編碼原理
Hex編碼就是把一個8位的字節(jié)數(shù)據(jù)用兩個十六進制數(shù)展示出來，編碼時氧猬，將8位二進制碼重新分組成兩個4位的字節(jié)壁涎，其中一個字節(jié)的低4位是原字節(jié)的高四位，另一個字節(jié)的低4位是原數(shù)據(jù)的低4位山林，高4位都補0房待，然后輸出這兩個字節(jié)對應十六進制數(shù)字作為編碼。Hex編碼后的長度是源數(shù)據(jù)的2倍驼抹，Hex編碼的編碼表為
0 0 1 1 2 2 3 3
4 4 5 5 6 6 7 7
8 8 9 9 10 a 11 b
12 c 13 d 14 e 15 f
比如ASCII碼A的Hex編碼過程為
ASCII碼：A (65)
二進制碼：0100_0001
重新分組：0000_0100 0000_0001
十六進制： 4 1
Hex編碼：41
丁
e4b881
代碼實現(xiàn)
使用Bouncy Castle的實現(xiàn)
下面的代碼使用開源軟件Bouncy Castle實現(xiàn)Hex編解碼桑孩，使用的版本是1.56。
import java.io.UnsupportedEncodingException;
import org.bouncycastle.util.encoders.Hex;
public class HexTestBC {
public static void main(String[] args)
throws UnsupportedEncodingException {
// 編碼
byte data[] = "A".getBytes("UTF-8");
byte[] encodeData = Hex.encode(data);
String encodeStr = Hex.toHexString(data);
System.out.println(new String(encodeData, "UTF-8"));
System.out.println(encodeStr);
// 解碼
byte[] decodeData = Hex.decode(encodeData);
byte[] decodeData2 = Hex.decode(encodeStr);
System.out.println(new String(decodeData, "UTF-8"));
System.out.println(new String(decodeData2, "UTF-8"));
}
}
程序輸出
41
41
A
A
使用Apache Commons Codec實現(xiàn)
下面的代碼使用開源軟件Apache Commons Codec實現(xiàn)Hex編解碼框冀，使用的版本是1.10流椒。
import java.io.UnsupportedEncodingException;
import org.apache.commons.codec.DecoderException;
import org.apache.commons.codec.binary.Hex;
public class HexTestCC {
public static void main(String[] args)
throws UnsupportedEncodingException,
DecoderException {
// 編碼
byte data[] = "A".getBytes("UTF-8");
char[] encodeData = Hex.encodeHex(data);
String encodeStr = Hex.encodeHexString(data);
System.out.println(new String(encodeData));
System.out.println(encodeStr);
// 解碼
byte[] decodeData = Hex.decodeHex(encodeData);
System.out.println(new String(decodeData, "UTF-8"));
}
}
源碼分析
Bouncy Castle實現(xiàn)源碼分析
Bouncy Castle實現(xiàn)Hex編解碼的是org.bouncycastle.util.encoders.HexEncoder類，實現(xiàn)編碼時首先定義了一個編碼表
protected final byte[] encodingTable =
{
(byte)'0', (byte)'1', (byte)'2', (byte)'3',
(byte)'4', (byte)'5', (byte)'6', (byte)'7',
(byte)'8', (byte)'9', (byte)'a', (byte)'b',
(byte)'c', (byte)'d', (byte)'e', (byte)'f'
};
然后編碼的代碼是
public int encode(
byte[] data,
int off,
int length,
OutputStream out)
throws IOException
{
for (int i = off; i < (off + length); i++)
{
int v = data[i] & 0xff;
out.write(encodingTable[(v >>> 4)]);
out.write(encodingTable[v & 0xf]);
}
return length * 2;
}
解碼的實現(xiàn)稍微復雜一點明也，在HexEncoder的構(gòu)造方法中會調(diào)用initialiseDecodingTable建立解碼表宣虾，代碼如下
protected final byte[] decodingTable = new byte[128];
protected void initialiseDecodingTable()
{
for (int i = 0; i < decodingTable.length; i++)
{
decodingTable[i] = (byte)0xff;
}
for (int i = 0; i < encodingTable.length; i++)
{
decodingTable[encodingTable[i]] = (byte)i;
}
decodingTable['A'] = decodingTable['a'];
decodingTable['B'] = decodingTable['b'];
decodingTable['C'] = decodingTable['c'];
decodingTable['D'] = decodingTable['d'];
decodingTable['E'] = decodingTable['e'];
decodingTable['F'] = decodingTable['f'];
}
解碼表是一個長度是128的字節(jié)數(shù)組，每個位置代表對應的ASCII碼温数，該位置上的值表示該ASCII碼對應的二進制碼绣硝。具體到Hex的解碼表，第48-59個位置撑刺，即ASCII碼0-9的位置保存了數(shù)字0-9鹉胖，第65-70個位置，即ASCII碼A-F的位置保存了數(shù)字10-15，第97-102個位置甫菠，即ASCII碼a-f同樣保存了數(shù)字10-15败许。解碼表為
比如array[65] = A
-1 -1 -1 -1 -1 -1 -1 -1
-1 -1 -1 -1 -1 -1 -1 -1
-1 -1 -1 -1 -1 -1 -1 -1
-1 -1 -1 -1 -1 -1 -1 -1
-1 ! -1 " -1 # -1 $ -1 % -1 & -1 ' -1
( -1 ) -1 * -1 + -1 , -1 - -1 . -1 / -1
0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7
8 8 9 9 : -1 ; -1 < -1 = -1 > -1 ? -1
