1.Feign是什么
Feign是一个声明式的web service客户端，它使得编写web service客户端更为容易。创建接口，为接口添加注解，即可使用Feign。Feign可以使用Feign注解或者JAX-RS注解，还支持热插拔的编码器和解码器。Spring Cloud为Feign添加了Spring MVC的注解支持，并整合了Ribbon和Eureka来为使用Feign时提供负载均衡。
feign源码的github地址：https: //github.com/OpenFeign/feign
2.feign的使用
1.引入依赖，feign作为springcloud五大组件之一，只需要简单引入下面依赖即可：
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
2.定义一个feign接口，通过@FeignClient来实现你要调用微服务中的哪个服务，哪个接口，代码示例如下：
/**
* @author fjzheng
* @version 1.0
* @date 2021/7/5 15:04
*/
@FeignClient(name = "${micro-server.mall-product}", path = "/api/mall/product")
public interface ProductService {
@GetMapping("/sku/{skuId}")
ResponseVO<ProductSku> getSku(@PathVariable("skuId") Long skuId);
@PostMapping("/sku/price")
ResponseVO<List<ProductSku>> getSkuPrice(@RequestBody SkuQuery query);
}如上订单服务需要调商品服务的两个接口，就按照上面写，@FeignClient注解的具体属性后面详细解析
3.开启feign功能，需要在服务的启动类上添加注解EnableFeignClients，如下所示：
@SpringBootApplication
@EnableDiscoveryClient
@MapperScan(basePackages = "com.shepherd.mallorder.dao")
@ComponentScan(basePackages = {"com.shepherd"})
@EnableFeignClients(basePackages = "com.shepherd.mallorder")
@EnableRabbit //要想监听队列接收消息必须开启此注解
public class MallOrderServiceApplication {
public static void main(String[] args) {
SpringApplication.run(MallOrderServiceApplication.class, args);
}
}完成以上步骤，只要商品服务注册到了微服务的服务中心了，那么订单服务就可以调用商品服务的相关接口了。
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交流探讨群：Shepherd_126
3.@FeignClient注解
注解@FeignClient的源码如下：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {
/**
* The name of the service with optional protocol prefix. Synonym for {@link #name()
* name}. A name must be specified for all clients, whether or not a url is provided.
* Can be specified as property key, eg: ${propertyKey}.
* @return the name of the service with optional protocol prefix
*/
@AliasFor("name")
String value() default "";
/**
* The service id with optional protocol prefix. Synonym for {@link #value() value}.
* @deprecated use {@link #name() name} instead
* @return the service id with optional protocol prefix
*/
@Deprecated
String serviceId() default "";
/**
* This will be used as the bean name instead of name if present, but will not be used
* as a service id.
* @return bean name instead of name if present
*/
String contextId() default "";
/**
* @return The service id with optional protocol prefix. Synonym for {@link #value()
* value}.
*/
@AliasFor("value")
String name() default "";
/**
* @return the <code>@Qualifier</code> value for the feign client.
*/
String qualifier() default "";
/**
* @return an absolute URL or resolvable hostname (the protocol is optional).
*/
String url() default "";
/**
* @return whether 404s should be decoded instead of throwing FeignExceptions
*/
boolean decode404() default false;
/**
* A custom configuration class for the feign client. Can contain override
* <code>@Bean</code> definition for the pieces that make up the client, for instance
* {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
*
* @see FeignClientsConfiguration for the defaults
* @return list of configurations for feign client
*/
Class<?>[] configuration() default {};
/**
* Fallback class for the specified Feign client interface. The fallback class must
* implement the interface annotated by this annotation and be a valid spring bean.
* @return fallback class for the specified Feign client interface
*/
Class<?> fallback() default void.class;
/**
* Define a fallback factory for the specified Feign client interface. The fallback
* factory must produce instances of fallback classes that implement the interface
* annotated by {@link FeignClient}. The fallback factory must be a valid spring bean.
*
* @see feign.hystrix.FallbackFactory for details.
