BASE64编码的图片在网页中的显示问题的解决

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我是靠谱客的博主清脆书本，最近开发中收集的这篇文章主要介绍BASE64编码的图片在网页中的显示问题的解决，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

转自: http://blog.sina.com.cn/s/blog_4ad7c2540100bemr.html

1.为什么要用到BASE64编码的图片信息
   Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一。Base64 主要不是加密，它主要的用途是把一些二进制数转成普通字符用于网络传输。由于一些二进制字符在传输协议中属于控制字符，不能直接传送需要转换一下。最常见的用途是作为电子邮件或WebService附件的传输编码.
2.base64编码定义
　　目前的internet e-mail标准--简单邮件传递协议(smtp)在rfc821中规定了两条重要但不难实现的限制：
       1)邮件的内容必须全部为7-比特的美国ascii码。
       2)每一行的长度不能超过1000的字符。
   因此为了通过smtp用e-mail进行传送,内存的序列化对象必须转化为和以上相容的格式。
   rfc1521提供了一个可行的方案。它定义了邮件的内容部分,使之能包涵多种形式的数据。这种标准就是目前众所周知的mime。
　　按照rfc1521编码过程为:输入是24个比特,输出是4个字节。24个比特输入组从左至右由3个8比特的输入组形成。这24个比特被看成4个连续的6比特组,而每个6比特输入组被翻译为base64码表中的一个数字。依次反复不断进行，直到全部输入数据转换完成。
   如果最后剩下两个输入数据，在编码结果后加1个“=”；如果最后剩下一个输入数据，编码结果后加2个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证资料还原的正确性。
   完整的base64定义可见 RFC1421和 RFC2045。编码后的数据比原始数据略长，为原来的4/3。在电子邮件中，根据RFC822规定，每76个字符，还需要加上一个回车换行。可以估算编码后数据长度大约为原长的135.1%。

3.如何进行base64编码
   Base64 使用US-ASCII子集的65个字符, 每个字符用6位表示
   对于文本串，编码过程如下。例如"men":
   先转成US-ASCII值.

   "m"十进制 109
   "e"十进制 101
   "n"十进制 110
   二进制 :
   m 01101101
   e 01100101
   n 01101110

三个8位连起来是24位
011011010110010101101110

然后分成4个6位
011011 010110 010101 101110

现在得到4个值，十进制为
27 22 21 46

  对应的 Base64 字符是 : b W V u
  编码总是基于3个字符，从而产生4个Base64字符。

  如果只是2个字符的数据，使用特殊字符"="补齐Base64的4字。
  如，编码"me"
  01101101 01100101
  0110110101100101
  011011 010110 0101
  111111 (与，补足6位)
  011011 010110 010100
  b W U
  b W U = ("=" 补足4字符)
  于是 "bWU=" 就是"me"的Base64值.

  如果只是2个字符的数据,如编码 "m"
  01101101
  011011 01
  111111
  011011 010000
  b Q = =
于是 "bQ==" 就是"m"的Base64值.

4.显示被存储为Base64编码字符串的图片的例子
1)使用data: URI直接在网页中嵌入.
data: URI定义于IETF标准的RFC 2397
data: URI的基本使用格式如下：
data:[<MIME-type>][;base64|charset=some_charset],<data>
mime-type是嵌入数据的mime类型，比如png图片就是image/png。
如果后面跟base64，说明后面的data是采用base64方式进行编码的
<html>
<body>
<img src='data:image/gif;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAkGBwgHBgkIBwgKCgkLDRYPDQwMDRsUFRAWIB0iIiAd
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2pkxuQyiQjtI5U0Fwcjwr1RQFFFFB//Z'/>

</body>
</html>

这种方式,Firfox、Opera、Safari和Konqueror这些浏览器都已经支持，但是IE直到7.0版本都还没有支持.所以,比较好的做法是在服务器端将base64编码的字符串转换成byte流.这里,我提供了java的实现方法.

2)字符串转换成byte流,然后显示出来
在java中,提供了一个用于编码和解码(encode/decode )base64字符串和数据流的Java开源类库Java Base64.

服务器端代码
package test;
import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;

public class ImgShow extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res)
   throws ServletException, IOException {
  String xmlImg="/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAkGBwgHBgkIBwgKCgkLDRYPDQwMDRsUFRAWIB0iIiAd
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2pkxuQyiQjtI5U0Fwcjwr1RQFFFFB//Z";
  try{
   res.setContentType("image/*"); // 设置返回的文件类型
   OutputStream toClient = res.getOutputStream();
   GenerateImage(xmlImg,toClient);
  }catch(Exception ex){
   System.out.println(ex.toString());
  }

}

       // 对字节数组字符串进行Base64解码并生成图片
public static boolean GenerateImage(String imgStr,OutputStream out) {
  if (imgStr == null) // 图像数据为空
   return false;
  BASE64Decoder decoder = new BASE64Decoder();
  try {
   // Base64解码
   byte[] b = decoder.decodeBuffer(imgStr);
   for (int i = 0; i < b.length; ++i) {
    if (b[i] < 0) {// 调整异常数据
     b[i] += 256;
    }
   }
//   OutputStream out = new FileOutputStream(imgFilePath);
   out.write(b);
   out.flush();
   out.close();
   return true;
  } catch (Exception e) {
   return false;
  }
}

}
客户端代码
<html>
<head><title>用servlet显示base64编码的图片</title></head>
<body>
<img src="http://localhost:8080/servlet/ImgShow">
</body>
</html>
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