core：Java功能增強 —— 攔截器

通過@LoadBalanced修飾就可以為應(yīng)用引入Ribbon框架來實現(xiàn)負(fù)載均衡。

 @Configuration
public class ConfigBean {

/**
 * 使用@LoadBalanced開啟負(fù)載均衡
 * RestTemplate 通過它來訪問服務(wù)
 * */
@LoadBalanced
@Bean
RestTemplate restTemplate() {
    return new RestTemplate();
}
}

使用時這樣(jServer1就是注冊在eureka的服務(wù)名)：

@RestController
@RequestMapping("/")
public class HelloController {
@Autowired
RestTemplate restTemplate;

@RequestMapping("/hello")
public String hello() {
    String result = restTemplate.getForEntity("http://jServer1/hello",String.class).getBody();
    System.out.println(result);
    return result;
}
}

所以Ribbon的核心在于@LoadBalanced，那它引入了哪些東西呢？進(jìn)入看一下：

image.png

立刻明白了什么，是老朋友攔截器啊，所以是攔截了請求，然后根據(jù)負(fù)載均衡策略找到合適的服務(wù)實例，再發(fā)請求出去么。

intercept方法如下：

public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {
    URI originalUri = request.getURI();
    String serviceName = originalUri.getHost();
    return (ClientHttpResponse)this.loadBalancer.execute(serviceName, new LoadBalancerRequest<ClientHttpResponse>() {
        public ClientHttpResponse apply(ServiceInstance instance) throws Exception {
            HttpRequest serviceRequest = LoadBalancerInterceptor.this.new ServiceRequestWrapper(request, instance);
            return execution.execute(serviceRequest, body);
        }
    });
}

進(jìn)到execute方法：

public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
    ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);

    //就是這里了?。?    Server server = this.getServer(loadBalancer);

    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));
        RibbonLoadBalancerContext context = this.clientFactory.getLoadBalancerContext(serviceId);
        RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

        try {
            T returnVal = request.apply(ribbonServer);
            statsRecorder.recordStats(returnVal);
            return returnVal;
        } catch (IOException var9) {
            statsRecorder.recordStats(var9);
            throw var9;
        } catch (Exception var10) {
            statsRecorder.recordStats(var10);
            ReflectionUtils.rethrowRuntimeException(var10);
            return null;
        }
    }
}

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

以最基本的BaseLoadBalancer為例(還有其他Balancer實現(xiàn))：

public Server chooseServer(Object key) {
    if (this.counter == null) {
        this.counter = this.createCounter();
    }

    this.counter.increment();
    if (this.rule == null) {
        return null;
    } else {
        try {
            //這里的rule = DEFAULT_RULE = new RoundRobinRule();
            return this.rule.choose(key);
        } catch (Throwable var3) {
            return null;
        }
    }
}

可以發(fā)現(xiàn)是根據(jù)IRule.choose(key)來選擇服務(wù)器實例的，而IRule就是負(fù)載均衡策略接口，可以看出已經(jīng)默認(rèn)實現(xiàn)了很多種策略：

image.png