/** * 使用RestTemplate实现负载均衡 * @return */ @Bean @LoadBalanced RestTemplate restTemplate() { return new RestTemplate(); }
首先我们来看看@LoadBalanced注解的源码:
/** * Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient * @author Spencer Gibb */ @Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD }) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Qualifier public @interface LoadBalanced { }
具体的配置是在哪里执行的呢?我们在LoadBalancerClient的包下面发现了一个类叫做LoadBalancerAutoConfiguration,看名字有点像是客户端负载均衡服务器的自动化配置类,我们来看看这个类的源码:
@Configuration @ConditionalOnClass(RestTemplate.class) @ConditionalOnBean(LoadBalancerClient.class) @EnableConfigurationProperties(LoadBalancerRetryProperties.class) public class LoadBalancerAutoConfiguration { @LoadBalanced @Autowired(required = false) private List<RestTemplate> restTemplates = Collections.emptyList(); @Bean public SmartInitializingSingleton loadBalancedRestTemplateInitializer( final List<RestTemplateCustomizer> customizers) { return new SmartInitializingSingleton() { @Override public void afterSingletonsInstantiated() { for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) { for (RestTemplateCustomizer customizer : customizers) { customizer.customize(restTemplate); } } } }; } @Autowired(required = false) private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList(); @Bean @ConditionalOnMissingBean public LoadBalancerRequestFactory loadBalancerRequestFactory( LoadBalancerClient loadBalancerClient) { return new LoadBalancerRequestFactory(loadBalancerClient, transformers); } @Configuration @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate") static class LoadBalancerInterceptorConfig { @Bean public LoadBalancerInterceptor ribbonInterceptor( LoadBalancerClient loadBalancerClient, LoadBalancerRequestFactory requestFactory) { return new LoadBalancerInterceptor(loadBalancerClient, requestFactory); } @Bean @ConditionalOnMissingBean public RestTemplateCustomizer restTemplateCustomizer( final LoadBalancerInterceptor loadBalancerInterceptor) { return new RestTemplateCustomizer() { @Override public void customize(RestTemplate restTemplate) { List<ClientHttpRequestInterceptor> list = new ArrayList<>( restTemplate.getInterceptors()); list.add(loadBalancerInterceptor); restTemplate.setInterceptors(list); } }; } } @Configuration @ConditionalOnClass(RetryTemplate.class) public static class RetryAutoConfiguration { @Bean public RetryTemplate retryTemplate() { RetryTemplate template = new RetryTemplate(); template.setThrowLastExceptionOnExhausted(true); return template; } @Bean @ConditionalOnMissingBean public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory() { return new LoadBalancedRetryPolicyFactory.NeverRetryFactory(); } } @Configuration @ConditionalOnClass(RetryTemplate.class) public static class RetryInterceptorAutoConfiguration { @Bean @ConditionalOnMissingBean public RetryLoadBalancerInterceptor ribbonInterceptor( LoadBalancerClient loadBalancerClient, LoadBalancerRetryProperties properties, LoadBalancedRetryPolicyFactory lbRetryPolicyFactory, LoadBalancerRequestFactory requestFactory) { return new RetryLoadBalancerInterceptor(loadBalancerClient, properties, lbRetryPolicyFactory, requestFactory); } @Bean @ConditionalOnMissingBean public RestTemplateCustomizer restTemplateCustomizer( final RetryLoadBalancerInterceptor loadBalancerInterceptor) { return new RestTemplateCustomizer() { @Override public void customize(RestTemplate restTemplate) { List<ClientHttpRequestInterceptor> list = new ArrayList<>( restTemplate.getInterceptors()); list.add(loadBalancerInterceptor); restTemplate.setInterceptors(list); } }; } } }
这个类的源码比较长,我们就来说一下这里的核心功能:
1.LoadBalancerAutoConfiguration类上有两个关键注解,分别是@ConditionalOnClass(RestTemplate.class)和@ConditionalOnBean(LoadBalancerClient.class),说明Ribbon如果想要实现负载均衡的自动化配置需要满足两个条件:第一个,RestTemplate类必须存在于当前工程的环境中;第二个,在Spring容器中必须有LoadBalancerClient的实现Bean。
2.ribbonInterceptor方法返回了一个拦截器叫做LoadBalancerInterceptor,这个拦截器的作用主要是在客户端发起请求时进行拦截,进而实现客户端负载均衡功能。
3.restTemplateCustomizer方法返回了一个RestTemplateCustomizer,这个方法主要用来给RestTemplate添加LoadBalancerInterceptor拦截器。
4.restTemplates是一个被@LoadBalanced注解修饰的RestTemplate对象列表,在loadBalancedRestTemplateInitializer方法中通过调用RestTemplateCustomizer中的customizef方法来给RestTemplate添加上LoadBalancerInterceptor拦截器。
从而实现了从 http://服务名/hello到 http://域名/hello的转换。
RestTemplate从一个简单的服务请求控件变成了具有客户端负载均衡功能的请求控件。简而言之,就是 RestTemplate发起一个请求,这个请求被LoadBalancerInterceptor给拦截了,拦截后将请求的地址中的服务逻辑名转为具体的服务地址,然后继续执行请求,就是这么一个过程。
转自: http://blog.csdn.net/u012702547/article/details/77940838