@Autowired与@Resource的区别

@Autowired与@Resource相同点 

两者都可以写在字段和setter方法上。两者如果都写在字段上，那么就不需要再写setter方法。

@Autowired与@Resource不同点

1.@Autowired(org.springframework.beans.factory.annotation.Autowired)属于Spring的注解，@Resource(javax.annotation.Resource)属于J2EE的注解。
2.@Autowired是通过byType注入的；@Resource是通过byName注入的；

@Resource有2个重要的属性分别是name和type,Spring将@Resource注解的name属性解析为bean的名字,而type则解析为bean的类型。所以使用name属性，则使用byName的自动注入策略，而使用byType属性时则使用byType自动注入策略。如果没用指定name或者type则默认通过byName自动注入策略。

注：最好是将@Resource放在setter方法上，因为这样更符合面向对象的思想，通过set、get去操作属性，而不是直接去操作属性。

@Resource装配顺序

①如果同时指定了name和type，则从Spring上下文中找到唯一匹配的bean进行装配，找不到则抛出异常。

②如果指定了name，则从上下文中查找名称（id）匹配的bean进行装配，找不到则抛出异常。

③如果指定了type，则从上下文中找到类似匹配的唯一bean进行装配，找不到或是找到多个，都会抛出异常。

④如果既没有指定name，又没有指定type，则自动按照byName方式进行装配；如果没有匹配，则回退为一个原始类型进行匹配，如果匹配则自动装配。

@Resource的作用相当于@Autowired，只不过@Autowired按照byType自动注入。

代码演示：

定义一个 接口  OrderMapper，二个实现类 OrderService01、OrderService02。一个 PayService 调用 orderMapper 接口中的方法。

orderMapper

package com.example.service;

/**
 * @author mingtian
 * @version V1.0
 * @Description
* @date 2019/12/9 16:57
 */
public interface OrderMapper {
/**
     * 新增订单方法
     */
void add();
}

OrderServiceImpl01

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.stereotype.Component;

/**
 * @author mingtian
 * @version V1.0
 * @Description 订单接口实现类
 * @date 2019/12/9 16:55
 */
@Component
public class OrderServiceImpl01 implements OrderMapper {
public void add() {
System.out.println("OrderService01");
}
}

OrderServiceImpl02

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.stereotype.Component;

/**
 * @author mingtian
 * @version V1.0
 * @Description 订单接口实现类
 * @date 2019/12/9 16:55
 */
@Component
public class OrderServiceImpl02 implements OrderMapper {
public void add() {
System.out.println("OrderService02");
}
}

PayService 使用 @Auwowried

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;

/**
 * @author mingtian
 * @version V1.0
 * @Description PayService
 * @date 2019/12/9 16:54
 */
@Component
public class PayService {

@Autowired
private OrderMapper orderMapper;

    public void add() {
orderMapper.add();
}
}

配置类

package com.example.config;

import com.example.entity.Order;
import com.example.entity.Win10;
import org.springframework.context.annotation.*;

/**
 * @author mingtian
 * @version V1.0
 * @Description
 * @ComponentScan("com.example.entity") //开启扫包范围
 * @date 2019/12/9 11:17
 */
@Configuration
@ComponentScan("com.example")
public class MyConfig {

}

测试类

package com.example.test;

import com.example.config.MyConfig;
import com.example.entity.FactoryBeanEntity;
import com.example.service.impl.PayService;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

/**
 * @author mingtian
 * @version V1.0
 * @Description 测试类
 * @date 2019/12/9 11:00
 */
public class Test005 {

public static AnnotationConfigApplicationContext context;

    public static void main(String[] args) {
        context = new AnnotationConfigApplicationContext(MyConfig.class);
PayService payService = (PayService) context.getBean("payService");

       payService.add();

System.out.println("bean:" + payService);

}
}

现在启动报错，错误信息如下：

十二月 09, 2019 5:23:50 下午 org.springframework.context.support.AbstractApplicationContext refresh

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'payService': Unsatisfied dependency expressed through field 'orderMapper'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.example.service.OrderMapper' available: expected single matching bean but found 2: orderServiceImpl01,orderServiceImpl02

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'payService': Unsatisfied dependency expressed through field 'orderMapper'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.example.service.OrderMapper' available: expected single matching bean but found 2: orderServiceImpl01,orderServiceImpl02

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643)

at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:116)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1422)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)

at org.springframework.beans.factory.support.AbstractBeanFactory$$Lambda$31/221036634.getObject(Unknown Source)

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:879)

at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878)

at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550)

at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:89)

at com.example.test.Test005.main(Test005.java:19)

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.example.service.OrderMapper' available: expected single matching bean but found 2: orderServiceImpl01,orderServiceImpl02

at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:220)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1265)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1207)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)

... 15 more

由上面报错的错误信息可知，通过 @Autowired 注解注入的 orderMapper 找到了二个实现类导致的错误。

解决办法：

方法1 使用 @Autowired + @Qualifier(value="orderServiceImpl01")  显示的指定一个实现类

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;

/**
 * @author mingtian
 * @version V1.0
 * @Description PayService
 * @date 2019/12/9 16:54
 */
@Component
public class PayService {

@Autowired
    @Qualifier(value = "orderServiceImpl01")
private OrderMapper orderMapper;

    public void add() {
orderMapper.add();
}
}

方法2  使用@Resource(name="orderServiceImpl01")

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;

/**
 * @author mingtian
 * @version V1.0
 * @Description PayService
 * @date 2019/12/9 16:54
 */
@Component
public class PayService {

@Resource(name = "orderServiceImpl01")
private OrderMapper orderMapper;

    public void add() {
orderMapper.add();
}
}

由上面的二种解决方法可知：

 @Autowired + @Qualifier(value="orderServiceImpl01") 等同于 @Resource(name="orderServiceImpl01")

方法3 使用 @Primary 注解 指定默认加载的类，这样的话 直接在PayService 中 直接使用 @Autowired  一个注解即可，默认启动就会加载 OrderServiceImpl01 这个类

package com.example.service.impl;

import com.example.service.OrderMapper;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

/**
 * @author mingtian
 * @version V1.0
 * @Description 订单接口实现类
 * @date 2019/12/9 16:55
 */
@Primary
@Component
public class OrderServiceImpl01 implements OrderMapper {
public void add() {
System.out.println("OrderService01");
}
}

@Primary 这个注解 在 SpringBoot 多数据源的时候经常使用 ，用来设置加载默认的数据源(优先级)。

注意：使用 @Component 注解的时候，记得开启扫包的范围(使用 @ComponentScan("com.example")指定扫包范围)，不然启动会报错的。

 OrderMapper，二个实现类 OrderService01、OrderService02、