Java Spring 自动装配

如果被注入的属性类型是Bean引用的话，那么可以在<bean>标签中使用autowire属性去配置自动注入方式，属性值有两个

• byName：通过属性名自动装配，即去匹配setXxx与id="xxx"(name="xxx")是否一致；
• byType：通过Bean的类型从容器中匹配，匹配出多个相同Bean类型，报错。

beans.xml

<bean class="org.example.UserDaoImpl" id="userDao"/>
    <bean class="org.example.UserServiceImpl" id="userService" autowire="byName"/>
    <bean class="org.example.UserServiceImpl" id="userService1" autowire="byType"/>

UserService

public interface UserService {
    public void show();
}

UserServiceImpl

public class UserServiceImpl implements UserService {
    private UserDao userDao;
    public void setUserDao(UserDao userDao){
        this.userDao = userDao;
    }

    @Override
    public void show() {
        System.out.println(userDao);
    }
}

主函数调用

public static void main(String[] args) {
    ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
    ((UserService) applicationContext.getBean("userService")).show();
    ((UserService) applicationContext.getBean("userService1")).show();
}

执行结果

org.example.UserDaoImpl@1f0f1111
org.example.UserDaoImpl@1f0f1111

如果xml文件自动装配中有多个同类型的bean容器将会报错

例如：

<bean class="org.example.UserDaoImpl" id="userDao"/>
    <bean class="org.example.UserDaoImpl" id="userDao1"/>
    <bean class="org.example.UserServiceImpl" id="userService" autowire="byName"/>
    <bean class="org.example.UserServiceImpl" id="userService1" autowire="byType"/>

IDEA报错提示

无法自动装配。存在多个 'UserDao' 类型的 Bean。Bean: userDao1,userDao。属性: 'userDao'

运行结果提示

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService1' defined in class path resource [beans.xml]: Unsatisfied dependency expressed through bean property 'userDao'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'org.example.UserDao' available: expected single matching bean but found 2: userDao,userDao1
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userService1' defined in class path resource [beans.xml]: Unsatisfied dependency expressed through bean property 'userDao'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'org.example.UserDao' available: expected single matching bean but found 2: userDao,userDao1