* @return fallback factory for the specified Feign client interface
*/
Class<?> fallbackFactory() default void.class;
/**
* @return path prefix to be used by all method-level mappings. Can be used with or
* without <code>@RibbonClient</code>.
*/
String path() default "";
/**
* @return whether to mark the feign proxy as a primary bean. Defaults to true.
*/
boolean primary() default true;
}
- FeignClient注解被@Target(ElementType.TYPE)修饰，表示FeignClient注解的作用目标在接口上@Retention(RetentionPolicy.RUNTIME)，注解会在class字节码文件中存在，在运行时可以通过反射获取到；@Documented表示该注解将被包含在javadoc中。
- feign 用于声明具有该接口的REST客户端的接口的注释应该是创建（例如用于自动连接到另一个组件。 如果功能区可用，那将是
用于负载平衡后端请求，并且可以配置负载平衡器使用与伪装客户端相同名称（即值）@RibbonClient 。 - 其中value()和name()一样，是被调用的 service的名称。url(),直接填写硬编码的url,decode404()即404是否被解码，还是抛异常；configuration()，标明FeignClient的配置类，默认的配置类为FeignClientsConfiguration类，可以覆盖Decoder、Encoder和Contract等信息，进行自定义配置。fallback(),填写熔断器的信息类。
从源码可以得知，name是value的别名，value也是name的别名。两者的作用是一致的，name指定FeignClient的名称，如果项目使用了Ribbon，name属性会作为微服务的名称，用于服务发现。其中，serviceId和value的作用一样，用于指定服务ID，已经废弃。
qualifier：该属性用来指定@Qualifier注解的值，该值是该FeignClient的限定词，可以使用改值进行引用
url：该属性一般用于调试程序，允许我们手动指定@FeignClient调用的地址
decode404：当发生http 404错误时，如果该字段位true，会调用decoder进行解码，否则抛出FeignException
configuration：Feign配置类，可以自定义Feign的Encoder、Decoder、LogLevel、Contract。
参考：https: //blog.csdn.net/weixin_38912281/article/details/104538676
从注解源码中可以得知，feign客户端默认的配置类为FeignClientsConfiguration，这个配置类注入了很多的相关配置的bean，包括feignRetryer、FeignLoggerFactory、FormattingConversionService等,其中还包括了Decoder、Encoder、Contract，如果这三个bean在没有注入的情况下，会自动注入默认的配置。
@Configuration(proxyBeanMethods = false)
public class FeignClientsConfiguration {
@Autowired
private ObjectFactory<HttpMessageConverters> messageConverters;
@Autowired(required = false)
private List<AnnotatedParameterProcessor> parameterProcessors = new ArrayList<>();
@Autowired(required = false)
private List<FeignFormatterRegistrar> feignFormatterRegistrars = new ArrayList<>();
@Autowired(required = false)
private Logger logger;
@Autowired(required = false)
private SpringDataWebProperties springDataWebProperties;
@Bean
@ConditionalOnMissingBean
public Decoder feignDecoder() {
return new OptionalDecoder(
new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
}
@Bean
@ConditionalOnMissingBean
@ConditionalOnMissingClass("org.springframework.data.domain.Pageable")
public Encoder feignEncoder() {
return new SpringEncoder(this.messageConverters);
}
@Bean
@ConditionalOnClass(name = "org.springframework.data.domain.Pageable")
@ConditionalOnMissingBean
public Encoder feignEncoderPageable() {
PageableSpringEncoder encoder = new PageableSpringEncoder(
new SpringEncoder(this.messageConverters));
if (springDataWebProperties != null) {
encoder.setPageParameter(
springDataWebProperties.getPageable().getPageParameter());
encoder.setSizeParameter(
springDataWebProperties.getPageable().getSizeParameter());
encoder.setSortParameter(
springDataWebProperties.getSort().getSortParameter());
}
return encoder;
}
@Bean
@ConditionalOnMissingBean
public Contract feignContract(ConversionService feignConversionService) {
return new SpringMvcContract(this.parameterProcessors, feignConversionService);
}
@Bean
public FormattingConversionService feignConversionService() {
FormattingConversionService conversionService = new DefaultFormattingConversionService();
for (FeignFormatterRegistrar feignFormatterRegistrar : this.feignFormatterRegistrars) {
feignFormatterRegistrar.registerFormatters(conversionService);
