如何解码可引用的字符(从quotable到char)？

Question

我有一个带引号的文本.以下是此类文本的示例(来自维基百科文章):

如果你相信真理= 3Dbeauty,那么肯定= 20 =
数学是哲学中最美丽的分支.

我正在寻找一个Java类,它将编码形式解码为chars,例如,= 20到一个空格.

更新:感谢The Elite Gentleman,我知道我需要使用QuotedPrintableCodec:

import org.apache.commons.codec.DecoderException;
import org.apache.commons.codec.net.QuotedPrintableCodec;
import org.junit.Test;

public class QuotedPrintableCodecTest { 
private static final String TXT =  "If you believe that truth=3Dbeauty, then surely=20=mathematics is the most beautiful branch of philosophy.";

    @Test
    public void processSimpleText() throws DecoderException
    {
        QuotedPrintableCodec.decodeQuotedPrintable( TXT.getBytes() );           
    }
}   

但是我一直得到以下异常:

org.apache.commons.codec.DecoderException: Invalid URL encoding: not a valid digit (radix 16): 109
    at org.apache.commons.codec.net.Utils.digit16(Utils.java:44)
    at org.apache.commons.codec.net.QuotedPrintableCodec.decodeQuotedPrintable(QuotedPrintableCodec.java:186)

我究竟做错了什么？

更新2:我在@ SO中找到了这个问题并了解了MimeUtility:

import javax.mail.MessagingException;
import javax.mail.internet.MimeUtility;

public class QuotedPrintableCodecTest {
    private static final String TXT =  "If you believe that truth=3Dbeauty, then surely=20= mathematics is the most beautiful branch of philosophy.";

    @Test
    public void processSimpleText() throws MessagingException, IOException  
    {
        InputStream is = new ByteArrayInputStream(TXT.getBytes());

            BufferedReader br = new BufferedReader ( new InputStreamReader(  MimeUtility.decode(is, "quoted-printable") ));         
            StringWriter writer = new StringWriter(); 

            String line;
            while( (line = br.readLine() ) != null )
            {
                writer.append(line);
            }
            System.out.println("INPUT:  "  + TXT);
            System.out.println("OUTPUT: " +  writer.toString() );       
    }
    }

但是输出仍然不完美,它包含'=':

INPUT:  If you believe that truth=3Dbeauty, then surely=20= mathematics is the most beautiful branch of philosophy.
OUTPUT: If you believe that truth=beauty, then surely = mathematics is the most beautiful branch of philosophy.

现在我做错了什么？

Answer 1

Apache Commons Codec QuotedPrintableCodec类确实是RFC 1521 Quoted-Printable部分的实现.

更新,您的quoted-printable字符串是错误的,因为维基百科上的示例使用Soft-line break.

软线断裂:

Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES
      that encoded lines be no more than 76 characters long. If longer
      lines are to be encoded with the Quoted-Printable encoding, 'soft'
      line breaks must be used. An equal sign as the last character on a
      encoded line indicates such a non-significant ('soft') line break
      in the encoded text. Thus if the "raw" form of the line is a
      single unencoded line that says:

          Now's the time for all folk to come to the aid of
          their country.

      This can be represented, in the Quoted-Printable encoding, as

          Now's the time =
          for all folk to come=
           to the aid of their country.

      This provides a mechanism with which long lines are encoded in
      such a way as to be restored by the user agent.  The 76 character
      limit does not count the trailing CRLF, but counts all other
      characters, including any equal signs.

所以你的文字应该如下:

private static final String CRLF = "\r\n";
private static final String S = "If you believe that truth=3Dbeauty, then surely=20=" + CRLF + "mathematics is the most beautiful branch of philosophy.";

Javadoc明确指出:

引用的可打印规范的规则#3,#4和#5尚未实现,因为完整的quoted-printable规范不适合面向byte []的编解码器框架.一旦可运行的编解码器框架准备就绪,完成编解码器.以部分形式提供编解码器背后的动机是,对于那些不需要可引用的可打印行格式(规则#3,#4,#5)的应用程序,例如Q编​​解码器,它已经派上用场了.

并且Apache QuotedPrintableCodec 记录了一个错误,因为它不支持软换行符.