base64編碼 淺析

Base64是網絡上最多見的用於傳輸8Bit字節代碼的編碼方式之一，你們能夠查看RFC2045～RFC2049，上面有MIME的詳細規範。Base64編碼可用於在HTTP環境下傳遞較長的標識信息。例如，在Java Persistence系統Hibernate中，就採用了Base64來將一個較長的惟一標識符（通常爲128-bit的UUID）編碼爲一個字符串，用做HTTP表單和HTTP GET URL中的參數。在其餘應用程序中，也經常須要把二進制數據編碼爲適合放在URL（包括隱藏表單域）中的形式。此時，採用Base64編碼不只比較簡短，同時也具備不可讀性，即所編碼的數據不會被人用肉眼所直接看到。

 

如下是新浪微博開放的源碼中提供的 Base64編碼：

/*
Copyright (c) 2007-2009, Yusuke Yamamoto
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
 * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.
 * Neither the name of the Yusuke Yamamoto nor the
      names of its contributors may be used to endorse or promote products
      derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY Yusuke Yamamoto ``AS IS'' AND ANY
EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL Yusuke Yamamoto BE LIABLE FOR ANY
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package com.weibo.weibo4j.http;

/**
 * A utility class encodes byte array into String using Base64 encoding scheme.
 * 
 * @see com.weibo.weibo4j.http.HttpClient
 * @author Yusuke Yamamoto - yusuke at mac.com
 */
public class BASE64Encoder
{
    private static final char last2byte = (char) Integer.parseInt("00000011", 2);
    private static final char last4byte = (char) Integer.parseInt("00001111", 2);
    private static final char last6byte = (char) Integer.parseInt("00111111", 2);
    private static final char lead6byte = (char) Integer.parseInt("11111100", 2);
    private static final char lead4byte = (char) Integer.parseInt("11110000", 2);
    private static final char lead2byte = (char) Integer.parseInt("11000000", 2);
    private static final char[] encodeTable = new char[] { '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 BASE64Encoder()
    {
    }

    public static String encode(byte[] from)
    {
        StringBuffer to = new StringBuffer((int) (from.length * 1.34) + 3);
        int num = 0;
        char currentByte = 0;
        for (int i = 0; i < from.length; i++)
        {
            num = num % 8;
            while (num < 8)
            {
                switch (num)
                {
                    case 0:
                        currentByte = (char) (from[i] & lead6byte);
                        currentByte = (char) (currentByte >>> 2);
                        break;
                    case 2:
                        currentByte = (char) (from[i] & last6byte);
                        break;
                    case 4:
                        currentByte = (char) (from[i] & last4byte);
                        currentByte = (char) (currentByte << 2);
                        if ((i + 1) < from.length)
                        {
                            currentByte |= (from[i + 1] & lead2byte) >>> 6;
                        }
                        break;
                    case 6:
                        currentByte = (char) (from[i] & last2byte);
                        currentByte = (char) (currentByte << 4);
                        if ((i + 1) < from.length)
                        {
                            currentByte |= (from[i + 1] & lead4byte) >>> 4;
                        }
                        break;
                }
                to.append(encodeTable[currentByte]);
                num += 6;
            }
        }
        if (to.length() % 4 != 0)
        {
            for (int i = 4 - to.length() % 4; i > 0; i--)
            {
                to.append("=");
            }
        }
        return to.toString();
    }
}

搞不清爲什麼寫那麼複雜，由於java的API中已經提供了對Base64編碼的封裝，直接調用就能夠了。 

示例代碼以下，已經在上線的項目用運用該編碼。

package com.common.util.CipherUtil;

import java.io.IOException;

import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;


public class Base64Util {
	
	/**
	 * 編碼
	 * @param bstr
	 * @return String
	 */
	public static String encodeBase64(byte[] bstr) 
	{
		return new BASE64Encoder().encode(bstr);
	}


	/**
	 * Base64解碼
	 * @param str
	 * @return string
	 */
	public static byte[] decodeBase64(String str) 
	{
		byte[] bt = null;
		try 
		{
			BASE64Decoder decoder = new BASE64Decoder();
			bt = decoder.decodeBuffer(str);
		}
		catch (IOException e)
		{
			e.printStackTrace();
		}
		return bt;
	}
	
	
	/**
	 * @param args
	 */
	public static void main(String[] args) 
	{
		String aa = "更多更多";
		
		String strEncode = encodeBase64(aa.getBytes());
		System.out.println("Base64加密後的:"+strEncode);
		
		String strDecode = new String(decodeBase64(strEncode));
		System.out.println("Base64解密後的:"+strDecode);
	}
	
	
	
}