Base64编码与解码的java实现

public class Base64 {

//码表

static private char[] alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="

.toCharArray();

//codes里存放的是值表

//可以这么理解根据码表中的值可以得到编码前的值

//如codes['A']=0,即为其索引值

static private byte[] codes = new byte[256];

static {

for (int i = 0; i < 256; i++)

codes[i] = -1;

for (int i = 'A'; i <= 'Z'; i++)

codes[i] = (byte) (i - 'A');

for (int i = 'a'; i <= 'z'; i++)

codes[i] = (byte) (26 + i - 'a');

for (int i = '0'; i <= '9'; i++)

codes[i] = (byte) (52 + i - '0');

codes['+'] = 62;

codes['/'] = 63;

}

/**

* 将原始数据编码为base64编码

*/

static public char[] encode(byte[] data) {

//对于原文的字节数量是3的倍数的时候,转换后的字符串理论上将要比原来的长1/3,即原来的4/3.

//当原文的字节数不是3的倍数的时候,不足的地方我们补全.

//故原文补全后的最大长度为 原字符串长度+2

//编码后的长度为(原字符串长度+2)*3/4

char[] out = new char[((data.length + 2) / 3) * 4];

//i指向原字符串,index指向编码后的字符串

for (int i = 0, index = 0; i < data.length; i += 3, index += 4) {

boolean second = false;//后第一字节是否存在

boolean third = false;//后第二字节是否存在

int val = (0xFF & (int) data[i]);

//左移8位

val <<= 8;

if ((i + 1) < data.length) {

val |= (0xFF & (int) data[i + 1]);

third = true;

}

val <<= 8;

if ((i + 2) < data.length) {

val |= (0xFF & (int) data[i + 2]);

second = true;

}

//0x3F为111111,val&0x3F就是取出最右边的六位,然后根据码表对其进行编码

//如果不存在则直接补=

out[index + 3] = alphabet[(second ? (val & 0x3F) : 64)];

val >>= 6;

out[index + 2] = alphabet[(third ? (val & 0x3F) : 64)];

val >>= 6;

out[index + 1] = alphabet[val & 0x3F];

val >>= 6;

out[index + 0] = alphabet[val & 0x3F];

}

return out;

}

/**

* 将base64编码的数据解码成原始数据

*/

static public byte[] decode(char[] data) {

int len = ((data.length + 3) / 4) * 3;

//去除补全位=后,原式数据的长度

if (data.length > 0 && data[data.length - 1] == '=')

--len;

if (data.length > 1 && data[data.length - 2] == '=')

--len;

byte[] out = new byte[len];

int shift = 0;

int accum = 0;

int index = 0;

for (int ix = 0; ix < data.length; ix++) {

//取得value值就为编码前的值

int value = codes[data[ix] & 0xFF];

if (value >= 0) {

accum <<= 6;

shift += 6;

accum |= value;

//有8位则输出

if (shift >= 8) {

shift -= 8;

//accum >> shift即为取出前8位

out[index++] = (byte) ((accum >> shift) & 0xff);

}

}

}

if (index != out.length)

throw new Error("miscalculated data length!");

return out;

}

public static void main(String[] args) throws Exception {

// 加密成base64

String strSrc = "123456";

String strEncode = new String(Base64.encode(strSrc.getBytes()));

System.out.println(strEncode);

String strDecode = new String(Base64.decode(strEncode.toCharArray()));

System.out.println(strDecode);

}

}