java setString() not working

问题

try{is = new FileInputStream(new File(s));
        PreparedStatement ps; 
        ps= cn.prepareStatement("Update instructor set name ='?' ,gender ='?', image ='?' where instructorID =?");
          JOptionPane.showMessageDialog(null, "5");
        ps.setString(1,name);
          JOptionPane.showMessageDialog(null, "4");
            ps.setString(2,gender);
            JOptionPane.showMessageDialog(null, "3");
            ps.setBlob(3, is);
            JOptionPane.showMessageDialog(null, "2");
          ps.setString(4, iden);
          JOptionPane.showMessageDialog(null, "1");
            ps.executeUpdate();
            JOptionPane.showMessageDialog(null, "successfully updated");
        }catch(Exception e ){
            JOptionPane.showMessage(null,"error1");
        }

So I want to insert these variables to the prepared statement, however I'm getting an error. After adding those JOptionPanes to debug, the program shows "5", and then "4" and then "error1". Which I think means that the line ps.setString(2,gender); failed to execute. However, I cannot find where did I get it wrong. Could anyone help?

回答1:

Remove quotes around question marks. Otherwise, '?' would be interpreted as a string literal with a single question mark in it:

ps= cn.prepareStatement("Update instructor set name =? ,gender =?, image =? where instructorID =?");

Currently, JDBC thinks that your query has a single parameter, corresponding to the question mark at the end. Trying to set parameter number two causes the exception.

来源：https://stackoverflow.com/questions/45770221/java-setstring-not-working