Connection ct = null;  
        Statement  sm = null;  
        ResultSet rs = null;
          

          try{     
            Class.forName("com.mysql.jdbc.Driver");     //连接mysql数据库 
            ct = DriverManager.getConnection("jdbc:mysql://localhost:3306/数据库名?useUnicode=true&characterEncoding=utf-8","用户名"              , "密码");
            sm = ct.createStatement();
            rs = sm.executeQuery("select *from 表名");
           
            JSONArray jsonData = JSONArray.fromObject(convertList(rs));      //先转成List格式，再转成json格式

            System.out.println(jsonData.toString());
            PrintWriter out = response.getWriter();    //把json数据传递到前端
            out.print(jsonData);
      } 
        catch(Exception ex)
       {  
               //error 代码
        }  

    private static List convertList(ResultSet rs) throws SQLException {
        List list = new ArrayList();
        ResultSetMetaData md = rs.getMetaData();
        int columnCount = md.getColumnCount();
        while (rs.next()) {
            Map rowData = new HashMap();
            for (int i = 1; i <= columnCount; i++) {
                rowData.put(md.getColumnName(i), rs.getObject(i));
            }
            list.add(rowData);
        }
        return list;
    }