spring的IoC（Inversion of Control）3

Part I: Spring's IoC (Inversion of Control) 2

In the previous article beans.xml, we specified the class object that needs to be instantiated. In Springthe IoCdefault construction method called to create an object is no-parameter construction.
Here is a good test:
modify the following UserServiceImplcategories, as follows:

public class UserServiceImpl implements UserService{
    
    

    private UserDao userDao;

    // set注入
    public void setUserDao(UserDao dao){
    
    
        this.userDao = dao;
    }

    public UserServiceImpl(){
    
    
        System.out.println("UserServiceImpl无参数构造！");
    }

    public UserServiceImpl(UserDao dao){
    
    
        this.userDao = dao;
        System.out.println("UserServiceImpl有参构造！");
    }

    public void getUserService() {
    
    
        userDao.getUser();
    }
}

Test at this time:

public class myTest {
    
    

    @Test
    public void Test(){
    
    
        // create and configure beans
        ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
        UserService userService = (UserService) context.getBean("userService");
        userService.getUserService();
    }
}

Still call Test, and then there is no problem, the test results are as follows: It

can be seen that the default constructor is no-argument constructor.
Then we beans.xmlpass in the change object in:

<?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="mySqlDao" class="com.dao.UserMySQLImpl"/>
    <bean id="oracleDao" class="com.dao.UserOracleImpl"/>

    <bean id="userService" class="com.service.UserServiceImpl">
        <constructor-arg type="com.dao.UserDao" ref="oracleDao"/>
    </bean>
</beans>

Specify the incoming type, and reference.
Then execute Test: the

constructor parameter passes the document address: https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#beans-constructor-injection

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Reference video address: https://www.bilibili.com/video/BV1WE411d7Dv?p=6