@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();
}

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 {
    }

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 ListRestTemplate> restTemplates = Collections.emptyList();

@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
final ListRestTemplateCustomizer> 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 ListLoadBalancerRequestTransformer> 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) {
ListClientHttpRequestInterceptor> list = new ArrayList>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
}
}

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 ListRestTemplate> restTemplates = Collections.emptyList();

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 {
@Autowired(required = false)
private ListUser> users = Collections.emptyList();
@Autowired(required = false)
private MapString,User> userMap = new HashMap>();
@RequestMapping("/list")
public Object listUsers() {return users;}
@RequestMapping("/map")public Object mapUsers() {
return userMap;
}}

在controller中通过:

@Autowired(required = false)
private ListUser> users = Collections.emptyList();

@Autowired(required = false)
private MapString,User> userMap = new HashMap>();

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"
    }
}

访问http://localhost:8080/list:

[
{
"id": "1",
"name": "a"
},
{
"id": "2",
"name": "b"
}
]

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");
}

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

@RestController
public class MyController {
@Autowired(required = false)
@Qualifier("valid")
private ListUser> users = Collections.emptyList();
@Autowired(required = false)
@Qualifier("invalid")
private MapString,User> userMap = new HashMap>();
@RequestMapping("/list")
public Object listUsers() {
return users;
}
@RequestMapping("/map")
public Object mapUsers() {
return userMap;
}
}

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 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"
}
]

访问http://localhost:8080/map:


{
"user2": {
"id": "2",
"name": "b"
}
}

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");
}

然后controller中给List users打上@LoadBalanced注解:

@Autowired(required = false)
@LoadBalanced
private ListUser> users = Collections.emptyList();
  • 1
  • 2
  • 3

访问http://localhost:8080/list:

[
{
"id": "1",
"name": "a"
}
]

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。

文章来源于互联网:@LoadBalanced注解原理

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