业务分析：编写一个简单的前端页面，选择用一个 <select>列表，页面加载完成后发送Ajax请求， 加载所有省份。
数据库环境
创建如下的数据库以及数据表：
create database province;
use province;
create table province(id int primary key not null auto_increment,name varchar(20) not null
);insert into province(name) value("北京");
insert into province(name) value("天津");
insert into province(name) value("上海");
insert into province(name) value("重庆");
配置环境
使用druid.properties文件：
driverClassName=com.mysql.cj.jdbc.Driver
url=jdbc:mysql:///province
username=root
password=123456
initialSize=5
maxActive=10
maxWait=3000
使用redis.properties文件：
redis.host=127.0.0.1
redis.port=6379
redis.maxTotal=30
redis.maxIdle=10
依赖文件pom.xml为：
<dependencies><dependency><groupId>commons-logging</groupId><artifactId>commons-logging</artifactId><version>1.2</version></dependency><dependency><groupId>commons-beanutils</groupId><artifactId>commons-beanutils</artifactId><version>1.7.0</version></dependency><dependency><groupId>com.alibaba</groupId><artifactId>druid</artifactId><version>1.2.6</version></dependency><dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-annotations</artifactId><version>2.12.2</version></dependency><dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-core</artifactId><version>2.12.1</version></dependency><dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.12.1</version></dependency><dependency><groupId>jstl</groupId><artifactId>jstl</artifactId><version>1.2</version></dependency><dependency><groupId>org.glassfish.web</groupId><artifactId>jstl-impl</artifactId><version>1.2</version></dependency><dependency><groupId>javax.servlet</groupId><artifactId>javax.servlet-api</artifactId><version>3.1.0</version></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.23</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-tx</artifactId><version>5.2.4.RELEASE</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-jdbc</artifactId><version>5.2.4.RELEASE</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-beans</artifactId><version>5.2.4.RELEASE</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-core</artifactId><version>5.2.4.RELEASE</version></dependency>
</dependencies>
其中，创建servlet需要的核心依赖为：
<dependency><groupId>javax.servlet</groupId><artifactId>javax.servlet-api</artifactId><version>3.1.0</version>
</dependency>
创建web包，并且引入jQuery框架包。在web包下编写主页代码，其中body主题如下所示：
<body>
<select id="province"><option>--请选择省份--</option>
</select>
</body>
整个业务逻辑：
HTML页面发送请求——Servlet接收请求处理——调用Service层——调用dao层——数据库查询
创建com.company.dao.ProvinceDao接口
public interface ProvinceDao {List<Province> findAll();
}
创建对应的实现类com.company.dao.impl.ProvinceDaoImpl类
public class ProvinceDaoImpl implements ProvinceDao {// 1. 声明JdbcTemplate对象private final JdbcTemplate jdbcTemplate = new JdbcTemplate(JDBCUtils.getDataSource());@Overridepublic List<Province> findAll() {// 2. 定义sqlString sql = "select * from province";// 3. 执行sql语句List<Province> provinces = jdbcTemplate.query(sql, new BeanPropertyRowMapper<Province>(Province.class));return provinces;}
}
创建实体类com.company.domain.Province类
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;}@Overridepublic String toString() {return "Province{" +"id=" + id +", name='" + name + '\'' +'}';}
}
创建service层com.company.service.ProvinceService接口
public interface ProvinceService {List<Province> findAll();
}
创建对应的实现类com.company.service.impl.ProvinceServiceImpl类
public class ProvinceServiceImpl implements ProvinceService {// 声明dao变量，service调用dao层private final ProvinceDao dao = new ProvinceDaoImpl();@Overridepublic List<Province> findAll() {return dao.findAll();}
}
创建jdbc工具类com.company.util.JDBCUtils类
