@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，可以使该RestTemplate在请求时拥有客户端负载均衡的能力。javascript
如下是该注解功能来源分析：html
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。java
搜索@LoadBalanced注解的使用，springcloud源码中有三个类使用了该注解：web
（由第三个的@ConditionalOnClass(RestTemplate.class)推断第三个才是此处有关的）spring
一、app
@Configuration
@ConditionalOnClass(WebClient.class)
@ConditionalOnBean(LoadBalancerClient.class)
public class ReactiveLoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List<WebClient.Builder> webClientBuilders = Collections.emptyList();
public List<WebClient.Builder> getBuilders() {
return webClientBuilders;
}
…………
二、负载均衡
/**
* 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<AsyncRestTemplate> restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedAsyncRestTemplateInitializer(
final List<AsyncRestTemplateCustomizer> customizers) {
return new SmartInitializingSingleton() {
@Override
public void afterSingletonsInstantiated() {
for (AsyncRestTemplate restTemplate : AsyncRestTemplateCustomizerConfig.this.restTemplates) {
for (AsyncRestTemplateCustomizer customizer : customizers) {
customizer.customize(restTemplate);
}
}
}
};
}
}
…………
3ide
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<RestTemplate> restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
return () -> restTemplateCustomizers.ifAvailable(customizers -> {
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 restTemplate -> {
List<ClientHttpRequestInterceptor> 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<ClientHttpRequestInterceptor> 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快捷键将其还原以后的代码为下：ui
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
return new SmartInitializingSingleton() {
@Override
public void afterSingletonsInstantiated() {
restTemplateCustomizers.ifAvailable(new Consumer<List<RestTemplateCustomizer>>() {
@Override
public void accept(List<RestTemplateCustomizer> customizers) {
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);
}
};
}
}
@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<ClientHttpRequestInterceptor> 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对全部标注了@LoadBalanced的RestTemplate Bean添加了一个LoadBalancerInterceptor拦截器，而这个拦截器的做用就是对请求的URI进行转换获取到具体应该请求哪一个服务实例ServiceInstance。this
@LoadBalanced @Autowired(required = false) private List<RestTemplate> restTemplates = Collections.emptyList();
这段代码能将全部标注了@LoadBalanced的RestTemplate自动注入进来，在于@LoadBalanced注解上的@Qualifier注解。
@Qualifier注解和平常使用的@Autowired注解：
一）、平常使用不少都是用@Autowired来注入一个bean,其实@Autowired还能够注入List和Map,好比我定义两个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<User> users = Collections.emptyList();
@Autowired(required = false)
private Map<String,User> userMap = new HashMap<>();
@RequestMapping("/list")
public Object listUsers() {
return users;
}
@RequestMapping("/map")
public Object mapUsers() {
return userMap;
}
}
Controller经过如下代码：
@Autowired(required = false)
private List<User> users = Collections.emptyList();
@Autowired(required = false)
private Map<String,User> userMap = new HashMap<>();
能够自动将两个bean注入进来，当注入map的时候，map的key必须是String类型，而后bean name将做为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）、而后咱们给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<User> users = Collections.emptyList();
@Autowired(required = false)
@Qualifier("invalid")
private Map<String,User> userMap = new HashMap<>();
@RequestMapping("/list")
public Object listUsers() {
return users;
}
@RequestMapping("/map")
public Object mapUsers() {
return userMap;
}
}
那么全部标注了@Qualifier("valid")的user bean都会自动注入到List<user> users中去(本例是user1)，全部标注了@Qualifier("invalid")的user bean都会自动注入到Map<String,User> userMap中去（本例是user2）。
访问http://localhost:8080/list:
[
{
"id": "1",
"name": "a"
}
]
访问http://localhost:8080/map:
{
"user2": {
"id": "2",
"name": "b"
}
}
三）、看到这里咱们能够理解@LoadBalanced的用处了，其实就是一个修饰符，和@Qualifier同样，好比咱们给user1打上@LoadBalanced:
@Bean("user1")
@LoadBalanced
User user1() {
return new User("1", "a");
}
@Bean("user2")
User user2() {
return new User("2", "b");
}
而后controller中给List<User> users打上@LoadBalanced注解:
@Autowired(required = false)
@LoadBalanced
private List<User> users = Collections.emptyList();
访问http://localhost:8080/list：
[
{
"id": "1",
"name": "a"
}
]
和@Qualifier注解效果同样，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，能够经过@Qualifier来进行区分，以便在注入的时候明确代表你要注入具体的哪一个bean,消除歧义。通常咱们都是@Qulifier("asd")指定id注入; 若是@Qulifier不指定value，则就是注入@Bean标有@Qualifier的全部对象；可是这里又区分注入类型是Map<K,T>，仍是集合类型 List<T>, 若是是Map<K,T>，容器中存在@Qulifier一个，则直接注入;若是有多个，看是否有beanid与字段name相同的，不然抛出异常 expected single matching bean but found n 。若是是List<T> 则将容器全部的@Qulifier注入到集合中。