Question

我试图从mysql数据库中获取一个图像，并将其存储为blob。到目前为止，我已经尝试了三种或多或少不同的方式，其中任何一种似乎都把我带到了任何地方我必须在这里犯一些愚蠢的错误。

    try {
          // create a java mysql database connection
          String myDriver = "org.gjt.mm.mysql.Driver";
          Class.forName(myDriver);
          Connection conn = DriverManager.getConnection(mySettings.dbConnect, mySettings.dbUser, mySettings.dbPass);
          PreparedStatement stmt = conn.prepareStatement("select up.userid, up.name, la.languages, up.photo, up.dob, up.edited from userprofile up join languages la on up.languages_id = la.languages_id where up.userid = ?");

          stmt.setString(1, userid);
          ResultSet rs = stmt.executeQuery();
          while (rs.next()) {
              // version 1
              InputStream photois = rs.getBinaryStream("photo");
              int ch;
              String str = new String();
              while ((ch = photois.read()) != -1) {
                  // here the exception occures when i InputStream.read().
                  // both Exception.getMessage() and Exception.getCause()
                  // of the caught exception yield null
                  str += (char)ch;
              }
              byte[] bdata=str.getBytes();
              byte[] img64 = Base64.encode(bdata);
              String photo64 = new String(img64);

              // version 2
              Blob b = rs.getBlob("photo");
              byte[] ba = b.getBytes(1, b.length()); 
              // b.length() throws exception, no message, no cause
              byte[] img64 = Base64.encode(ba);
              String photo64 = new String(img64);

              // version 3
              InputStream photois = rs.getBinaryStream("photo");
              byte[] buf = IOUtils.toByteArray(photois); 
              // this throws an exception, message and cause both null as above 
              String photo64 = DatatypeConverter.printBase64Binary(buf);

          }
          conn.close();
    }
    catch (Exception e) {
          System.err.println("Got an exception! (reading userprofile from db)");
          System.err.println(e.getMessage());
          System.err.println(e.getCause());
    }

在所有情况下，控制台都给我这个：
System.err

Answer 1

getMessage（）和getCause（）中的null异常是NullPointerException。

ResultSet#getBinaryStream()的JavaDoc声明它为包含SQL NULL的列返回null。尽管ResultSet#getBlob()的JavaDoc省略了相同的内容，但我认为测试行不包含blob列中的任何数据，即blob在数据库中为NULL。

如何在java中将java.sql.blob转换为base64编码的字符串

1 个答案: