Java spring webmvc如何实现控制反转

public interface UserDao {
  String getUser();
}
public class UserDaoMSSqlImpl implements UserDao {
  public String getUser() {
    return "从SQL Server查询用户";
  }
}
public class UserDaoMySqlImpl implements UserDao {
  public String getUser() {
    return "从MySQL查询用户";
  }
}

public interface UserService {
  void getUser();
}
public class UserServiceImpl implements UserService {
  // 如果想更换数据库，这个地方需要改一下
  private UserDao userDao = new UserDaoMSSqlImpl();

  public void getUser() {
    System.out.println(userDao.getUser());
  }
}

public class UserTest {
  public static void main(String[] args) {
    UserService userService = new UserServiceImpl();
    userService.getUser();
  }
}

<dependencies>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.2.8.RELEASE</version>
  </dependency>
</dependencies>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    https://www.springframework.org/schema/beans/spring-beans.xsd">

  <bean id="mssqlImpl" class="com.example.dao.UserDaoMSSqlImpl"></bean>
  <bean id="mysqlImpl" class="com.example.dao.UserDaoMySqlImpl"></bean>

  <bean id="userServiceImpl" class="com.example.service.UserServiceImpl">
    <!-- 更新ref：mssqlImpl或mysqlImpl，即可实现数据库切换 -->
    <property name="userDao" ref="mysqlImpl"></property>
  </bean>
</beans>

public class UserServiceImpl implements UserService {
  private UserDao userDao;

  public void getUser() {
    System.out.println(userDao.getUser());
  }

  // 程序被动接受入口，依赖注入就是利用set方法注入
  public void setUserDao(UserDao userDao) {
    this.userDao = userDao;
  }
}

public class UserTest {
  public static void main(String[] args) {
    // 获取Spring上下文对象
    ApplicationContext context = new ClassPathXmlApplicationContext("services.xml");
    UserService userService1 = (UserService)context.getBean("userServiceImpl");
    userService1.getUser();
  }
}