Spring 基于 XML 的自动装配：byName工作原理

public class UserDao {
    public void save() {
        System.out.println("UserDao.save()");
    }
}
public class UserService {
    private UserDao userDao;
    // setter 方法名决定了属性名：userDao
    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }
    public void register() {
        userDao.save();
    }
}

<bean id="userDao" class="com.example.UserDao"/>
<bean id="userService" class="com.example.UserService" autowire="byName"/>

<bean id="userDao" class="com.example.UserDao"/>
<bean id="userService" class="com.example.UserService">
    <property name="userDao" ref="userDao"/>
</bean>

public class UserService {
    private UserDao userDao;         // 匹配 id="userDao" 的 Bean
    private EmailService emailService;  // 匹配 id="emailService" 的 Bean
}

<bean id="userDao" class="com.example.UserDao"/>
<bean id="emailService" class="com.example.EmailService"/>
<bean id="userService" class="com.example.UserService" autowire="byName"/>

public class UserService {
    private UserDao myUserDao;  // 属性名是 myUserDao
    public void setMyUserDao(UserDao myUserDao) {
        this.myUserDao = myUserDao;
    }
}

<bean id="userDao" class="com.example.UserDao"/>
<bean id="userService" class="com.example.UserService" autowire="byName"/>

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

<!-- 没有定义 userDao -->
<bean id="userService" class="com.example.UserService" autowire="byName"/>

public class UserService {
    private UserDao userDao;
    // 没有 setter 方法
}

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="..."
       default-autowire="byName">
    <bean id="userDao" class="com.example.UserDao"/>
    <bean id="emailService" class="com.example.EmailService"/>
    <!-- 都会自动按名称装配 -->
    <bean id="userService" class="com.example.UserService"/>
    <bean id="orderService" class="com.example.OrderService"/>
</beans>

<bean id="specialService" class="com.example.SpecialService" autowire="no"/>

// 简化逻辑
if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
    autowireByName(beanName, mbd, bw, newPvs);
}
protected void autowireByName(String beanName, AbstractBeanDefinition mbd, 
                              BeanWrapper bw, MutablePropertyValues pvs) {
    // 1. 获取所有需要注入的属性名
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        // 2. 检查容器中是否有同名 Bean
        if (containsBean(propertyName)) {
            // 3. 获取 Bean 实例
            Object bean = getBean(propertyName);
            // 4. 添加到属性值列表
            pvs.add(propertyName, bean);
        }
    }
}

<bean id="userDaoA" class="com.example.UserDao"/>
<bean id="userDaoB" class="com.example.UserDao"/>
<bean id="userService" class="com.example.UserService" autowire="byName">
    <!-- 显式指定使用 userDaoB，覆盖自动装配 -->
    <property name="userDao" ref="userDaoB"/>
</bean>

@Required
public void setUserDao(UserDao userDao) {
    this.userDao = userDao;
}