Base64的java實作

Base64 使用US-ASCII子集的65個字元, 每個字元用6位表示 

是以"m"的Base64值為38, 二進制形式是 100110. 

對于文本串，編碼過程如下。例如"men": 

先轉成US-ASCII值. 

"m"十進制 109 

"e"十進制 101 

"n"十進制 110 

二進制 : 

m 01101101 

e 01100101 

n 01101110 

三個8位連起來是24位 

011011010110010101101110 

然後分成4個6位 

011011 010110 010101 101110 

現在得到4個值，十進制為 

27 22 21 46 

對應的 Base64 字元是 : 

b W V u 

編碼總是基于3個字元，進而産生4個Base64字元。 

如果隻剩1或2個字元，使用特殊字元"="補齊Base64的4字。 

如，編碼"me" 

01101101 01100101 

0110110101100101 

011011 010110 0101 

111111 (與，補足6位) 

011011 010110 010100 

b W U 

b W U = ("=" 補足4字元) 

于是 "bWU=" 就是"me"的Base64值. 

再如編碼 "m" 

01101101 

011011 01 

111111 

011011 010000 

b Q = = 

于是 "bQ==" 就是"m"的Base64值. 

值得注意的是，MIME規定一行最多76個字元.

import java.io.*; 

public class MIMEBase64 { 

static String BaseTable[] = { 

"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P", 

"Q","R","S","T","U","V","W","X","Y","Z","a","b","c","d","e","f", 

"g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v", 

"w","x","y","z","0","1","2","3","4","5","6","7","8","9","+","/" 

}; 

public static void encode(String filename, BufferedWriter out) { 

try { 

File f = new File(filename); 

FileInputStream fin = new FileInputStream(filename); 

// 讀檔案到BYTE數組 

byte bytes[] = new byte[(int)(f.length())]; 

int n = fin.read(bytes); 

if (n < 1) return; // 沒有内容 

byte buf[] = new byte[4]; // base64 字元數組 

int n3byt = n / 3; // 3 bytes 組數 

int nrest = n % 3; // 分組後剩餘 bytes 

int k = n3byt * 3; // 

int linelength = 0; // 行長 

int i = 0; // 指針 

// 3-bytes 分組 ... 

while ( i < k ) { 

buf[0] = (byte)(( bytes[i] & 0xFC) >> 2); 

buf[1] = (byte)(((bytes[i] & 0x03) << 4) | 

((bytes[i+1] & 0xF0) >> 4)); 

buf[2] = (byte)(((bytes[i+1] & 0x0F) << 2) | 

((bytes[i+2] & 0xC0) >> 6)); 

buf[3] = (byte)( bytes[i+2] & 0x3F); 

send(out, BaseTable[buf[0]]); 

send(out, BaseTable[buf[1]]); 

send(out, BaseTable[buf[2]]); 

send(out, BaseTable[buf[3]]); 

if ((linelength += 4) >= 76) { 

send(out, "/r/n"); 

linelength = 0; 

} 

i += 3; 

} 

// 處理尾部 ... 

if (nrest==2) { 

// 2 bytes left 

buf[0] = (byte)(( bytes[k] & 0xFC) >> 2); 

buf[1] = (byte)(((bytes[k] & 0x03) << 4) | 

((bytes[k+1] & 0xF0) >> 4)); 

buf[2] = (byte)(( bytes[k+1] & 0x0F) << 2); 

} 

else if (nrest==1) { 

// 1 byte left 

buf[0] = (byte)((bytes[k] & 0xFC) >> 2); 

buf[1] = (byte)((bytes[k] & 0x03) << 4); 

} 

if (nrest > 0) { 

// 發送尾部 

if ((linelength += 4) >= 76) send(out, "/r/n"); 

send(out, BaseTable[buf[0]]); 

send(out, BaseTable[buf[1]]); 

// 

if (nrest==2) { 

send(out, BaseTable[buf[2]]); 

} 

else { 

send(out, "="); 

} 

send(out, "="); 

} 

out.flush(); 

//這裡用到的send方法，請大家根據需要，自己寫。可以是把結果輸出到控制台，或發送郵件。 

} 

catch (Exception e) { 

e.printStackTrace(); 

} 

} 

}