PreparedStatement问号的问题

Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            conn = DBUtil.getConnection();
            String sql = "select ?, ?, ? from dept";
            ps = conn.prepareStatement(sql);
            ps.setString(1, "deptno");
            ps.setString(2, "dname");
            ps.setString(3, "loc");
            rs = ps.executeQuery();
            ArrayList<Dept> depts = new ArrayList<>();
            while (rs.next()) {
                String deptno = rs.getString("deptno");
                String dname = rs.getString("dname");
                String loc = rs.getString("loc");
                System.out.printf("%s %s %s\n", deptno, dname, loc);
                final Dept dept = new Dept(deptno, dname, loc);
                depts.add(dept);
            }
            request.setAttribute("depts", depts);
            request.getRequestDispatcher("/list.jsp").forward(request, response);
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            DBUtil.close(conn, ps, rs);
        }

结果

deptno dname loc
deptno dname loc
deptno dname loc
deptno dname loc
deptno dname loc
deptno dname loc
deptno dname loc
deptno dname loc

总结

PreparedStatement只能用来为可以加引号’的参数（如参数值）设置动态参数，即用?占位，不可用于表名、字段名等。