core：Java功能增强 —— 拦截器
通过@LoadBalanced修饰就可以为应用引入Ribbon框架来实现负载均衡。
@Configuration
public class ConfigBean {/**
* 使用@LoadBalanced开启负载均衡
* RestTemplate 通过它来访问服务
* */
@LoadBalanced
@Bean
RestTemplate restTemplate() {
return new RestTemplate();
}
}
使用时这样(jServer1就是注册在eureka的服务名)：
@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，那它引入了哪些东西呢？进入看一下：
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立刻明白了什么，是老朋友拦截器啊，所以是拦截了请求，然后根据负载均衡策略找到合适的服务实例，再发请求出去么。
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() {
public ClientHttpResponse apply(ServiceInstance instance) throws Exception {
HttpRequest serviceRequest = LoadBalancerInterceptor.this.new ServiceRequestWrapper(request, instance);
return execution.execute(serviceRequest, body);
}
});
}
进到execute方法：
public T execute(String serviceId, LoadBalancerRequest request) throws IOException {
ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);
//就是这里了！！
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为例(还有其他Balancer实现)：
public Server chooseServer(Object key) {
if (this.counter == null) {
this.counter = this.createCounter();
}this.counter.increment();
if (this.rule == null) {
return null;
} else {
try {
//这里的rule = DEFAULT_RULE = new RoundRobinRule();
return this.rule.choose(key);
} catch (Throwable var3) {
return null;
}
}
}
【Ribbon|Ribbon 负载均衡】可以发现是根据IRule.choose(key)来选择服务器实例的，而IRule就是负载均衡策略接口，可以看出已经默认实现了很多种策略：
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