889~891 分析，环境搭建，实现查询数据库，redis缓存优化

案例：

　　案例需求：

　　　　1.提供index.html页面，页面中有一个省份，下拉列表

　　　　2.当页面加载完成后，发送ajax请求，加载说有省份

CREATE DATABASE day23; -- 创建数据库
USE day23;             -- 使用数据库
CREATE TABLE province(   -- 创建表
    id INT PRIMARY KEY AUTO_INCREMENT,
    NAME VARCHAR(20) NOT NULL
     
);
-- 插入数据
INSERT INTO province VALUES(NULL,'北京');
INSERT INTO province VALUES(NULL,'上海');
INSERT INTO province VALUES(NULL,'广州');
INSERT INTO province VALUES(NULL,'陕西');

public class Province {
 
    private int id;
    private String name;
 
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}

public interface ProvinceDao {
 
    public List<Province> findAll();
}

public class ProvinceDaoImpl implements ProvinceDao {
 
    //1.声明成员变量 jdbctemplement
    private JdbcTemplate template = new JdbcTemplate(JDBCUtils.getDataSource());
 
    @Override
    public List<Province> findAll() {
        //1.定义sql
        String sql = "select * from province ";
        //2.执行sql
        List<Province> list = template.query(sql, new BeanPropertyRowMapper<Province>(Province.class));
        return list;
    }
}

/**
 * JDBC工具类 使用Durid连接池
 */
public class JDBCUtils {
 
    private static DataSource ds ;
 
    static {
        try {
            //1.加载配置文件
            Properties pro = new Properties();
            //使用ClassLoader加载配置文件，获取字节输入流
            InputStream is = JDBCUtils.class.getClassLoader().getResourceAsStream("druid.properties");
            pro.load(is);
 
            //2.初始化连接池对象
            ds = DruidDataSourceFactory.createDataSource(pro);
 
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
 
    /**
     * 获取连接池对象
     */
    public static DataSource getDataSource(){
        return ds;
    }
 
 
    /**
     * 获取连接Connection对象
     */
    public static Connection getConnection() throws SQLException {
        return  ds.getConnection();
    }
}

public interface ProvinceService {
 
    public List<Province> findAll();
 
    public String findAllJson();
}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/jquery-3.3.1.min.js"></script>
    <script>
        $(function () {
            //发送ajax请求，加载所有省份数据
            $.get("provinceServlet",{},function (data) {
                //[{"id":1,"name":"北京"},{"id":2,"name":"上海"},{"id":3,"name":"广州"},{"id":4,"name":"陕西"}]
                //1.获取select
                var province = $("#province");
                //2.遍历json数组
                $(data).each(function () {
                    //3.创建<option>
                    var option = "<option name='"+this.id+"'>"+this.name+"</option>";
                    //4.调用select的append追加option
                    province.append(option);
                });
            });
        });
    </script>
</head>
<body>
        <select id="province">
            <option>--请选择省份--</option>
 
        </select>
</body>
</html>

package com.example.itcast.service.impl;
 
 
import com.example.itcast.dao.ProvinceDao;
import com.example.itcast.dao.impl.ProvinceDaoImpl;
import com.example.itcast.domain.Province;
import com.example.itcast.jedis.util.JedisPoolUtils;
import com.example.itcast.service.ProvinceService;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import redis.clients.jedis.Jedis;
 
import java.util.List;
 
public class ProvinceServiceImpl implements ProvinceService {
    //声明dao
    private ProvinceDao dao = new ProvinceDaoImpl();
 
    @Override
    public List<Province> findAll() {
        return dao.findAll();
    }
 
    /**
        使用redis缓存
     */
 
    @Override
    public String findAllJson() {
        //1.先从redis中查询数据
        //1.1获取redis客户端连接
        Jedis jedis = JedisPoolUtils.getJedis();
        String province_json = jedis.get("province");
 
        //2判断 province_json 数据是否为null
        if(province_json == null || province_json.length() == 0){
            //redis中没有数据
            System.out.println("redis中没数据，查询数据库...");
            //2.1从数据中查询
            List<Province> ps = dao.findAll();
            //2.2将list序列化为json
            ObjectMapper mapper = new ObjectMapper();
            try {
                province_json = mapper.writeValueAsString(ps);
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            }
 
            //2.3 将json数据存入redis
            jedis.set("province",province_json);
            //归还连接
            jedis.close();
 
        }else{
            System.out.println("redis中有数据，查询缓存...");
        }
        return province_json;
    }
}

package com.example.itcast.web.servlet;
 
import com.example.itcast.service.ProvinceService;
import com.example.itcast.service.impl.ProvinceServiceImpl;
 
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
 
@WebServlet("/provinceServlet")
public class ProvinceServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       /* //1.调用service查询
        ProvinceService service = new ProvinceServiceImpl();
        List<Province> list = service.findAll();
        //2.序列化list为json
        ObjectMapper mapper = new ObjectMapper();
        String json = mapper.writeValueAsString(list);*/
 
        //1.调用service查询
        ProvinceService service = new ProvinceServiceImpl();
        String json = service.findAllJson();
 
 
        System.out.println(json);
        //3.响应结果
        response.setContentType("application/json;charset=utf-8");
        response.getWriter().write(json);
 
    }
 
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}