Spring的applicationContext默认使用无参构造创建对象

public class User {
    private String name;
　　
    public User() {
        System.out.println("User的无参构造");
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

<?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="user" class="com.rzk.pojo.User">
        <property name="name" value="小明"/>默认使用无参设置值
    </bean>


</beans>

public class Mytext {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        User user = (User) context.getBean("user");
        <!--System.out.println(user.getName());-->
　　　　　application一注册就实例化
    }
}

当我们创建ApplicationContext对象的时候 同时会将 配置文件中配置的bean对象一起创建

（饿汉模式 ）这时候无参构造也被加载

 上面是使用无参设置值

-------------

下面是有参设置

public class User {
    private String name;

    public User(String name) {
        this.name = name;
        System.out.println("User的有参构造");
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

直接通过参数名设置值

 <?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="user" class="com.rzk.pojo.User">
 <!--        <property name="name" value="小明"/>-->
         <constructor-arg name="name"  value="小明" />使用有参注入值
     </bean>
 </beans>

-------------

通过下标赋值

public class User {
    private String name;
    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;
        System.out.println("User的有参构造");
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

<?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="user" class="com.rzk.pojo.User">
        <constructor-arg index="0"  value="小明" />
        <constructor-arg index="1" value="12"/>
    </bean>
</beans>