@ -1 A 10 B 11 C 12 D 13 E 14 F 15 G -1
H -1 I -1 J -1 K -1 L -1 M -1 N -1 O -1
P -1 Q -1 R -1 S -1 T -1 U -1 V -1 W -1
X -1 Y -1 Z -1 [ -1 \ -1 ] -1 ^ -1 _ -1
` -1 a 10 b 11 c 12 d 13 e 14 f 15 g -1
h -1 i -1 j -1 k -1 l -1 m -1 n -1 o -1
p -1 q -1 r -1 s -1 t -1 u -1 v -1 w -1
x -1 y -1 z -1 { -1 | -1 } -1 ~ -1 -1
解碼的過程實際上就是獲取連續(xù)兩個字節(jié)，取這兩個字節(jié)解碼表中對應的數(shù)值淑蔚，然后將這兩個數(shù)值拼接成一個8位二進制碼，作為解碼的輸出愕撰。源碼如下：
public int decode(
byte[] data,
int off,
int length,
OutputStream out)
throws IOException
{
byte b1, b2;
int outLen = 0;
int end = off + length;
while (end > off)
{
if (!ignore((char)data[end - 1]))
{
break;
}
end--;
}
int i = off;
while (i < end)
{
while (i < end && ignore((char)data[i]))
{
i++;
}
b1 = decodingTable[data[i++]];
while (i < end && ignore((char)data[i]))
{
i++;
}
b2 = decodingTable[data[i++]];
if ((b1 | b2) < 0)
{
throw new IOException("invalid
characters encountered in Hex data");
}
out.write((b1 << 4) | b2);
outLen++;
}
return outLen;
}
其中ignore方法的代碼如下刹衫，解碼時會忽略首、尾及中間的空白搞挣。
private static boolean ignore(
char c)
{
return c == '\n' || c =='\r' || c == '\t' || c == ' ';
}
示例代碼中的Hex工具類持有HexEncoder的實例带迟，并通過ByteArrayOutputStream類實現(xiàn)對byte數(shù)組的操作，此外不再贅述囱桨。
public class Hex
{
private static final Encoder encoder = new HexEncoder();
public static byte[] encode(
byte[] data,
int off,
int length)
{
ByteArrayOutputStream bOut = new ByteArrayOutputStream();
try
{
encoder.encode(data, off, length, bOut);
}
catch (Exception e)
{
throw new EncoderException("exception encoding Hex string: "
+ e.getMessage(), e);
}
return bOut.toByteArray();
}
......
}
Apache Commons Codec實現(xiàn)源碼分析
Apache Commons Codec實現(xiàn)Hex編碼的步驟是直接創(chuàng)建一個兩倍源數(shù)據(jù)長度的字符數(shù)組仓犬，然后分別將源數(shù)據(jù)的每個字節(jié)轉(zhuǎn)換成兩個字節(jié)放到目標字節(jié)數(shù)組中，Apache Commons Codec支持設(shè)置的要轉(zhuǎn)換為大寫還是小寫舍肠。
private static final char[] DIGITS_LOWER =
{'0', '1', '2', '3', '4', '5', '6', '7',
'8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
private static final char[] DIGITS_UPPER =
{'0', '1', '2', '3', '4', '5', '6', '7',
'8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
public static char[] encodeHex(final byte[] data) {
return encodeHex(data, true);
}
public static char[] encodeHex(final byte[] data,
final boolean toLowerCase) {
return encodeHex(data,
toLowerCase ? DIGITS_LOWER : DIGITS_UPPER);
}
protected static char[] encodeHex(final byte[] data,
final char[] toDigits) {
final int l = data.length;
final char[] out = new char[l << 1];
// two characters form the hex value.
for (int i = 0, j = 0; i < l; i++) {
out[j++] = toDigits[(0xF0 & data[i]) >>> 4];
out[j++] = toDigits[0x0F & data[i]];
}
return out;
}
Apache Commons Codec實現(xiàn)Hex解碼的步驟是首先創(chuàng)建一個原字符串一半長度的字節(jié)數(shù)組搀继，然后依次將兩個連續(xù)的十六進制數(shù)轉(zhuǎn)換為一個字節(jié)數(shù)據(jù)，轉(zhuǎn)換時使用了JDK的Character.digit方法翠语。
public static byte[] decodeHex(final char[] data)
throws DecoderException {
final int len = data.length;
if ((len & 0x01) != 0) {
throw new DecoderException("Odd number of characters.");
}
final byte[] out = new byte[len >> 1];
// two characters form the hex value.
for (int i = 0, j = 0; j < len; i++) {
int f = toDigit(data[j], j) << 4;
j++;
f = f | toDigit(data[j], j);
j++;
out[i] = (byte) (f & 0xFF);
}
return out;
}
protected static int toDigit(final char ch, final int index)
throws DecoderException {
final int digit = Character.digit(ch, 16);
if (digit == -1) {
throw new DecoderException(""
+ "Illegal hexadecimal character "
+ ch + " at index " + index);
}
return digit;
}