}
return conversionService;
}
@Bean
@ConditionalOnMissingBean
public Retryer feignRetryer() {
return Retryer.NEVER_RETRY;
}
@Bean
@Scope("prototype")
@ConditionalOnMissingBean
public Feign.Builder feignBuilder(Retryer retryer) {
return Feign.builder().retryer(retryer);
}
@Bean
@ConditionalOnMissingBean(FeignLoggerFactory.class)
public FeignLoggerFactory feignLoggerFactory() {
return new DefaultFeignLoggerFactory(this.logger);
}
@Bean
@ConditionalOnClass(name = "org.springframework.data.domain.Page")
public Module pageJacksonModule() {
return new PageJacksonModule();
}
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ HystrixCommand.class, HystrixFeign.class })
protected static class HystrixFeignConfiguration {
@Bean
@Scope("prototype")
@ConditionalOnMissingBean
@ConditionalOnProperty(name = "feign.hystrix.enabled")
public Feign.Builder feignHystrixBuilder() {
return HystrixFeign.builder();
}
}
}
从配置类源码中可以知道，我们自定义注入该配置类中bean，如果我们注入自定义的bean，那么此配置类的默认bean将不注入。比如FeignClientsConfiguration的默认重试次数为Retryer.NEVER_RETRY，即不重试，那么如果希望重试，那么就可以重写这个bean，即自定义，注入feignRetryer的bean,代码如下：
@Configuration
public class FeignConfig {
@Bean
public Retryer feignRetryer() {
return new Retryer.Default(100, SECONDS.toMillis(1), 5);
}
}
4.feign的工作原理
一切冲项目启动类加上注解@EnableFeignClients开始，该组件开启feign客户端扫描，同时已代表开启feign组件功能：
通过包扫描注入FeignClient的bean，该源码在FeignClientsRegistrar类：首先在启动配置上检查是否有@EnableFeignClients注解，如果有该注解，则开启包扫描，扫描被@FeignClient注解接口，描被@FeignClient注解接口。扫描出该注解后，通过beanDefinition注入到IOC容器中，方便后续被调用使用
可以发现这个FeignClientsRegistrar类实现了 ImportBeanDefinitionRegistrar，阅读过Spring源码的同学们应该很清楚，这个类是Spring提供的一个扩展点，提供给外界去动态扩展自己的需要的Bean，这里面有一个关键的方法#registerBeanDefinitions，这个方法会在Spring初始化上下文 refresh方法进行调用。
接下来是开启包扫描和注入bean的核心源码：大致的流程是初始化一个扫描器Scanner 完成包名下面扫描所有的带有FeignClent注解的类，最后调用registerFeignClient方法
1.方法registerFeignClients
public void registerFeignClients(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
ClassPathScanningCandidateComponentProvider scanner = getScanner();
scanner.setResourceLoader(this.resourceLoader);
Set<String> basePackages;
Map<String, Object> attrs = metadata
.getAnnotationAttributes(EnableFeignClients.class.getName());
AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
FeignClient.class);
final Class<?>[] clients = attrs == null ? null
: (Class<?>[]) attrs.get("clients");
if (clients == null || clients.length == 0) {
scanner.addIncludeFilter(annotationTypeFilter);
basePackages = getBasePackages(metadata);
}
else {
final Set<String> clientClasses = new HashSet<>();
basePackages = new HashSet<>();
for (Class<?> clazz : clients) {
basePackages.add(ClassUtils.getPackageName(clazz));
clientClasses.add(clazz.getCanonicalName());
}
AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {
@Override
protected boolean match(ClassMetadata metadata) {
String cleaned = metadata.getClassName().replaceAll("\\$", ".");
return clientClasses.contains(cleaned);
}
};
scanner.addIncludeFilter(
new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));
}
for (String basePackage : basePackages) {
Set<BeanDefinition> candidateComponents = scanner
.findCandidateComponents(basePackage);
for (BeanDefinition candidateComponent : candidateComponents) {
if (candidateComponent instanceof AnnotatedBeanDefinition) {
// verify annotated class is an interface
AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
Assert.isTrue(annotationMetadata.isInterface(),
"@FeignClient can only be specified on an interface");
