负载均衡-ribbon源码解析

1 @LoadBalanced注解

java 复制代码

/**
 * 基于ribbon调用服务及负载均衡
 * @return
 */
@LoadBalanced
@Bean
public RestTemplate restTemplate(){
    return new RestTemplate();
}

java 复制代码

@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {
    return (restTemplate) -> {
        // 获取拦截器
        List<ClientHttpRequestInterceptor> list = new ArrayList(restTemplate.getInterceptors());
        // 添加一个新的拦截器（负载均衡的拦截器，该拦截器就是在restTemplate发送请求之前执行）
        list.add(loadBalancerInterceptor);
        // 将拦截器设置到restTemplate中  该restTemplate就是在OrderApplication中添加了@LoadBalanced的注解
        restTemplate.setInterceptors(list);
    };
}

总上所属：@LoadBalanced就是对template起到标识作用，加了@LoadBalanced注解的template开启负载均衡

2 源码解析

1）通过restTemplate发起请求

java 复制代码

Result result = restTemplate.getForObject("http://product-server/product/" + id, Result.class);

2）执行拦截器的intercept方法

java 复制代码

public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {
    URI originalUri = request.getURI();
    String serviceName = originalUri.getHost();
    Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
    // 调用loadBalancer的execute方法  loadBalancer：负载均衡器
    // serviceName：就是product-server
    return (ClientHttpResponse)this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution));
}

3）查看RibbonLoadBalancerClient中的execute方法

java 复制代码

public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint) throws IOException {
    // 通过负载均衡器 
    ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);
    // 通过负载均衡器 接 负载均衡算法 获取服务实例
    Server server = this.getServer(loadBalancer, hint);
    if (server == null) {
        throw new IllegalStateException("No instances available for " + serviceId);
    } else {
        RibbonLoadBalancerClient.RibbonServer ribbonServer = new RibbonLoadBalancerClient.RibbonServer(serviceId, server, this.isSecure(server, serviceId), this.serverIntrospector(serviceId).getMetadata(server));
        return this.execute(serviceId, (ServiceInstance)ribbonServer, (LoadBalancerRequest)request);
    }
}

4）根据负载均衡算法获取服务实例

java 复制代码

protected Server getServer(ILoadBalancer loadBalancer, Object hint) {
    return loadBalancer == null ? null : loadBalancer.chooseServer(hint != null ? hint : "default");
}

java 复制代码

public Server chooseServer(Object key) {
    if (this.counter == null) {
        this.counter = this.createCounter();
    }
    this.counter.increment();
    if (this.rule == null) {
        return null;
    } else {
        try {
            // rule就是配置的负载均衡算法
            // choose就是根据指定的负载均衡算法获取服务实例
            return this.rule.choose(key);
        } catch (Exception var3) {
            logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", new Object[]{this.name, key, var3});
            return null;
        }
    }
}

java 复制代码

// 默认的负载均衡器
private static final IRule DEFAULT_RULE = new RoundRobinRule();

}

```java
// 默认的负载均衡器
private static final IRule DEFAULT_RULE = new RoundRobinRule();