@AutoWired specific explanation

Use @Autowired to autowire Beans

@Autowired annotation autowires a single bean attribute with compatible types

Constructors, ordinary fields (even non-public), all methods with parameters can be applied @Authwired annotation

By default, all attributes annotated with @Autowired need to be set. When Spring cannot find a matching bean assembly attribute, an exception will be thrown . If a certain attribute is not allowed to be set, you can set the required attribute annotated with @Autowired False

By default, when there are multiple types of compatible beans in the IOC container, auto- wiring through types will not work. At this time, you can provide the name of the bean in the @Qualifier annotation. Spring allows you to annotate the input parameters of the method @Qualifiter Specify the name of the injected bean

The @Autowired annotation can also be applied to array-type attributes, at this time Spring will automatically wire all matching beans.

The @Autowired annotation can also be applied to collection properties . At this time, Spring reads the type information of the collection and then automatically assembles all compatible beans. 

@Autowired annotation is used on java.util.Map , if the key value of the Map is String, then Spring will automatically assemble a Bean compatible with its Map value type, then the name of the bean is used as the key value

package com.learn.spring.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

import com.learn.spring.service.UserService;

/**
 * 表现层的组件
 * 
 * @Controller的作用:
 * 	  等价于在xml配置文件中: <bean id="userController" class="com.learn.spring.controller.UserController"></bean>
 * @Controller注解默认的id值是类名首字母小写,如果想要自己指定，可以使用value属性来指定
 * @Controller(value="id") / @Controller("id")
 */
@Controller  
//@Component
public class UserController {
	
	@Autowired(required=false)  // 默认情况下，required=true,代表@AutoWired标注的属性必须被装配.
	                            //可以改为required=false，有就装配，没有就算了.
	private UserService userService ;
	
	public void login(){
		System.out.println("UserController  login .....");
		userService.Handlelogin();
	}

}

package com.learn.spring.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Service;

import com.learn.spring.dao.UserDao;

/**
 * 业务层的组件
 *
 */
@Service
public class UserService {
	
	/**
	 *@AutoWired 优先采用类型的匹配的方式进行bean的装配. 如果有多个类型兼容的bean匹配了,会使用
	 *			 属性名 与bean的id值进行匹配. 
	 */
	@Autowired
	@Qualifier("userDaoMyBatisImpl") //进一步的指定要装配的bean的id值.
	private UserDao userDao ;
	
	public void Handlelogin(){
		System.out.println("UserService  handleLogin ....");
		//处理登录的逻辑
		userDao.getUserByUsernameAndPassword();
	}
}

package com.learn.spring.dao;

public interface UserDao {
	public void getUserByUsernameAndPassword();
		
	
}

package com.learn.spring.dao;

import org.springframework.stereotype.Repository;

@Repository
public class UserDaoMyBatisImpl  implements UserDao{
	
	public void getUserByUsernameAndPassword(){
		System.out.println("UserDaoMyBatisImpl getUserByUsernameAndPassword......");
	}
}