Nacos Feign调用研究

FeignClient调用原理

要启用FeignClient首先必须在启动类上加上注解@EnableFeignClients，EnableFeignClients代码如下

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {
  ....

注意到注解@Import(FeignClientsRegistrar.class)，FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar，在启动时会执行registerBeanDefinitions动态注册

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar {
  @Override
    public void registerBeanDefinitions(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        registerDefaultConfiguration(metadata, registry);
        registerFeignClients(metadata, registry);
    }
  ...
}

• registerDefaultConfiguration

我们首先看registerDefaultConfiguration，代码不多，直接贴代码

private void registerDefaultConfiguration(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        Map<String, Object> defaultAttrs = metadata
                .getAnnotationAttributes(EnableFeignClients.class.getName(), true);

        if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
            String name;
            if (metadata.hasEnclosingClass()) {
                name = "default." + metadata.getEnclosingClassName();
            }
            else {
                name = "default." + metadata.getClassName();
            }
            registerClientConfiguration(registry, name,
                    defaultAttrs.get("defaultConfiguration"));
        }
    }

• 获取配置信息defaultAttrs
• 注册默认配置类信息，配置类从defaultConfiguration中获取并且名称为 "default." + metadata.getClassName() 比如启动类为TestApplication，那么名称为default.com.test.TestApplication

registerClientConfiguration将配置信息注册为FeignClientSpecification

private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
            Object configuration) {
        BeanDefinitionBuilder builder = BeanDefinitionBuilder
                .genericBeanDefinition(FeignClientSpecification.class);
        builder.addConstructorArgValue(name);
        builder.addConstructorArgValue(configuration);
        registry.registerBeanDefinition(
                name + "." + FeignClientSpecification.class.getSimpleName(),
                builder.getBeanDefinition());
    }

FeignClientSpecification其实就是一个key-value结构体，key就是配置名称，value就是配置类

FeignClientSpecification(String name, Class<?>[] configuration) {
        this.name = name;
        this.configuration = configuration;
    }

在EnableFeignClients注解中参数defaultConfiguration为全局配置类，如果FeignClient没有配置则会获取defaultConfiguration的配置，defaultConfiguration可以配置为任意类，比如

@EnableFeignClients(defaultConfiguration = GlobalFeignClientConfiguration.class)

GlobalFeignClientConfiguration该如何指定配置呢？我们知道可以通过feign.Builder来手动创建FeignClient，在feign.Builder中有以下变量

public static class Builder {
    private Logger.Level logLevel = Logger.Level.NONE;
    private Contract contract = new Contract.Default();
    private Client client = new Client.Default(null, null);
    private Retryer retryer = new Retryer.Default();
    private Logger logger = new NoOpLogger();
    private Encoder encoder = new Encoder.Default();
    private Decoder decoder = new Decoder.Default();
    private QueryMapEncoder queryMapEncoder = new FieldQueryMapEncoder();
    private ErrorDecoder errorDecoder = new ErrorDecoder.Default();
    private Options options = new Options();
  ...
}

这些都可以通过defaultConfiguration重新定义，比如下面这个指定了Loger.Level

public class GlobalFeignClientConfiguration {
    @Bean
    public Level level() {
        return Level.FULL;
    }
}

当然也可以通过配置文件进行配置

feign:
  client:
    config:
      feignName:
        connectTimeout: 5000
        readTimeout: 5000
        loggerLevel: full
        errorDecoder: com.example.SimpleErrorDecoder
        retryer: com.example.SimpleRetryer
        requestInterceptors:
          - com.example.FooRequestInterceptor
          - com.example.BarRequestInterceptor
        decode404: false
        encoder: com.example.SimpleEncoder
        decoder: com.example.SimpleDecoder
        contract: com.example.SimpleContract

• registerFeignClients

registerFeignClients先扫描所有带注解FeignClient的类，并注册到Spring容器中

public void registerFeignClients(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {

        LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();
        Map<String, Object> attrs = metadata
                .getAnnotationAttributes(EnableFeignClients.class.getName());
        AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
                FeignClient.class);
        //...扫描FeignClient类（部分代码省略）

        for (BeanDefinition candidateComponent : candidateComponents) {
            if (candidateComponent instanceof AnnotatedBeanDefinition) {
        
        //... 注册Feign客户端（部分代码省略）
                Map<String, Object> attributes = annotationMetadata
                        .getAnnotationAttributes(FeignClient.class.getCanonicalName());

                String name = getClientName(attributes);
                registerClientConfiguration(registry, name,
                        attributes.get("configuration"));

                registerFeignClient(registry, annotationMetadata, attributes);
            }
        }
    }

会将Client注册为FeignClientFactoryBean，这样Spring Boot在获取Feign实例时就会调用FeignClientFactoryBean.getTarget方法

private void registerFeignClient(BeanDefinitionRegistry registry,
            AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
        String className = annotationMetadata.getClassName();
        BeanDefinitionBuilder definition = BeanDefinitionBuilder
                .genericBeanDefinition(FeignClientFactoryBean.class);
        //...省略部分代码
      AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
        BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
                new String[] { alias });
        BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
    }

