负载均衡LoadBalancer

nacos2.x已不支持ribbon，使用nacos2.x需要手动引入LoadBalancer

1 手写随机负载均衡算法

@Resource
private RestTemplate restTemplate;

//通过restTemplate访问随机一个url地址
@RequestMapping("/test")
public String loadbalance(){
    List<ServiceInstance> instances = discoveryClient.getInstance("order");
    List<String> uris = instances.stream().map(instance -> instance.getUri().toString() + "/order/query").collect(Collections.toList());
    int i = ThreadLocalRandom.current().nextInt(uris.size());
    String uri = uris.get(i);
    System.out.printLn("访问地址为" + uri);
    return this.restTemplate.getForObject(uri, String.calss);
}

2 LoadBalancerClient API

https://spring.io/guides/gs/spring-cloud-loadbalancer/

（1）引入依赖

<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-starter-loadbalancer</artifactId>
</dependency>

（2）写代码

@Resource
private LoadBalancerClient loadBalancerClient;

@RequestMapping("/loadbalancerclient-api")
public String loadbalancerclientApi(){
	ServiceInstance serviceInstance = loadBalancerClient.choose("order");
	String url = serviceInstance.getUri().toString()+"/order/query";
	return restTemplate.getForObject(url,String.class);
}

3 @LoadBalanced注解方式

（1）引入loadbalancer starter

<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-starter-loadbalancer</artifactId>
</dependency>

（2）自定义一个@LoadBalanced注解的RestTemplate

@Bean
@LoadBalanced // 相当于为restTemplate整合了lb的能力
public RestTemplate restTemplateLoadBalancer(){
	return new RestTemplate();
}

（3）写代码

@Resource
private RestTemplate restTemplateLoadBalancer;

@RequestMapping("/loadbalancerclient-annotation")
public String loadbalancerclientAnnotation(){
	return this.restTemplateLoadBalancer.getForObject("http://order/order/query", String.class);
}

4 更改默认负载均衡方式

https://docs.spring.io/spring-cloud-commons/docs/current/reference/html/#switching-between-the-load-balancing-algorithms

默认的负载均衡为轮询

（1）定义配置类

public class CustomLoadBalancerConfiguration {
	@Bean
	ReactorLoadBalancer<ServiceInstance> randomLoadBalancer(Environment environment, LoadBalancerClientFactory loadBalancerClientFactory) {
	String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
	return new RandomLoadBalancer(loadBalancerClientFactory.getLazyProvider(name,ServiceInstanceListSupplier.class), name);
	}
}

（2）在启动类上通过@LoadBalancerClients注解使用自定义的配置类

@SpringBootApplication
@LoadBalancerClients(defaultConfiguration = CustomLoadBalancerConfiguration.class)
public class UserApplication {

5 核心源码分析

实际上，loadbalancer注解就是给RestTemplate添加了一个拦截器，调用过程简单来说，就是RestTemplate加上loadbalancer注解后，在创建请求之前添加了一个拦截器，在拦截器中通过服务名称调用LoadBalancer的choose方法，获取到真实的ip:port然后再拼接上具体的接口，替换原有uri，再通过RestTemplate进行远程调用。

5.1 LoadBalancerClient#choose

（1）LoadBalancerClient的自动装配

BlockingLoadBalancerClientAutoConfiguration

@Bean
@ConditionalOnBean(LoadBalancerClientFactory.class)
@ConditionalOnMissingBean
public LoadBalancerClient blockingLoadBalancerClient(LoadBalancerClientFactory loadBalancerClientFactory) {
	return new BlockingLoadBalancerClient(loadBalancerClientFactory);
}

（2）LoadBalancerClient-choose源码分析

5.2 @LoadBalanced和RestTemplate

5.2.1 RestTamplate调用过程

5.2.2 RestTemplate使用@LoadBalanced

（1）向Spring IoC容器中注入RestTemplate的Bean

@Bean
@LoadBalanced // 相当于为RestTemplate整合了LoadBalancer的功能
public RestTemplate restTemplateLoadBalancer(){
	return new RestTemplate();
}

（2）依赖注入

@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
	@LoadBalanced
	@Autowired(required = false) // 获取到restTemplateLoadBalancer，然后给其添加拦截器
	private List<RestTemplate> restTemplates = Collections.emptyList();

（3）创建拦截器

@Bean
public LoadBalancerInterceptor loadBalancerInterceptor(LoadBalancerClient loadBalancerClient,
                                                       LoadBalancerRequestFactory requestFactory) {
	return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
}

（4）给指定的restTemplate添加拦截器属性

@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {
	return restTemplate -> {
        List<ClientHttpRequestInterceptor> list = new ArrayList<>(restTemplate.getInterceptors());
		list.add(loadBalancerInterceptor);
		restTemplate.setInterceptors(list);
	};
}

（5）拥有了拦截器能力的RestTemplate