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 { ....留神到注解@Import(FeignClientsRegistrar.class)，FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar，在启动时会执行registerBeanDefinitions动静注册
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar { @Override public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) { registerDefaultConfiguration(metadata, registry); registerFeignClients(metadata, registry); } ...}- registerDefaultConfiguration
咱们首先看registerDefaultConfiguration，代码不多，间接贴代码
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()比方启动类为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构造体，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能够配置为任意类，比方
@EnableFeignClients(defaultConfiguration = GlobalFeignClientConfiguration.class)GlobalFeignClientConfiguration该如何指定配置呢？咱们晓得能够通过feign.Builder来手动创立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(); ...}这些都能够通过defaultConfiguration从新定义，比方上面这个指定了Loger.Level
public class GlobalFeignClientConfiguration { @Bean public Level level() { return Level.FULL; }}当然也能够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（局部代码省略） for (BeanDefinition candidateComponent : candidateComponents) { if (candidateComponent instanceof AnnotatedBeanDefinition) { //... 注册Feign客户端（局部代码省略） 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办法
private void registerFeignClient(BeanDefinitionRegistry registry, AnnotationMetadata annotationMetadata, Map<String, Object> attributes) { String className = annotationMetadata.getClassName(); BeanDefinitionBuilder definition = BeanDefinitionBuilder .genericBeanDefinition(FeignClientFactoryBean.class); //...省略局部代码 AbstractBeanDefinition beanDefinition = definition.getBeanDefinition(); BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className, new String[] { alias }); BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry); }最终会通过targeter.target创立一个代理对象
<T> T getTarget() { FeignContext context = applicationContext.getBean(FeignContext.class); Feign.Builder builder = feign(context); //...省略局部代码 Targeter targeter = get(context, Targeter.class); return (T) targeter.target(this, builder, context, new HardCodedTarget<>(type, name, url)); }创立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副实现
Nacos FeignClient调用原理
如果应用Nacos作为注册核心，只有在依赖中增加nacos的starter并且在配置文件中指定nacos的地址就接入实现
<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配置核心获取获取服务信息，比拟重要的就是服务的地址和端口，那么这所有时如何实现的，咱们就从spring-cloud-starter-alibaba-nacos-discovery开始，spring-cloud-starter-alibaba-nacos-discovery自身没有蕴含任何的代码（不晓得为何这样设计），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>这外面蕴含starter相干的代码，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.NacosConfigServerAutoConfigurationorg.springframework.cloud.bootstrap.BootstrapConfiguration=\com.alibaba.cloud.nacos.discovery.configclient.NacosDiscoveryClientConfigServiceBootstrapConfiguration咱们这里关注NacosDiscoveryClientConfigServiceBootstrapConfiguration
@ConditionalOnClass(ConfigServicePropertySourceLocator.class)@ConditionalOnProperty(value = "spring.cloud.config.discovery.enabled", matchIfMissing = false)@Configuration@ImportAutoConfiguration({ NacosDiscoveryClientAutoConfiguration.class, NacosDiscoveryAutoConfiguration.class })public class NacosDiscoveryClientConfigServiceBootstrapConfiguration {}这里应用了注解ImportAutoConfiguration，该注解会主动导入配置的类。 咱们进入NacosDiscoveryClientAutoConfiguration
@Configuration@ConditionalOnNacosDiscoveryEnabled@AutoConfigureBefore({ SimpleDiscoveryClientAutoConfiguration.class, CommonsClientAutoConfiguration.class })public class NacosDiscoveryClientAutoConfiguration { @Bean public DiscoveryClient nacosDiscoveryClient( NacosDiscoveryProperties discoveryProperties) { return new NacosDiscoveryClient(discoveryProperties); } //...省略局部代码}这里会注入一个NacosDiscoveryClient对象，该对象实现接口DiscoveryClient。DiscoveryClient在spring cloud中负责从注册核心获取服务列表，也就是说底层Feign并不是间接和注册核心打交道，而是通过DiscoveryClient发现服务，那么只有实现了DiscoveryClient就能实现自定义注册核心。
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); } } //...省略局部代码}以获取服务实例getInstances为例，discoveryProperties.namingServiceInstance()获取了一个NamingService对象，这个对象蕴含nacos api所有的操作，有了这个对象就能和nacos通信。