最终会通过targeter.target创建一个代理对象

<T> T getTarget() {
        FeignContext context = applicationContext.getBean(FeignContext.class);
        Feign.Builder builder = feign(context);
      //...省略部分代码
        Targeter targeter = get(context, Targeter.class);
        return (T) targeter.target(this, builder, context,
                new HardCodedTarget<>(type, name, url));
    }

创建代理对象的代码位于feign.Feign.target中，至此整个Feign从初始化到最终实例化就全部完成

Nacos FeignClient调用原理

如果使用Nacos作为注册中心，只要在依赖中添加nacos的starter并且在配置文件中指定nacos的地址就接入完成

<dependency>
    <groupId>com.alibaba.cloud</groupId>
    <artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
    <version>2.1.1.RELEASE</version>
</dependency>

接入nacos后，Feign调用将从nacos配置中心获取获取服务信息，比较重要的就是服务的地址和端口，那么这一切时如何实现的，我们就从spring-cloud-starter-alibaba-nacos-discovery开始，spring-cloud-starter-alibaba-nacos-discovery本身没有包含任何的代码（不知道为何这样设计），但依赖了spring-cloud-alibaba-nacos-discovery

<dependencies>
    <dependency>
        <groupId>com.alibaba.cloud</groupId>
        <artifactId>spring-cloud-alibaba-nacos-discovery</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>
</dependencies>

这里面包含starter相关的代码，spring.factories定义如下

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.alibaba.cloud.nacos.NacosDiscoveryAutoConfiguration,\
  com.alibaba.cloud.nacos.ribbon.RibbonNacosAutoConfiguration,\
  com.alibaba.cloud.nacos.endpoint.NacosDiscoveryEndpointAutoConfiguration,\
  com.alibaba.cloud.nacos.discovery.NacosDiscoveryClientAutoConfiguration,\
  com.alibaba.cloud.nacos.discovery.configclient.NacosConfigServerAutoConfiguration
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
com.alibaba.cloud.nacos.discovery.configclient.NacosDiscoveryClientConfigServiceBootstrapConfiguration

我们这里关注NacosDiscoveryClientConfigServiceBootstrapConfiguration

@ConditionalOnClass(ConfigServicePropertySourceLocator.class)
@ConditionalOnProperty(value = "spring.cloud.config.discovery.enabled", matchIfMissing = false)
@Configuration
@ImportAutoConfiguration({ NacosDiscoveryClientAutoConfiguration.class,
        NacosDiscoveryAutoConfiguration.class })
public class NacosDiscoveryClientConfigServiceBootstrapConfiguration {

}

这里使用了注解ImportAutoConfiguration，该注解会自动导入配置的类。 我们进入NacosDiscoveryClientAutoConfiguration

@Configuration
@ConditionalOnNacosDiscoveryEnabled
@AutoConfigureBefore({ SimpleDiscoveryClientAutoConfiguration.class,
        CommonsClientAutoConfiguration.class })
public class NacosDiscoveryClientAutoConfiguration {
    @Bean
    public DiscoveryClient nacosDiscoveryClient(
            NacosDiscoveryProperties discoveryProperties) {
        return new NacosDiscoveryClient(discoveryProperties);
    }
  //...省略部分代码
}

这里会注入一个NacosDiscoveryClient对象，该对象实现接口DiscoveryClient。DiscoveryClient在spring cloud中负责从注册中心获取服务列表，也就是说底层Feign并不是直接和注册中心打交道，而是通过DiscoveryClient发现服务，那么只要实现了DiscoveryClient就能实现自定义注册中心。

NacosDiscoveryClient.java

public class NacosDiscoveryClient implements DiscoveryClient {

    private static final Logger log = LoggerFactory.getLogger(NacosDiscoveryClient.class);
    public static final String DESCRIPTION = "Spring Cloud Nacos Discovery Client";

    private NacosDiscoveryProperties discoveryProperties;

    public NacosDiscoveryClient(NacosDiscoveryProperties discoveryProperties) {
        this.discoveryProperties = discoveryProperties;
    }

    @Override
    public String description() {
        return DESCRIPTION;
    }

    @Override
    public List<ServiceInstance> getInstances(String serviceId) {
        try {
            String group = discoveryProperties.getGroup();
            List<Instance> instances = discoveryProperties.namingServiceInstance()
                    .selectInstances(serviceId, group, true);
            return hostToServiceInstanceList(instances, serviceId);
        }
        catch (Exception e) {
            throw new RuntimeException(
                    "Can not get hosts from nacos server. serviceId: " + serviceId, e);
        }
    }
  //...省略部分代码

}

以获取服务实例getInstances为例，discoveryProperties.namingServiceInstance()获取了一个NamingService对象，这个对象包含nacos api所有的操作，有了这个对象就能和nacos通信。