源码探索之@LoadBalanced注解工作原理

蔚落 2023-10-07 20:01 15阅读 0赞

## 源码探索之@LoadBalanced注解工作原理 ##

### @LoadBalanced所在依赖包 ###

测试项目所用包spring-cloud-commons：

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### @LoadBalanced源码 ###

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

### @LoadBalanced的使用 ###

在使用微服务框架时，通过`ribbon`实现多服务之间的负载均衡，需要对`RestTemplate`bean对象加上`@LoadBalanced`注解即可，如下：

@Configuration
    public class RestTemplateConfig {
        
    
        @Bean
        @LoadBalanced
        public RestTemplate getRestTemplate() {
        
            return new RestTemplate();
        }
    }

### @LoadBalanced注解加上后有了什么变化？ ###

查找该注解的使用类，可以看到有2个

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#### 1.AsyncLoadBalancerAutoConfiguration类 ####

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#### 2.LoadBalancerAutoConfiguration类 ####

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可以看到，这2个`负载均衡自动配置类`有个明显的注解差别，配置类2的注解中多了一个`@ConditionalOnClass(RestTemplate.class)`，该注解的意思是：当项目中有`RestTemplate.class`类时，该配置类才会生效！

#### 3.关注LoadBalancerAutoConfiguration类 ####

##### 3.1代码 #####

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标记1处，注入有`@LoadBalanced`注解的`RestTemplate`bean对象；

标记2处，给`RestTemplate`注入拦截器`loadBalancerInterceptor`；

标记4处，注入负载均衡实例。

##### 3.2拦截器loadBalancerInterceptor #####

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标记3处，执行负载均衡对象请求服务实例的动作。

Debug模式看看上述`标记4`处该负载均衡对象是哪个实例，如下：

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可以看的是由Ribbon实现的负载均衡。

### 总结 ###

1.  使用`Ribbon`做负载均衡，代码配置`RestTemplate`，并加上`@LoadBalanced`注解
2.  被`@LoadBalanced`注解的`RestTemplate`bean对象被注入到`LoadBalancerAutoConfiguration`类管理
3.  `LoadBalancerAutoConfiguration`类给`RestTemplate`对象添加默认拦截器`loadBalancerInterceptor`
4.  拦截器`loadBalancerInterceptor`用注入的`Ribbon负载均衡客户端`实例实现`负载均衡`

[img-O71V3reP-1664418259269_C_Users_xwy_AppData_Roaming_Typora_typora-user-images_image-20220928155139897.png_pic_center]: https://img-blog.csdnimg.cn/ddd64b30bf0541b3b488079ebff16acb.png
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