@LoadBalanced注解原理

在使用springcloud ribbon客户端负载均衡的时候，可以给RestTemplate bean 加一个@LoadBalanced注解，就能让这个RestTemplate在请求时拥有客户端负载均衡的能力：

@Bean
@LoadBalanced
RestTemplate restTemplate() {
    return new RestTemplate();
}
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是不是很神奇?打开@LoadBalanced的注解源码，并没有什么特殊的东东

package org.springframework.cloud.client.loadbalancer;
import org.springframework.beans.factory.annotation.Qualifier;
import java.lang.annotation.Documented;

import java.lang.annotation.ElementType;

import java.lang.annotation.Inherited;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;
/**

Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient
@author Spencer Gibb

*/

@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@Qualifier

public @interface LoadBalanced {

}

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唯一不同的地方就是多了一个@Qulifier注解.

搜索@LoadBalanced注解的使用地方，发现只有一处使用了,在LoadBalancerAutoConfiguration这个自动装配类中：

@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList();
@Bean

public SmartInitializingSingleton loadBalancedRestTemplateInitializer(

final List<RestTemplateCustomizer> customizers) {

return new SmartInitializingSingleton() {

@Override

public void afterSingletonsInstantiated() {

for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {

for (RestTemplateCustomizer customizer : customizers) {

customizer.customize(restTemplate);

}

}

}

};

}
@Autowired(required = false)

private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();
@Bean

@ConditionalOnMissingBean

public LoadBalancerRequestFactory loadBalancerRequestFactory(

LoadBalancerClient loadBalancerClient) {

return new LoadBalancerRequestFactory(loadBalancerClient, transformers);

}
@Configuration

@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 new RestTemplateCustomizer() {

@Override

public void customize(RestTemplate restTemplate) {

List<ClientHttpRequestInterceptor> list = new ArrayList<>(

restTemplate.getInterceptors());

list.add(loadBalancerInterceptor);

restTemplate.setInterceptors(list);

}

};

}

}
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这段自动装配的代码的含义不难理解，就是利用了RestTempllate的拦截器，使用RestTemplateCustomizer对所有标注了@LoadBalanced的RestTemplate Bean添加了一个LoadBalancerInterceptor拦截器，而这个拦截器的作用就是对请求的URI进行转换获取到具体应该请求哪个服务实例ServiceInstance。

那么为什么

@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList();
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这个restTemplates能够将所有标注了@LoadBalanced的RestTemplate自动注入进来呢？这就要说说@Autowired注解和@Qualifier这两个注解了。

大家日常使用很多都是用@Autowired来注入一个bean,其实@Autowired还可以注入List和Map,比如我定义两个Bean:

@RestController
public class MyController {
<span class="token annotation punctuation">@Autowired</span><span class="token punctuation">(</span>required <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">User</span><span class="token punctuation">&gt;</span></span> users <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">emptyList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token annotation punctuation">@Autowired</span><span class="token punctuation">(</span>required <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> <span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span><span class="token class-name">User</span><span class="token punctuation">&gt;</span></span> userMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token annotation punctuation">@RequestMapping</span><span class="token punctuation">(</span><span class="token string">"/list"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">listUsers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">return</span> users<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@RequestMapping</span><span class="token punctuation">(</span><span class="token string">"/map"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">mapUsers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">return</span> userMap<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
}
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在controller中通过:

@Autowired(required = false)
private List<User> users = Collections.emptyList();
@Autowired(required = false)

private Map<String,User> userMap = new HashMap<>();
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就可以自动将两个bean注入进来，当注入map的时候，map的key必须是String类型，然后bean name将作为map的key,本例，map中将有两个key分别为user1和user2,value分别为对应的User Bean实例。

访问http://localhost:8080/map:

{
    "user1": {
        "id": "1",
        "name": "a"
    },
    "user2": {
        "id": "2",
        "name": "b"
    }
}
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访问http://localhost:8080/list:

[
    {
        "id": "1",
        "name": "a"
    },
    {
        "id": "2",
        "name": "b"
    }
]
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然后我们给user1和user2分别打上@Qualifier修饰符:

@Bean("user1")
@Qualifier("valid")
User user1() {
    return new User("1", "a");
}
@Bean("user2")

@Qualifier("invalid")

User user2() {

return new User("2", "b");

}
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然后将controller中的user list 和user map分别也打上@Qualifier修饰符:

@RestController
public class MyController {
<span class="token annotation punctuation">@Autowired</span><span class="token punctuation">(</span>required <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@Qualifier</span><span class="token punctuation">(</span><span class="token string">"valid"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">User</span><span class="token punctuation">&gt;</span></span> users <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">emptyList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token annotation punctuation">@Autowired</span><span class="token punctuation">(</span>required <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@Qualifier</span><span class="token punctuation">(</span><span class="token string">"invalid"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> <span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span><span class="token class-name">User</span><span class="token punctuation">&gt;</span></span> userMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token annotation punctuation">@RequestMapping</span><span class="token punctuation">(</span><span class="token string">"/list"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">listUsers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">return</span> users<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@RequestMapping</span><span class="token punctuation">(</span><span class="token string">"/map"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">mapUsers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
    <span class="token keyword">return</span> userMap<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
}
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那么所有标注了@Qualifier(“valid”)的user bean都会自动注入到List users中去(本例是user1)，所有标注了@Qualifier(“invalid”)的user bean都会自动注入到Map<String,User> userMap中去（本例是user2），我们再次访问上面两个url:

访问http://localhost:8080/list:

[
    {
        "id": "1",
        "name": "a"
    }
]
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访问http://localhost:8080/map:


{
    "user2": {
        "id": "2",
        "name": "b"
    }
}
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看到这里我们可以理解@LoadBalanced的用处了，其实就是一个修饰符，和@Qualifier一样，比如我们给user1打上@LoadBalanced:

@Bean("user1")
 @LoadBalanced
 User user1() {
     return new User("1", "a");
 }
@Bean("user2")

User user2() {

return new User("2", "b");

}
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然后controller中给List users打上@LoadBalanced注解:

@Autowired(required = false)
@LoadBalanced
private List<User> users = Collections.emptyList();
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访问http://localhost:8080/list：

[
    {
        "id": "1",
        "name": "a"
    }
]
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和@Qualifier注解效果一样，只有user1被注入进了List，user2没有修饰符，没有被注入进去。

另外当spring容器中有多个相同类型的bean的时候，可以通过@Qualifier来进行区分，以便在注入的时候明确表明你要注入具体的哪个bean,消除歧义。

DiligentCoder

@LoadBalanced注解原理