Here you can find the source of decode(String s)
| Parameter | Description |
|---|---|
| s | the string to decode (decoded in default encoding) |
public static byte[] decode(String s) throws Base64DecoderException
// Licensed under the Apache License, Version 2.0 (the "License"); public class Main{ /** The equals sign (=) as a byte. */ private final static byte EQUALS_SIGN = (byte) '='; /** The new line character (\n) as a byte. */ private final static byte NEW_LINE = (byte) '\n'; /**// w w w. j a va 2 s . c o m * Translates a Base64 value to either its 6-bit reconstruction value * or a negative number indicating some other meaning. **/ private final static byte[] DECODABET = { -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 0 - 8 -5, -5, // Whitespace: Tab and Linefeed -9, -9, // Decimal 11 - 12 -5, // Whitespace: Carriage Return -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26 -9, -9, -9, -9, -9, // Decimal 27 - 31 -5, // Whitespace: Space -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 42 62, // Plus sign at decimal 43 -9, -9, -9, // Decimal 44 - 46 63, // Slash at decimal 47 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine -9, -9, -9, // Decimal 58 - 60 -1, // Equals sign at decimal 61 -9, -9, -9, // Decimal 62 - 64 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N' 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z' -9, -9, -9, -9, -9, -9, // Decimal 91 - 96 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm' 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z' -9, -9, -9, -9, -9 // Decimal 123 - 127 /* ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 128 - 139 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 140 - 152 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 153 - 165 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 166 - 178 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 179 - 191 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 192 - 204 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 205 - 217 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 218 - 230 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9, // Decimal 231 - 243 -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9 // Decimal 244 - 255 */ }; private final static byte WHITE_SPACE_ENC = -5; private final static byte EQUALS_SIGN_ENC = -1; /** * Decodes data from Base64 notation. * * @param s the string to decode (decoded in default encoding) * @return the decoded data * @since 1.4 */ public static byte[] decode(String s) throws Base64DecoderException { byte[] bytes = s.getBytes(); return decode(bytes, 0, bytes.length); } /** * Decodes Base64 content in byte array format and returns * the decoded byte array. * * @param source The Base64 encoded data * @return decoded data * @since 1.3 * @throws Base64DecoderException */ public static byte[] decode(byte[] source) throws Base64DecoderException { return decode(source, 0, source.length); } /** * Decodes Base64 content in byte array format and returns * the decoded byte array. * * @param source the Base64 encoded data * @param off the offset of where to begin decoding * @param len the length of characters to decode * @return decoded data * @since 1.3 * @throws Base64DecoderException */ public static byte[] decode(byte[] source, int off, int len) throws Base64DecoderException { return decode(source, off, len, DECODABET); } /** * Decodes Base64 content using the supplied decodabet and returns * the decoded byte array. * * @param source the Base64 encoded data * @param off the offset of where to begin decoding * @param len the length of characters to decode * @param decodabet the decodabet for decoding Base64 content * @return decoded data */ public static byte[] decode(byte[] source, int off, int len, byte[] decodabet) throws Base64DecoderException { int len34 = len * 3 / 4; byte[] outBuff = new byte[2 + len34]; // Upper limit on size of output int outBuffPosn = 0; byte[] b4 = new byte[4]; int b4Posn = 0; int i = 0; byte sbiCrop = 0; byte sbiDecode = 0; for (i = 0; i < len; i++) { sbiCrop = (byte) (source[i + off] & 0x7f); // Only the low seven bits sbiDecode = decodabet[sbiCrop]; if (sbiDecode >= WHITE_SPACE_ENC) { // White space Equals sign or better if (sbiDecode >= EQUALS_SIGN_ENC) { // An equals sign (for padding) must not occur at position 0 or 1 // and must be the last byte[s] in the encoded value if (sbiCrop == EQUALS_SIGN) { int bytesLeft = len - i; byte lastByte = (byte) (source[len - 1 + off] & 0x7f); if (b4Posn == 0 || b4Posn == 1) { throw new Base64DecoderException( "invalid padding byte '=' at byte offset " + i); } else if ((b4Posn == 3 && bytesLeft > 2) || (b4Posn == 4 && bytesLeft > 1)) { throw new Base64DecoderException( "padding byte '=' falsely signals end of encoded value " + "at offset " + i); } else if (lastByte != EQUALS_SIGN && lastByte != NEW_LINE) { throw new Base64DecoderException( "encoded value has invalid trailing byte"); } break; } b4[b4Posn++] = sbiCrop; if (b4Posn == 4) { outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet); b4Posn = 0; } } } else { throw new Base64DecoderException( "Bad Base64 input character at " + i + ": " + source[i + off] + "(decimal)"); } } // Because web safe encoding allows non padding base64 encodes, we // need to pad the rest of the b4 buffer with equal signs when // b4Posn != 0. There can be at most 2 equal signs at the end of // four characters, so the b4 buffer must have two or three // characters. This also catches the case where the input is // padded with EQUALS_SIGN if (b4Posn != 0) { if (b4Posn == 1) { throw new Base64DecoderException( "single trailing character at offset " + (len - 1)); } b4[b4Posn++] = EQUALS_SIGN; outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet); } byte[] out = new byte[outBuffPosn]; System.arraycopy(outBuff, 0, out, 0, outBuffPosn); return out; } /** * Decodes four bytes from array <var>source</var> * and writes the resulting bytes (up to three of them) * to <var>destination</var>. * The source and destination arrays can be manipulated * anywhere along their length by specifying * <var>srcOffset</var> and <var>destOffset</var>. * This method does not check to make sure your arrays * are large enough to accommodate <var>srcOffset</var> + 4 for * the <var>source</var> array or <var>destOffset</var> + 3 for * the <var>destination</var> array. * This method returns the actual number of bytes that * were converted from the Base64 encoding. * * * @param source the array to convert * @param srcOffset the index where conversion begins * @param destination the array to hold the conversion * @param destOffset the index where output will be put * @param decodabet the decodabet for decoding Base64 content * @return the number of decoded bytes converted * @since 1.3 */ private static int decode4to3(byte[] source, int srcOffset, byte[] destination, int destOffset, byte[] decodabet) { // Example: Dk== if (source[srcOffset + 2] == EQUALS_SIGN) { int outBuff = ((decodabet[source[srcOffset]] << 24) >>> 6) | ((decodabet[source[srcOffset + 1]] << 24) >>> 12); destination[destOffset] = (byte) (outBuff >>> 16); return 1; } else if (source[srcOffset + 3] == EQUALS_SIGN) { // Example: DkL= int outBuff = ((decodabet[source[srcOffset]] << 24) >>> 6) | ((decodabet[source[srcOffset + 1]] << 24) >>> 12) | ((decodabet[source[srcOffset + 2]] << 24) >>> 18); destination[destOffset] = (byte) (outBuff >>> 16); destination[destOffset + 1] = (byte) (outBuff >>> 8); return 2; } else { // Example: DkLE int outBuff = ((decodabet[source[srcOffset]] << 24) >>> 6) | ((decodabet[source[srcOffset + 1]] << 24) >>> 12) | ((decodabet[source[srcOffset + 2]] << 24) >>> 18) | ((decodabet[source[srcOffset + 3]] << 24) >>> 24); destination[destOffset] = (byte) (outBuff >> 16); destination[destOffset + 1] = (byte) (outBuff >> 8); destination[destOffset + 2] = (byte) (outBuff); return 3; } } }