpub -->
    {"lsquo", "8216"}, // left single quotation mark,U+2018 ISOnum -->
    {"rsquo", "8217"}, // right single quotation mark,U+2019 ISOnum -->
    {"sbquo", "8218"}, // single low-9 quotation mark, U+201A NEW -->
    {"ldquo", "8220"}, // left double quotation mark,U+201C ISOnum -->
    {"rdquo", "8221"}, // right double quotation mark,U+201D ISOnum -->
    {"bdquo", "8222"}, // double low-9 quotation mark, U+201E NEW -->
    {"dagger", "8224"}, // dagger, U+2020 ISOpub -->
    {"Dagger", "8225"}, // double dagger, U+2021 ISOpub -->
    {"permil", "8240"}, // per mille sign, U+2030 ISOtech -->
    {"lsaquo", "8249"}, // single left-pointing angle quotation mark,U+2039 ISO proposed -->
    // <!-- lsaquo is proposed but not yet ISO standardized -->
    {"rsaquo", "8250"}, // single right-pointing angle quotation mark,U+203A ISO proposed -->
    // <!-- rsaquo is proposed but not yet ISO standardized -->
    {"euro", "8364"}, // -- euro sign, U+20AC NEW -->
};

再扩展下

从前面可以看到转义字符中间的那段数字是unicode，那么 这个 转移字符可以 随便构造了 ，并不限于上面的定义，比如 中的unicode是20013，那么构造一个转移字符中,经过浏览器的渲染 就变回中了 ,虽然不必这么绕为了显示一个字符，但如果在一些不方便传输特殊字符的场景 就可以派上用途了