public class JDBCUtils {public static DataSource ds;static {try {// 1. 加载配置文件Properties pro = new Properties();// 2. 获取字节输入流InputStream stream = JDBCUtils.class.getClassLoader().getResourceAsStream("druid.properties");pro.load(stream);// 3. 初始化连接池对象ds = DruidDataSourceFactory.createDataSource(pro);} catch (Exception e) {e.printStackTrace();}}// 获取数据源对象public static DataSource getDataSource() {return ds;}// 获取连接对象public static Connection getConnection() throws SQLException {return ds.getConnection();}}
创建com.company.web.servlet.ProvinceServlet类
@WebServlet(name = "ProvinceServlet", value = "/provinceServlet")
public class ProvinceServlet extends HttpServlet {@Overrideprotected void doGet(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jsonObjectMapper mapper = new ObjectMapper();String json = mapper.writeValueAsString(list);System.out.println(json);// 3. 响应结果response.setContentType("application/json;charset=utf-8");response.getWriter().write(json);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}
}
在主页中编写响应的请求代码，发起Ajax连接。
<script src="js/jquery-3.5.1.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":"天津"}// {"id":3,"name":"上海"},{"id":4,"name":"重庆"}]// 1. 获取selectlet province = $("#province");// 2. 遍历json数组$(data).each(function () {// 3. 创建option标签let option = "<option name='" + this.id + "'>" + this.name + "</option>";// 4. 调用select的append方法，追加optionprovince.append(option);});});});
</script>
使用Redis对整个业务进行优化，主要的使用方式是使用Redis的缓存优化。
在整个业务的Service层进行操作，逻辑如下：
这样的操作，可以减少数据库的查询次数，加快整个业务的运行速度。
导入新的jedis的依赖包：
<dependency><groupId>redis.clients</groupId><artifactId>jedis</artifactId><version>3.6.0</version>
</dependency>
编写jedis连接池工具类com.company.util.JedisPoolUtils类：
public class JedisPoolUtils {private static JedisPool jedisPool;static {// 读取配置文件InputStream resource = JedisPoolUtils.class.getClassLoader().getResourceAsStream("redis.properties");Properties properties = new Properties();try {properties.load(resource);} catch (IOException e) {e.printStackTrace();}// 获取数据，设置到JedisPoolConfig中JedisPoolConfig jedisPoolConfig = new JedisPoolConfig();jedisPoolConfig.setMaxTotal(Integer.parseInt(properties.getProperty("redis.maxTotal")));jedisPoolConfig.setMaxIdle(Integer.parseInt(properties.getProperty("redis.maxIdle")));// 初始化JedisPool对象jedisPool = new JedisPool(jedisPoolConfig, properties.getProperty("redis.host"), Integer.parseInt(properties.getProperty("redis.port")));}/*** 获取连接方法* @return 返回Jedis连接对象*/public static Jedis getJedis() {return jedisPool.getResource();}
}
在service接口层添加新的方法：
public interface ProvinceService { List<Province> findAll(); String findAllJson();}
实现类中添加新的方法：
/*** 使用redis缓存优化业务*/
@Override
public String findAllJson() {// 先从redis中查询数据// 创建jedis对象Jedis jedis = JedisPoolUtils.getJedis();System.out.println("连接成功");String provinceJson = jedis.get("province");// 判断provinceJson是否有值if (provinceJson == null || provinceJson.length() == 0) {System.out.println("redis中无数据，查询数据库...");// redis中没有数据，需要从数据库中获取List<Province> provinces = dao.findAll();// 将list序列化成jsonObjectMapper mapper = new ObjectMapper();try {provinceJson = mapper.writeValueAsString(provinces);System.out.println(provinceJson);} catch (JsonProcessingException e) {e.printStackTrace();}try {// 将json数据存入redis中String set = jedis.set("province", provinceJson);if ("OK".equals(set)) {System.out.println("数据存入成功");}} catch (Exception e) {e.printStackTrace();} finally {// 释放资源jedis.close();}} else {System.out.println("redis中有数据，查询缓存...");}return provinceJson;
}
最后在servlet中替换新的方法调用：
// 1. 查询json
ProvinceService service = new ProvinceServiceImpl();
// 2. 序列化结果
String json = service.findAllJson();
System.out.println(json);
// 3. 响应结果
response.setContentType("application/json;charset=utf-8");
response.getWriter().write(json);
使用redis缓存一些内容不经常发生变化的数据更加合适
遇见问题：
java.lang.ClassNotFoundException: org.apache.commons.pool2.impl.GenericObjectPoolConfig
原因是缺少必要的jar包：
<dependency><groupId>org.apache.commons</groupId><artifactId>commons-pool2</artifactId><version>2.6.0</version>
</dependency>
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