网络——Base64Encode（转：自定义Base64编码器——Base64Encode）

【0】README

0.1） 本文source code 转自 core java volume 2 ， 旨在了解 如何定义一个 编码器， 如Base64Encode  ；

0.2）注意： 区别自定义的 Base64Encode 和 java.util.Base64 编码器

0.3）for souce code, please visit  自定义 Base64Encode 源代码

【1】 Base64Encode 编码器源代码如下： 

package com.corejava.chapter3;

import java.io.ByteArrayOutputStream;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.OutputStream;

public class Base64Encode
{
/**
* Computes the Base64 encoding of a string.
* @param s a string
* @return the Base64 encoding of s
*/
public static String base64Encode(String s)
{
ByteArrayOutputStream bOut = new ByteArrayOutputStream();
Base64OutputStream out = new Base64OutputStream(bOut);
try
{
out.write(s.getBytes());
out.flush();
}
catch (IOException e)
{
}
return bOut.toString();
}
}

/**
* This stream filter converts a stream of bytes to their Base64 encoding.
*
* Base64 encoding encodes 3 bytes into 4 characters. |11111122|22223333|33444444| Each set of 6
* bits is encoded according to the toBase64 map. If the number of input bytes is not a multiple of
* 3, then the last group of 4 characters is padded with one or two = signs. Each output line is at
* most 76 characters.
*/
class Base64OutputStream extends FilterOutputStream
{
private static char[] toBase64 = { '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', '+', '/' };

private int col = 0;
private int i = 0;
private int[] inbuf = new int[3];

/**
* Constructs the stream filter.
* @param out the stream to filter
*/
public Base64OutputStream(OutputStream out)
{
super(out);
}

public void write(int c) throws IOException
{
inbuf[i] = c;
i++;
if (i == 3)
{
if (col >= 76)
{
super.write('\n');
col = 0;
}
super.write(toBase64[(inbuf[0] & 0xFC) >> 2]);
super.write(toBase64[((inbuf[0] & 0x03) << 4) | ((inbuf[1] & 0xF0) >> 4)]);
super.write(toBase64[((inbuf[1] & 0x0F) << 2) | ((inbuf[2] & 0xC0) >> 6)]);
super.write(toBase64[inbuf[2] & 0x3F]);
col += 4;
i = 0;
}
}
public void flush() throws IOException
{
if (i > 0 && col >= 76)
{
super.write('\n');
col = 0;
}
if (i == 1)
{
super.write(toBase64[(inbuf[0] & 0xFC) >> 2]);
super.write(toBase64[(inbuf[0] & 0x03) << 4]);
super.write('=');
super.write('=');
}
else if (i == 2)
{
super.write(toBase64[(inbuf[0] & 0xFC) >> 2]);
super.write(toBase64[((inbuf[0] & 0x03) << 4) | ((inbuf[1] & 0xF0) >> 4)]);
super.write(toBase64[(inbuf[1] & 0x0F) << 2]);
super.write('=');
}
}
}