1.简单使用
1.1.在启动类中，声明一个RestTemplate的bean，用@LoadBalanced注解修饰：
@Bean
@LoadBalanced
public RestTemplate restTemplate() {
return new RestTemplate();
}
1.2.在进行http调用的地方，通过@Autowired方式，注入RestTemplate，再通过restTemplate调用。调用时，写服务名就可以，比如按照下面这种方式调用。
return restTemplate.getForObject("http://MICOSERICE-USER/user/getuser",ResponseDto.class);
通过这两步操作，进行调用时，就可以通过服务名，得到多个服务实例的信息，通过负载均衡的方式进行调用。
从上面的步骤看来，使用负载均衡的开发量还是挺简单的，不过为什么在restTemplate上加个注解，通过restTemplate进行调用，就可以做到负载均衡呢，这里就需要研究下源码才能知道原因了。
2.源码分析
进入到@LoadBalanced注解，如下：
@Target({ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}
这个也看不出啥，在源码中LoadBalancerAutoConfiguration中使用了这个注解，如下：
@LoadBalanced
@Autowired(
required = false
)
private List restTemplates = Collections.emptyList(); //定义了一个restTemplates列表
在Spring注入当前类的loadBalancedRestTemplateInitializer的Bean时有如下方法
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(final List customizers) {
return new SmartInitializingSingleton() {
public void afterSingletonsInstantiated() {
Iterator var1 = LoadBalancerAutoConfiguration.this.restTemplates.iterator();
while(var1.hasNext()) {
RestTemplate restTemplate = (RestTemplate)var1.next();
Iterator var3 = customizers.iterator();
while(var3.hasNext()) {
RestTemplateCustomizer customizer = (RestTemplateCustomizer)var3.next();
customizer.customize(restTemplate); //调用RestTemplateCustomizer的customize方法
}
}
}
};
}
@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {
return new RestTemplateCustomizer() {
public void customize(RestTemplate restTemplate) {
List list = new ArrayList(restTemplate.getInterceptors());
list.add(loadBalancerInterceptor); //添加了一个loadBalancerInterceptor拦截器
restTemplate.setInterceptors(list);
}
};
}
loadBalancerInterceptor拦截器定义的拦截方法如下：
public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
URI originalUri = request.getURI(); //获取url
String serviceName = originalUri.getHost(); //获取host
Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
return (ClientHttpResponse)this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution)); //loadBalancer执行execute方法
}
loadBalancer执行execute方法如下：
public T execute(String serviceId, LoadBalancerRequest request) throws IOException {
ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId); //根据serviceId获取loadBalancer
Server server = this.getServer(loadBalancer); //获取server
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, ribbonServer, request); //执行相应的请求
}
}
其中getServer方法如下：
protected Server getServer(ILoadBalancer loadBalancer) {
return loadBalancer == null ? null : loadBalancer.chooseServer("default"); //选择server
}
这里默认执行的是ZoneAwareLoadBalancer下的chooseServer方法
public Server chooseServer(Object key) {
if (ENABLED.get() && this.getLoadBalancerStats().getAvailableZones().size() > 1) {
Server server = null;
try {
LoadBalancerStats lbStats = this.getLoadBalancerStats();
Map zoneSnapshot = ZoneAvoidanceRule.createSnapshot(lbStats);
logger.debug("Zone snapshots: {}", zoneSnapshot);
if (this.triggeringLoad == null) {
this.triggeringLoad = DynamicPropertyFactory.getInstance().getDoubleProperty("ZoneAwareNIWSDiscoveryLoadBalancer." + this.getName() + ".triggeringLoadPerServerThreshold", 0.2D);
}
if (this.triggeringBlackoutPercentage == null) {
this.triggeringBlackoutPercentage = DynamicPropertyFactory.getInstance().getDoubleProperty("ZoneAwareNIWSDiscoveryLoadBalancer." + this.getName() + ".avoidZoneWithBlackoutPercetage", 0.99999D);
}
Set availableZones = ZoneAvoidanceRule.getAvailableZones(zoneSnapshot, this.triggeringLoad.get(), this.triggeringBlackoutPercentage.get());
logger.debug("Available zones: {}", availableZones);
if (availableZones != null && availableZones.size() < zoneSnapshot.keySet().size()) {
String zone = ZoneAvoidanceRule.randomChooseZone(zoneSnapshot, availableZones);
logger.debug("Zone chosen: {}", zone);
if (zone != null) {
BaseLoadBalancer zoneLoadBalancer = this.getLoadBalancer(zone);
server = zoneLoadBalancer.chooseServer(key); //选择server
}
}
} catch (Exception var8) {
logger.error("Error choosing server using zone aware logic for load balancer={}", this.name, var8);
}
if (server != null) {
return server;
} else {
logger.debug("Zone avoidance logic is not invoked.");
return super.chooseServer(key);
}
} else {
logger.debug("Zone aware logic disabled or there is only one zone");
return super.chooseServer(key);
}
}
public Server chooseServer(Object key) {
if (this.counter == null) {
this.counter = this.createCounter();
}
this.counter.increment();
if (this.rule == null) {
return null;
} else {
try {
return this.rule.choose(key); //根据规则选择Server，默认是RoundRobinRule，即轮询规则
} catch (Exception var3) {
logger.warn("LoadBalancer [{}]: Error choosing server for key {}", new Object[]{this.name, key, var3});
return null;
}
}
}
IRule的实现有以下：
AvailabilityFilteringRule过滤器规则
BestAvailableRule 选择最小请求数
ClientConfigEnabledRoundRobinRule 轮询
RandomRule 随机选择一个server
RoundRobinRule 轮询选择server
RetryRule 根据轮询的方式重试
WeightedResponseTimeRule 根据响应时间去分配一个weight ，weight越低，被选择的可能性就越低
ZoneAvoidanceRule 根据server的zone区域和可用性来轮询选择