Spring注解Qualifier使用场景，并剖析在SpringCloud-Ribbon负载均衡组件中的应用

一、当Spring上下文中含有某个Bean的多个实例，可以指定获取某个实例：

 代码示例：

@Configuration
public class MyTest {


    @Bean("myExample1")
    @Qualifier
    MyExample myExample1()
    {
        return new MyExample("myExample1","Instance myExample1");
    }
    @Bean("myExample2")
    @Qualifier
    MyExample myExample2()
    {
        return new MyExample("myExample2","Instance myExample2");
    }
    @Bean
    @Qualifier
    MyTest myTest()
    {
        return new MyTest();
    }

}



@RestController
public class TestController {

    @Qualifier("myExample1")
    @Autowired
    List mysExampleList= Collections.emptyList();


    @GetMapping("/test")
    public Object test()
    {
        return mysExampleList;
    }
}

运行结果：

            

二、获取Spring上下文中，某个Bean的创建，有使用Qualifier注解标记的对象集合：

修改场景一种的Controller类，去掉Qualifier指定，代码改造如下：

package com.jason.example.userservice.qualifier.TestController;

import com.jason.example.userservice.qualifier.MyExample;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Collection;
import java.util.Collections;
import java.util.List;

/**
 * @program: userservice
 * @description
 * @author: 大龄程序猿
 * @create: 2020-07-02 22:07
 **/
@RestController
public class TestController {

    @Qualifier//获取spring上下文中，所有MyExample对象的创建有使用Qualifier注解标记的集合。
    @Autowired
    List mysExampleList= Collections.emptyList();


    @GetMapping("/test")
    public Object test()
    {
        return mysExampleList;
    }
}

运行结果：

从运行结果看，在去掉控制器中Qualifier别名的指定后，默认将Spring上下文中，所有MyExample创建使用Qualifier的对象，注入到集合中。

 

三、剖析在SpringCloud-ribbon组件中的运用：

Spring上下文中，所有使用了LoadBalancer注解标记的 RestTemplate对象，其中LoadBalancer 注解内部使用Qualifier注解：

@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {

}

 示意图： 

 

源码：

 

/*
 * Copyright 2012-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.loadbalancer;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.retry.support.RetryTemplate;
import org.springframework.web.client.RestTemplate;

/**
 * Auto-configuration for Ribbon (client-side load balancing).
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Will Tran
 * @author Gang Li
 */
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

        //获取所有使用LoadBalanced注解标记的  RestTemplate的对象。
	@LoadBalanced
	@Autowired(required = false)
	private List restTemplates = Collections.emptyList();

	@Autowired(required = false)
	private List transformers = Collections.emptyList();

	@Bean
	public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
			final ObjectProvider> restTemplateCustomizers) {
		return () -> restTemplateCustomizers.ifAvailable(customizers -> {
			for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
				for (RestTemplateCustomizer customizer : customizers) {
					customizer.customize(restTemplate);
				}
			}
		});
	}

	@Bean
	@ConditionalOnMissingBean
	public LoadBalancerRequestFactory loadBalancerRequestFactory(
			LoadBalancerClient loadBalancerClient) {
		return new LoadBalancerRequestFactory(loadBalancerClient, this.transformers);
	}

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
	static class LoadBalancerInterceptorConfig {

		@Bean
		public LoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRequestFactory requestFactory) {
			return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
		}

		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final LoadBalancerInterceptor loadBalancerInterceptor) {
			return restTemplate -> {
				List list = new ArrayList<>(
						restTemplate.getInterceptors());
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}

	}

	/**
	 * Auto configuration for retry mechanism.
	 */
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass(RetryTemplate.class)
	public static class RetryAutoConfiguration {

		@Bean
		@ConditionalOnMissingBean
		public LoadBalancedRetryFactory loadBalancedRetryFactory() {
			return new LoadBalancedRetryFactory() {
			};
		}

	}

	/**
	 * Auto configuration for retry intercepting mechanism.
	 */
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass(RetryTemplate.class)
	public static class RetryInterceptorAutoConfiguration {

		@Bean
		@ConditionalOnMissingBean
		public RetryLoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRetryProperties properties,
				LoadBalancerRequestFactory requestFactory,
				LoadBalancedRetryFactory loadBalancedRetryFactory) {
			return new RetryLoadBalancerInterceptor(loadBalancerClient, properties,
					requestFactory, loadBalancedRetryFactory);
		}

		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
			return restTemplate -> {
				List list = new ArrayList<>(
						restTemplate.getInterceptors());
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}

	}

}