springcloud ribbon的负载均衡注解 @LoadBalanced的实现原理（@Qulifier注解的使用）
@Configuration
public class MainConfig {
@Bean
@LoadBalanced
public RestTemplate restTemplate() {
return new RestTemplate();
}
}
如上，在使用springcloud ribbon的rest服务客户端的时候，可给RestTemplate的bean添加@LoadBalanced注解，可使该RestTemplate在请求时拥有客户端负载均衡的能力。
以下是该注解功能来源分析：
一、@LoadBalanced的源码：
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 {
}该注解的特殊之处在于使用了@Qulifier注解。
搜索@LoadBalanced注解的使用，springcloud源码中有三个类使用了该注解：
（由第三个的@ConditionalOnClass(RestTemplate.class)推断第三个才是此处有关的）
1、
@Configuration
@ConditionalOnClass(WebClient.class)
@ConditionalOnBean(LoadBalancerClient.class)
public class ReactiveLoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List webClientBuilders = Collections.emptyList();
public List getBuilders() {
return webClientBuilders;
}
………… 2、
/**
* Auto configuration for Ribbon (client side load balancing).
*
* @author Rob Worsnop
*/
@Configuration
@ConditionalOnBean(LoadBalancerClient.class)
@ConditionalOnClass(AsyncRestTemplate.class)
public class AsyncLoadBalancerAutoConfiguration {
@Configuration
static class AsyncRestTemplateCustomizerConfig {
@LoadBalanced
@Autowired(required = false)
private List restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedAsyncRestTemplateInitializer(
final List customizers) {
return new SmartInitializingSingleton() {
@Override
public void afterSingletonsInstantiated() {
for (AsyncRestTemplate restTemplate : AsyncRestTemplateCustomizerConfig.this.restTemplates) {
for (AsyncRestTemplateCustomizer customizer : customizers) {
customizer.customize(restTemplate);
}
}
}
};
}
}
………… 3
package org.springframework.cloud.client.loadbalancer;
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.backoff.BackOffPolicy;
import org.springframework.retry.support.RetryTemplate;
import org.springframework.web.client.RestTemplate;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
/**
* Auto configuration for Ribbon (client side load balancing).
*
* @author Spencer Gibb
* @author Dave Syer
* @author Will Tran
* @author Gang Li
*/
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List restTemplates = 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);
}
}
});
}
@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 restTemplate -> {
List list = new ArrayList<>(
restTemplate.getInterceptors());//把已有的拦截器列表取到。
list.add(loadBalancerInterceptor);//把负载均衡拦截器放进去。
restTemplate.setInterceptors(list);//把最新的拦截器列表放进去。
};
}
}
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public LoadBalancedRetryFactory loadBalancedRetryFactory() {
return new LoadBalancedRetryFactory() {};
}
}
@Configuration
@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);
};
}
}
} 以上第3个代码中的Lambda表达式，在idea中使用源码，用Alt+Enter快捷键将其还原之后的代码为下：
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
final ObjectProvider> restTemplateCustomizers) {
return new SmartInitializingSingleton() {
@Override
public void afterSingletonsInstantiated() {
restTemplateCustomizers.ifAvailable(new Consumer>() {
@Override
public void accept(List customizers) {
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);
}
};
}
}
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public LoadBalancedRetryFactory loadBalancedRetryFactory() {
return new LoadBalancedRetryFactory() {};
}
}
@Configuration
@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 new RestTemplateCustomizer() {
@Override
public void customize(RestTemplate restTemplate) {
List list = new ArrayList<>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
}
}
} LoadBalancerAutoConfiguration类的代码中，用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。
二、起作用的原理：
@LoadBalanced
@Autowired(required = false)
private List restTemplates = Collections.emptyList(); 这段代码能将所有标注了@LoadBalanced的RestTemplate自动注入进来，在于@LoadBalanced注解上的@Qualifier注解。
@Qualifier注解和日常使用的@Autowired注解：
一）、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");
}Controller:
@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;
}
} Controller通过以下代码：
@Autowired(required = false)
private List users = Collections.emptyList();
@Autowired(required = false)
private Map userMap = new HashMap<>(); 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"
}
]二）、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");
}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 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;
}
} 那么所有标注了@Qualifier("valid")的user bean都会自动注入到List
访问http://localhost:8080/list:
[
{
"id": "1",
"name": "a"
}
]访问http://localhost:8080/map:
{
"user2": {
"id": "2",
"name": "b"
}
}三）、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。一般我们都是@Qulifier("asd")指定id注入; 如果@Qulifier不指定value，则就是注入@Bean标有@Qualifier的所有对象；但是这里又区分注入类型是Map