如何在Java中将java.sql.blob转换为base64编码的字符串

Question

i am trying to get an image out of a mysql database where it is stored as a blob. so far i have tried three more or less different ways, non of which seem to take me anywhere. i must be making some stupid mistake here.

    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());
    }

in all cases the console gives me this:

Answer 1

The Exception with null in getMessage() and getCause() is the NullPointerException.

The JavaDoc for ResultSet#getBinaryStream() states that it returns null for the column containing SQL NULL. Even though the JavaDoc for the ResultSet#getBlob() omits to mention the same, it looks to me that the test rows do not contain any data in the blob column, ie the blob is NULL in database.