月份：2018年10月

import java.sql.*;
import java.util.*;

public class BaseDao {
    private static String url = "jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf-8&useSSL=true";
    private static String user = "root";
    private static String pwd = "123456";
    private static Connection conn;
    private static PreparedStatement pstmt;
    private static ResultSet rs;

    // 1.加载驱动（静态代码块）
    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    //2.建立连接
    private static void getConn() {
        if (conn == null) {
            try {
                conn = DriverManager.getConnection(url, user, pwd);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    //读
    public static ResultSet query(String sql, List<Object> params) {
        getConn();
        try {
            pstmt = conn.prepareStatement(sql);
            if (params != null && params.size() > 0) {
                for (int i = 0; i < params.size(); i++) {
                    pstmt.setObject(i + 1, params.get(i));
                }
            }
            rs = pstmt.executeQuery();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return rs;
    }

    //写
    public static int update(String sql, List<Object> params) {
        int result = 0;
        getConn();
        try {
            pstmt = conn.prepareStatement(sql);
            if (params != null && params.size() > 0) {
                for (int i = 0; i < params.size(); i++) {
                    pstmt.setObject(i + 1, params.get(i));
                }
            }
            result = pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            close();
        }
        return result;
    }

    //关闭连接
    public static void close() {
        try {
            if (rs != null) {
                rs.close();
                rs = null;
            }
            if (pstmt != null) {
                pstmt.close();
                pstmt = null;
            }
            if (conn != null) {
                conn.close();
                conn = null;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    //获取数据库所有表
    public static List<String> getTables(){
        getConn();
        List<String> tables = new ArrayList<String>();
        try {
            DatabaseMetaData dbMetaData = conn.getMetaData();
            ResultSet rs = dbMetaData.getTables(null, null, null,new String[] { "TABLE" });
            while (rs.next()){
                tables.add(rs.getString("TABLE_NAME"));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return tables;
    }

    //获取数据库某表的字段
    public static List<String> getColumns(String table){
        getConn();
        List<String> columns = new ArrayList<String>();
        String sql = "select * from " + table;
        getConn();
        try {
            PreparedStatement ps = conn.prepareStatement(sql);
            ResultSet rs = ps.executeQuery();
            ResultSetMetaData meta = rs.getMetaData();
            int columeCount = meta.getColumnCount();
            for (int i = 1; i < columeCount + 1; i++) {
                columns.add(meta.getColumnName(i));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return columns;
    }

    //测试
    public static void main(String[] args) throws SQLException {

//        String sql="insert into metric(kkey,value)values(?,?)";
//        List<Object> params=new ArrayList<>();
////        params.add(1);
//        params.add("66666");
//        params.add("aaaaa5655aaaaaaaaaaaaaa");
//        System.out.println(update(sql,params));

        //查找
//        String sql="select id,kkey,value,name from metric where name=?";
//        List<Object> params=new ArrayList<>();
//        params.add("process");
//        ResultSet rs=query(sql,params);
//        while (rs.next()){
//            int id=rs.getInt("id");
//            String key=rs.getString("kkey");
//            String value=rs.getString("value");
//            String nm=rs.getString("name");
//
//            System.out.println("id:"+id+"name:"+nm+" key:"+key+" value:"+value);
//        }
        List<String> tables=getTables();
        List<String> columns=getColumns("admin");
        System.out.println("done");
    }
}

Error：可能无法update，原因是因为sql语句书写不正确，正确的写法见下面（致谢）

    //写2
    public static int update2(String sql) {
        int result = 0;
        getConn();
        try {
            pstmt = conn.prepareStatement(sql);
            result = pstmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            close();
        }
        return result;
    } 
   public String updateStatus(@RequestParam("id") String id, @RequestParam("field") String field, @RequestParam("state") String state,@RequestParam("table") String table) {
        String sql = "update "+table+" set "+field+" = '"+state +"' where id='"+id+"'";
        System.out.println(sql);
        int rs=PatternDao.update2(sql);
        System.out.println("rs:"+rs);
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("msg", "");
        if(rs>0)
            jsonObject.put("code", 0);
        else
            jsonObject.put("code", -1);

        return JSONObject.toJSONString(jsonObject);
    }

jdbc：https://dev.mysql.com/downloads/connector/j/

ref：https://www.cnblogs.com/zhangdiIT/p/6772543.html#commentform

一.JSON的介绍主页以及各种语言对其的encoding

http://www.json.org/

二.JSON主流的几种JAVA处理操作

JSON：fastjson

JSON： Gson

JSON：org.json

JSON：json-lib

注意：最后一种不推荐使用，主推前三者。