Spring框架学习（10）：基于注解的方式配置Bean

使用组件扫描的方式可以让Spring从classpath下自动扫描，侦测和实例化具有特定注解的组件。这些注解分别是：

@Component：基本注解，标识一个受Spring管理的组件

@Respository：标识持久层组件

@Service：标记服务层（业务层）组件

@Controller：标识表示层组件

这些注解是建议大家在对应的层使用对应的组件，但实际上全部使用Component也没报错

对于扫描到的组件，Spring有默认的命名策略：使用非限定类名，第一个字母小写，我们也可以在注解中通过value属性标识组件的名称

用一个简单的例子来展示一下：

1.表示层组件

package annotation.controller;

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

import annotation.service.UserService;

@Controller
public class UserController {
	@Autowired
	private UserService userService;
	
	public void execute() {
		System.out.println("USerControllor excute...");
		userService.add();
	}
}

2.服务层组件

package annotation.service;

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

import annotation.repository.UserRepository;

@Service
public class UserService {
	@Autowired
	private UserRepository userRepository;
	
	//Autowire可以放在变量前也可以放在方法前
	//@Autowired
	//public void setUserRepository(UserRepository userRepository) {
	//	this.userRepository = userRepository;
	//}
	
	public void add() {
		System.out.println("USerService add...");
		userRepository.save();
	}
}

3.持久层组件

package annotation.repository;

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

@Repository
public class UserRepository {
	
	@Override
	public void save() {
		// TODO Auto-generated method stub
		System.out.println("UserRepository Save..");
	}
	
}

4.在bean的配置文件中：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd">

	<!-- 指定spring IOC容器扫描的包 -->
	 
	 <context:component-scan base-package="annotation" ></context:component-scan>
	 
</beans>

5,用main函数测试一下

package annotation;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import annotation.controller.UserController;
import annotation.repository.UserRepository;
import annotation.service.UserService;

public class Main {
	public static void main(String[] args) {
		ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-annotation.xml");		
		
		UserController userController = (UserController)ctx.getBean("userController");
		System.out.println(userController);
		userController.execute();
		
	}
}

在控制台上输出了以下内容：

annotation.controller.UserController@157696f
USerControllor excute...
USerService add...
UserRepository Save..

如果UserRepository是一个接口类，而且有多个实现类的话，应该怎么破呢？
让我们来试一试看看会出什么事

package annotation.repository;

public interface UserRepository {
	void save();
}

再写两个实现类

package annotation.repository;

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


@Repository
public class UserRepositoryImpl implements UserRepository {
	
	@Override
	public void save() {
		// TODO Auto-generated method stub
		System.out.println("UserRepository Save..");
	}
	
}

package annotation.repository;

import org.springframework.stereotype.Repository;

@Repository
public class UserJdbcRepository implements UserRepository {

	@Override
	public void save() {
		// TODO Auto-generated method stub
		System.out.println("UserJdbcRepository save..");
	}

}

再次运行上面的main函数会发现报错了
nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [annotation.repository.UserRepository] is defined: expected single matching bean but found 2: userJdbcRepository,userRepositoryImpl

它说没有唯一的实现类，为了解决这个问题，我们可以在UserService中指定用哪个类来实例化bean

package annotation.service;

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

import annotation.repository.UserRepository;

@Service
public class UserService {
	@Autowired
	//声明使用指定的类来实例化bean
	@Qualifier("userRepositoryImpl")
	private UserRepository userRepository;
	
	//Autowire可以放在变量前也可以放在方法前
	//@Autowired
	//@Qualifier("userRepositoryImpl")
	//public void setUserRepository(UserRepository userRepository) {
	//	this.userRepository = userRepository;
	//}
	
	public void add() {
		System.out.println("USerService add...");
		userRepository.save();
	}
}

再次运行就可以发现没问题了

在Autowire中设置required=fasle可以让Spring在没找到对应的bean时不进行报错，这里就不展示了