都市淘沙者

用Base64编码与解码

    转换后，我们用一个码表来得到我们想要的字符串（也就是最终的Base64编码），这个表是这样的：（摘自RFC2045）


                            Table 1: The Base64 Alphabet

      Value Encoding  Value Encoding  Value Encoding  Value Encoding
           0 A                  17 R                 34 i                  51 z
           1 B                  18 S                 35 j                  52 0
           2 C                  19 T                 36 k                 53 1
           3 D                  20 U                 37 l                  54 2
           4 E                  21 V                 38 m                55 3
           5 F                  22 W                 39 n                56 4
           6 G                  23 X                  40 o                57 5
           7 H                  24 Y                 41 p                 58 6
           8 I                   25 Z                 42 q                 59 7
           9 J                   26 a                 43 r                  60 8
          10 K                 27 b                 44 s                  61 9
          11 L                 28 c                  45 t                  62 +
          12 M                29 d                  46 u                  63 /
          13 N                30 e                  47 v
          14 O                31 f                  48 w                (pad) =
          15 P                32 g                  49 x
          16 Q                33 h                  50 y


让我们再来看一个实际的例子，加深印象！

所以上面的24位编码，编码后的Base64值为 rbp2
解码同理，把 rbq2 的二进制位连接上再重组得到三个8位值，得出原码。
（解码只是编码的逆过程，在此我就不多说了，另外有关MIME的RFC还是有很多的，如果需要详细情况请自行查找。）

下面的Base64编码/解码程序来自网上。

一、用SUN的API解码和编码

import java.io.*;
import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;
public class Base64
{
  public static void main(String[] args) {
    if (args.length == 2) {
      String string = Base64.encode(args[0]);
      Base64.decode(string, args[1]);
    } else
      System.out.println("usage: java Base64 inputFile outputFile");
  }
  public static String encode(String fileName) {
    String string = null;
    try {
      InputStream in = new FileInputStream(fileName);
      // in.available()返回文件的字节长度
      byte[] bytes = new byte[in.available()];
      // 将文件中的内容读入到数组中
      in.read(bytes);
      string = new BASE64Encoder().encode(bytes);
      in.close();
    } catch (FileNotFoundException fe) {
      fe.printStackTrace();
    } catch(IOException ioe) {
      ioe.printStackTrace();
    }
    return string;
  }
  public static void decode(String string, String fileName) {
    try {
      // 解码，然后将字节转换为文件
      byte[] bytes = new BASE64Decoder().decodeBuffer(string);
      ByteArrayInputStream in = new ByteArrayInputStream(bytes);
      byte[] buffer = new byte[1024];
      FileOutputStream out = new FileOutputStream(fileName);
      int bytesum = 0;
      int byteread = 0;
      while ((byteread = in.read(buffer)) != -1) {
        bytesum += byteread;
        out.write(buffer, 0, byteread); 
      }
    } catch(IOException ioe) {
      ioe.printStackTrace();
    }
  }
}

/* @author CuCuChen 
 * @version $Id$ 
 */ 
import java.io.*; 
class Base64Helper { 

//从文本文件对象中读取内容并转换为字符数组 
public static char[] readChars(File file) 
{ 
     CharArrayWriter caw = new CharArrayWriter(); 
     try 
     { 
         Reader fr = new FileReader(file); 
        Reader in = new BufferedReader(fr); 
         int count = 0; 
         char[] buf = new char[16384]; 
        while ((count=in.read(buf)) != -1) { 
             if (count > 0) caw.write(buf, 0, count); 
         } 
         in.close(); 
     } 
     catch (Exception e) { e.printStackTrace(); } 
     return caw.toCharArray(); 
} 

//从字符串对象中读取内容并转换为字符数组 
public static char[] readChars(String string) 
{ 
     CharArrayWriter caw = new CharArrayWriter(); 
     try 
     { 
         Reader sr = new StringReader(string.trim()); 
        Reader in = new BufferedReader(sr); 
         int count = 0; 
         char[] buf = new char[16384]; 
        while ((count=in.read(buf)) != -1) { 
             if (count > 0) caw.write(buf, 0, count); 
         } 
         in.close(); 
     } 
     catch (Exception e) { e.printStackTrace(); } 
     return caw.toCharArray(); 
}
 
//从二进制文件对象中读取内容并转换为字节数组 
public static byte[] readBytes(File file) 
{ 
     ByteArrayOutputStream baos = new ByteArrayOutputStream(); 
     try 
     { 
         InputStream fis = new FileInputStream(file); 
         InputStream is = new BufferedInputStream(fis); 
        int count = 0; 
         byte[] buf = new byte[16384]; 
         while ((count=is.read(buf)) != -1) { 
             if (count > 0) baos.write(buf, 0, count); 
         } 
         is.close(); 
    } 
     catch (Exception e) { e.printStackTrace(); } 
     return baos.toByteArray(); 
} 