Map<String, Object> attributes = annotationMetadata
.getAnnotationAttributes(
FeignClient.class.getCanonicalName());
String name = getClientName(attributes);
registerClientConfiguration(registry, name,
attributes.get("configuration"));
registerFeignClient(registry, annotationMetadata, attributes);
}
}
}
}2.方法：registerFeignClient
private void registerFeignClient(BeanDefinitionRegistry registry,
AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
String className = annotationMetadata.getClassName();
BeanDefinitionBuilder definition = BeanDefinitionBuilder
.genericBeanDefinition(FeignClientFactoryBean.class);
validate(attributes);
definition.addPropertyValue("url", getUrl(attributes));
definition.addPropertyValue("path", getPath(attributes));
String name = getName(attributes);
definition.addPropertyValue("name", name);
String contextId = getContextId(attributes);
definition.addPropertyValue("contextId", contextId);
definition.addPropertyValue("type", className);
definition.addPropertyValue("decode404", attributes.get("decode404"));
definition.addPropertyValue("fallback", attributes.get("fallback"));
definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
String alias = contextId + "FeignClient";
AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
boolean primary = (Boolean) attributes.get("primary"); // has a default, won't be
// null
beanDefinition.setPrimary(primary);
String qualifier = getQualifier(attributes);
if (StringUtils.hasText(qualifier)) {
alias = qualifier;
}
BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
new String[] { alias });
BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}启动后通过包扫描，当类有@FeignClient注解，将注解的信息取出，连同类名一起取出，赋给BeanDefinitionBuilder，然后根据BeanDefinitionBuilder得到beanDefinition，最后beanDefinition式注入到ioc容器中。在registerFeignClient方法中首先封装了一个FeignClientFactoryBean的，然后调用BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry)将FeignClientFactoryBean注册到Spring IOC容器里面，这里需要注意的是 FeignClientFactoryBean实现了FactoryBean，所以在程序真正调用这个FeignClient注解对应的类时候，实际上是会调用FeignClientFactoryBean里面的 getObject方法返回的对象。
@Override
public Object getObject() throws Exception {
return getTarget();
}
/**
* @param <T> the target type of the Feign client
* @return a {@link Feign} client created with the specified data and the context
* information
*/
<T> T getTarget() {
FeignContext context = this.applicationContext.getBean(FeignContext.class);
Feign.Builder builder = feign(context);
if (!StringUtils.hasText(this.url)) {
if (!this.name.startsWith("http")) {
this.url = "http://" + this.name;
}
else {
this.url = this.name;
}
this.url += cleanPath();
return (T) loadBalance(builder, context,
new HardCodedTarget<>(this.type, this.name, this.url));
}
if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
this.url = "http://" + this.url;
}
String url = this.url + cleanPath();
Client client = getOptional(context, Client.class);
if (client != null) {
if (client instanceof LoadBalancerFeignClient) {
// not load balancing because we have a url,
// but ribbon is on the classpath, so unwrap
client = ((LoadBalancerFeignClient) client).getDelegate();
}
if (client instanceof FeignBlockingLoadBalancerClient) {
// not load balancing because we have a url,
// but Spring Cloud LoadBalancer is on the classpath, so unwrap
client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
}
builder.client(client);
}
Targeter targeter = get(context, Targeter.class);
