<!-- 注册userService --> <bean id="userService" class="com.lyu.spring.service.impl.UserService"> 　　<constructor-arg ref="userDaoJdbc"></constructor-arg> </bean> <!-- 注册jdbc实现的dao --> <bean id="userDaoJdbc" class="com.lyu.spring.dao.impl.UserDaoJdbc"></bean>

public class UserService implements IUserService {private IUserDao userDao; public UserService(IUserDao userDao) { this.userDao = userDao;} public void loginUser() {userDao.loginUser();}}

@Test
public void testDI() { ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");// 获取bean对象UserService userService = ac.getBean(UserService.class, "userService"); // 模拟用户登录userService.loginUser();
}

public class UserService implements IUserService {private IUserDao userDao; private User user; public UserService(IUserDao userDao) { System.out.println("这是有一个参数的构造方法"); this.userDao = userDao;}public UserService(IUserDao userDao, User user) { System.out.println("这是有两个参数的构造方法"); this.userDao = userDao; this.user = user; } public void loginUser() {userDao.loginUser(); }
}

<!-- 注册userService --><bean id="userService" class="com.lyu.spring.service.impl.UserService"> <!-- 写法一 --> <!-- <property name="UserDao" ref="userDaoMyBatis"></property> --><!-- 写法二 --> <property name="userDao" ref="userDaoMyBatis"></property> </bean><!-- 注册mybatis实现的dao -->
<bean id="userDaoMyBatis" class="com.lyu.spring.dao.impl.UserDaoMyBatis"></bean>

public class UserService implements IUserService { private IUserDao userDao1;public void setUserDao(IUserDao userDao1) { this.userDao1 = userDao1; }public void loginUser() {userDao1.loginUser();}
}

@Resource
@Qualifier("userDaoMyBatis")
private IUserDao userDao;public UserService(){}

// candidateBeans 为上一步通过类型匹配到的多个bean，该 Map 中至少有两个元素。 protected String determineAutowireCandidate(Map<String, Object> candidateBeans, DependencyDescriptor descriptor) {//  requiredType 为匹配到的接口的类型
Class<?> requiredType = descriptor.getDependencyType();// 1. 先找 Bean 上有@Primary 注解的，有则直接返回 String primaryCandidate = this.determinePrimaryCandidate(candidateBeans, requiredType);if (primaryCandidate != null) { return primaryCandidate; } else { // 2.再找 Bean 上有 @Order，@PriorityOrder 注解的，有则返回 String               priorityCandidate = this.determineHighestPriorityCandidate(candidateBeans, requiredType); if (priorityCandidate != null) { return priorityCandidate; } else {Iterator var6 = candidateBeans.entrySet().iterator(); String candidateBeanName; Object beanInstance; do {if (!var6.hasNext()) { return null; } // 3. 再找 bean 的名称匹配的 Entry<String, Object> entry = (Entry)var6.next(); candidateBeanName = (String)entry.getKey(); beanInstance = entry.getValue();}          while(!this.resolvableDependencies.values().contains(beanInstance) && !this.matchesBeanName(candidateBeanName,         descriptor.getDependencyName()));return candidateBeanName; }}}