//写字节数组内容到二进制文件 
public static void writeBytes(File file, byte[] data) { 
     try { 
         OutputStream fos = new FileOutputStream(file); 
         OutputStream os = new BufferedOutputStream(fos); 
         os.write(data); 
         os.close(); 
     } 
     catch (Exception e) { e.printStackTrace(); } 
} 
//写字符数组内容到文本文件 
public static void writeChars(File file, char[] data) { 
     try { 
         Writer fos = new FileWriter(file); 
         Writer os = new BufferedWriter(fos); 
        os.write(data); 
         os.close(); 
     } 
     catch (Exception e) { e.printStackTrace(); } 
} 
} 
 
public class Base64{ 
//编码文件对象所指的文件 
static public char[] encode(File file){ 
  if (!file.exists()) { 
  System.err.println("错误:文件不存在！"); 
  return null; 
  } 
return encode(Base64Helper.readBytes(file)); 
} 

//编码文件名所指的文件 
static public char[] encode(String filename){ 
File file = new File(filename); 
  if (!file.exists()) { 
  System.err.println("错误:文件“"+filename+"”不存在！"); 
  return null; 
  } 
return encode(Base64Helper.readBytes(file)); 
} 
//编码传入的字节数组，输出编码后的字符数组 
static public char[] encode(byte[] data) 
{ 
  char[] out = new char[((data.length + 2) / 3) * 4]; 
// 
  // 对字节进行Base64编码,每三个字节转化为4个字符. 
  // 输出总是能被4整除的偶数个字符 
  // 
  for (int i=0, index=0; i< data.length; i+=3, index+=4) { 
  boolean quad = false; 
  boolean trip = false; 
  int val = (0xFF & (int) data[i]); 
  val <<= 8; 
  if ((i+1) < data.length) { 
  val |= (0xFF & (int) data[i+1]); 
  trip = true; 
  } 
  val <<= 8; 
  if ((i+2) < data.length) { 
  val |= (0xFF & (int) data[i+2]); 
  quad = true; 
  } 
  out[index+3] = alphabet[(quad? (val & 0x3F): 64)]; 
  val >>= 6; 
  out[index+2] = alphabet[(trip? (val & 0x3F): 64)]; 
  val >>= 6; 
  out[index+1] = alphabet[val & 0x3F]; 
  val >>= 6; 
  out[index+0] = alphabet[val & 0x3F]; 
  } 
    return out; 
} 
static public byte[] decode(char[] data) 
{ 
  // 程序中有判断如果有回车、空格等非法字符，则要去掉这些字符 
  // 这样就不会计算错误输出的内容 
  int tempLen = data.length; 
  for( int ix=0; ix< data.length; ix++ ) 
  { 
  if( (data[ix] > 255) || codes[ data[ix] ] < 0 ) 
  --tempLen; // 去除无效的字符 
  } 
  // 计算byte的长度 
  // -- 每四个有效字符输出三个字节的内容 
  // -- 如果有额外的3个字符，则还要加上2个字节, 
  // 或者如果有额外的2个字符，则要加上1个字节 
  int len = (tempLen / 4) * 3; 
  if ((tempLen % 4) == 3) len += 2; 
  if ((tempLen % 4) == 2) len += 1; 
  byte[] out = new byte[len]; 
  int shift = 0; 
  int accum = 0; 
  int index = 0; 
  // 一个一个字符地解码（注意用的不是tempLen的值进行循环） 
  for (int ix=0; ix< data.length; ix++) 
  { 
  int value = (data[ix]>255)? -1: codes[ data[ix] ]; 
  if ( value >= 0 ) // 忽略无效字符 
  { 
  accum <<= 6; 
  shift += 6; 
  accum |= value; 
  if ( shift >= 8 ) 
  { 
  shift -= 8; 
  out[index++] = 
  (byte) ((accum >> shift) & 0xff); 
  } 
  } 
  } 
  //如果数组长度和实际长度不符合，那么抛出错误 
  if( index != out.length) 
  { 
  throw new Error("数据长度不一致(实际写入了 " + index + "字节，但是系统指示有" + out.length + "字节)"); 
  } 
  return out; 
} 
// 
// 用于编码的字符 
// 
static private char[] alphabet ="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=".toCharArray(); 
// 
// 用于解码的字节（0-255） 
// 
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; 
} 
 public static void main (String [] args){ 
  String key = new String("Spider"); 
  byte[] a = key.getBytes(); 
  char[] b = Base64.encode(a) ; 
  System.out.println(new String(b));
   //for(int i=0;i< b.length;i++){ 
   //  System.out.println(b[i]); 
   //} 
  byte[] c = Base64. decode(b);
  System.out.println(new String(c));
} 
}  
 运行结果：
C:\java>java Base64
U3BpZGVy
Spider

posted on 2006-11-08 08:34 都市淘沙者 阅读(1026) 评论(0)  编辑  收藏 所属分类: Java Basic/Lucene/开源资料