return (T) targeter.target(this, builder, context,
new HardCodedTarget<>(this.type, this.name, url));
}至此，feign客户端接口的代理类已经生成，可供后续@Autowired 或者@Resource自动装配使用，使用时的对象示例如下：
这里假如引入了hystrix熔断机制，feign会通过代理模式， 自动将所有的方法用 hystrix 进行包装。其代理类为：HystrixInvocationHandler。开启配置如下：
feign:
hystrix:
enabled: true
# httpclient:
# connection-timeout: 10000
client:
config:
default: # 指定feignclients对应的名称 如果指定的是default 表示全局所有的client 的超时时间设置
connectTimeout: 10000
readTimeout: 10000当请求Feign Client的方法时会被拦截，如果没有开启hystrix，那么其代理类为：ReflectiveFeign
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if ("equals".equals(method.getName())) {
try {
Object otherHandler =
args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
return equals(otherHandler);
} catch (IllegalArgumentException e) {
return false;
}
} else if ("hashCode".equals(method.getName())) {
return hashCode();
} else if ("toString".equals(method.getName())) {
return toString();
}
return dispatch.get(method).invoke(args);
}在SynchronousMethodHandler类进行拦截处理，当被FeignClient的方法被拦截会根据参数生成RequestTemplate对象，该对象就是http请求的模板，代码如下：
@Override
public Object invoke(Object[] argv) throws Throwable {
RequestTemplate template = buildTemplateFromArgs.create(argv);
Options options = findOptions(argv);
Retryer retryer = this.retryer.clone();
while (true) {
try {
return executeAndDecode(template, options);
} catch (RetryableException e) {
try {
retryer.continueOrPropagate(e);
} catch (RetryableException th) {
Throwable cause = th.getCause();
if (propagationPolicy == UNWRAP && cause != null) {
throw cause;
} else {
throw th;
}
}
if (logLevel != Logger.Level.NONE) {
logger.logRetry(metadata.configKey(), logLevel);
}
continue;
}
}
}5.微服务直接feign调用相关问题
1.Feign远程调用丢失请求头问题：即当我们登录之后，登录信息是放在请求头里面，但是当我们从订单服务使用feign调用用户服务获取收货地址啥的这时候登录信息丢失了，因为此时请求头为空，最根本的原因是使用feign调用第三方服务时，feign组件是根据相关信息生成一个全新请求去调用第三方服务接口，自然没有之前携带登录信息的请求头了，解决方案如下：实现RequestInterceptor拦截器接口
@Component
@Slf4j
public class FeignInterceptor implements RequestInterceptor {
@Override
public void apply(RequestTemplate requestTemplate) {
//spring的上下文对象
ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
if (requestAttributes != null) {
HttpServletRequest request = requestAttributes.getRequest();
Enumeration<String> headerNames = request.getHeaderNames();
if (headerNames != null) {
while (headerNames.hasMoreElements()) {
String name = headerNames.nextElement();
// 跳过 content-length
if (Objects.equals("content-length", name)){
continue;
}
String value = request.getHeader(name);
requestTemplate.header(name, value);
}
}
}
}
}
2.Feign异步情况丢失上下文问题，根本原因是上下文对象的属性是使用threallocal存储的
解决办法：1）使用feign不使用多线程
2）开启新线程之前先获取上下文信息，开启线程之后把上下文信息放入新的新线程中：
RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
CompletableFuture<Void> itemAndStockFuture = CompletableFuture.supplyAsync(() -> {
RequestContextHolder.setRequestAttributes(requestAttributes);
//1. 查出所有选中购物项
List<OrderItemVo> checkedItems = cartFeignService.getCheckedItems();
confirmVo.setItems(checkedItems);
return checkedItems;
}, executor).thenAcceptAsync((items) -> {
//4. 库存
List<Long> skuIds = items.stream().map(OrderItemVo::getSkuId).collect(Collectors.toList());
Map<Long, Boolean> hasStockMap = wareFeignService.getSkuHasStocks(skuIds).stream().collect(Collectors.toMap(SkuHasStockVo::getSkuId, SkuHasStockVo::getHasStock));
confirmVo.setStocks(hasStockMap);
}, executor);3.在feign接口中封装调用get接口方法时，不能用一个对象当做参数，否则在feign客户端组装请求时最后变成post请求，这时候即时多个参数我们也应该用@RequestParam注解去一一映射对象里面的属性参数。
