由springcloud ribbon的 @LoadBalanced注解的使用理解
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 List restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
final List 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 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 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 List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。restTemplates = Collections.emptyList();
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:
@Bean("user1")
User user1() {
return new User("1", "a");
}
@Bean("user2"))
User user2() {
return new User("2", "b");
}然后我写一个Controller:
import com.example.demo.domain.User;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
@RestController
public class MyController {
@Autowired(required = false)
private List users = Collections.emptyList();
@Autowired(required = false)
private Map 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 List users = Collections.emptyList();
@Autowired(required = false)
private Map 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:
import com.example.demo.domain.User;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
@RestController
public class MyController {
@Autowired(required = false)
@Qualifier("valid")
private List users = Collections.emptyList();
@Autowired(required = false)
@Qualifier("invalid")
private Map userMap = new HashMap<>();
@RequestMapping("/list")
public Object listUsers() {
return users;
}
@RequestMapping("/map")
public Object mapUsers() {
return userMap;
}
} 访问http://localhost:8080/list:
[
{
"id": "1",
"name": "a"
}
]{
"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
@Autowired(required = false)
@LoadBalanced
private List users = Collections.emptyList(); 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。