Feign Client 原理和使用

最近一个新项目在做后端HTTP库技术选型的时候对比了Spring WebClient,Spring RestTemplate,Retrofit,Feign,Okhttp。综合考虑最终选择了上层封装比较好的Feign,尽管我们的App没有加入微服务，但是时间下来Feign用着还是很香的。

我们的sytyale针对Feign的底层原理和源码进行了解析，最后用一个小例子总结怎么快速上手。

本文作者：sytyale，另外一个聪明好学的同事

一、原理

Feign 是一个 Java 到 HTTP 的客户端绑定器，灵感来自于 Retrofit 和 JAXRS-2.0 以及 WebSocket。Feign 的第一个目标是降低将 Denominator 无变化的绑定到 HTTP APIs 的复杂性，而不考虑 ReSTfulness。

Feign 使用 Jersey 和 CXF 等工具为 ReST 或 SOAP 服务编写 java 客户端。此外，Feign 允许您在 Apache HC 等http 库之上编写自己的代码。Feign 以最小的开销将代码连接到 http APIs，并通过可定制的解码器和错误处理（可以写入任何基于文本的 http APIs）将代码连接到 http APIs。

Feign 通过将注解处理为模板化请求来工作。参数在输出之前直接应用于这些模板。尽管 Feign 仅限于支持基于文本的 APIs，但它极大地简化了系统方面，例如重放请求。此外，Feign 使得对转换进行单元测试变得简单。

Feign 10.x 及以上版本是在 Java 8上构建的，应该在 Java 9、10 和 11上工作。对于需要 JDK 6兼容性的用户，请使用 Feign 9.x

二、处理过程图

三、Http Client 依赖

feign 在默认情况下使用 JDK 原生的 URLConnection 发送HTTP请求。(没有连接池，保持长连接) 。

可以通过修改 client 依赖换用底层的 client，不同的 http client 对请求的支持可能有差异。具体使用示例如下:

feign: 
  httpclient:
    enable: false
  okhttp:
    enable: true

AND

<!-- Support PATCH Method-->
<dependency>    
  <groupId>org.apache.httpcomponents</groupId>    
  <artifactId>httpclient</artifactId> 
</dependency>
      
<!-- Do not support PATCH Method -->
<dependency>
    <groupId>io.github.openfeign</groupId>
    <artifactId>feign-okhttp</artifactId>
</dependency>

四、Http Client 配置

• okhttp 配置源码

@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(okhttp3.OkHttpClient.class)
public class OkHttpFeignConfiguration {

 private okhttp3.OkHttpClient okHttpClient;
  
 @Bean
 @ConditionalOnMissingBean(ConnectionPool.class)
 public ConnectionPool httpClientConnectionPool( FeignHttpClientProperties httpClientProperties, OkHttpClientConnectionPoolFactory connectionPoolFactory) {
  Integer maxTotalConnections = httpClientProperties.getMaxConnections();
  Long timeToLive = httpClientProperties.getTimeToLive();
  TimeUnit ttlUnit = httpClientProperties.getTimeToLiveUnit();
  return connectionPoolFactory.create(maxTotalConnections, timeToLive, ttlUnit);
 }

 @Bean
 public okhttp3.OkHttpClient client(OkHttpClientFactory httpClientFactory, ConnectionPool connectionPool, FeignHttpClientProperties httpClientProperties) {
  Boolean followRedirects = httpClientProperties.isFollowRedirects();
  Integer connectTimeout = httpClientProperties.getConnectionTimeout();
  this.okHttpClient = httpClientFactory
    .createBuilder(httpClientProperties.isDisableSslValidation())
    .connectTimeout(connectTimeout, TimeUnit.MILLISECONDS)
    .followRedirects(followRedirects).connectionPool(connectionPool).build();
  return this.okHttpClient;
 }

 @PreDestroy
 public void destroy() {
  if (this.okHttpClient != null) {
   this.okHttpClient.dispatcher().executorService().shutdown();
   this.okHttpClient.connectionPool().evictAll();
  }
 }
}

• HttpClient 配置源码

@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(CloseableHttpClient.class)
public class HttpClientFeignConfiguration {

 private final Timer connectionManagerTimer = new Timer(
   "FeignApacheHttpClientConfiguration.connectionManagerTimer", true);

 private CloseableHttpClient httpClient;

 @Autowired(required = false)
 private RegistryBuilder registryBuilder;

 @Bean
 @ConditionalOnMissingBean(HttpClientConnectionManager.class)
 public HttpClientConnectionManager connectionManager(
   ApacheHttpClientConnectionManagerFactory connectionManagerFactory,
   FeignHttpClientProperties httpClientProperties) {
  final HttpClientConnectionManager connectionManager = connectionManagerFactory
    .newConnectionManager(httpClientProperties.isDisableSslValidation(),
      httpClientProperties.getMaxConnections(),
      httpClientProperties.getMaxConnectionsPerRoute(),
      httpClientProperties.getTimeToLive(),
      httpClientProperties.getTimeToLiveUnit(), this.registryBuilder);
  this.connectionManagerTimer.schedule(new TimerTask() {
   @Override
   public void run() {
    connectionManager.closeExpiredConnections();
   }
  }, 30000, httpClientProperties.getConnectionTimerRepeat());
  return connectionManager;
 }

 @Bean
 @ConditionalOnProperty(value = "feign.compression.response.enabled", havingValue = "true")
 public CloseableHttpClient customHttpClient(
   HttpClientConnectionManager httpClientConnectionManager,
   FeignHttpClientProperties httpClientProperties) {
  HttpClientBuilder builder = HttpClientBuilder.create().disableCookieManagement()
    .useSystemProperties();
  this.httpClient = createClient(builder, httpClientConnectionManager,
    httpClientProperties);
  return this.httpClient;
 }

 @Bean
 @ConditionalOnProperty(value = "feign.compression.response.enabled", havingValue = "false", matchIfMissing = true)
 public CloseableHttpClient httpClient(ApacheHttpClientFactory httpClientFactory,
   HttpClientConnectionManager httpClientConnectionManager,
   FeignHttpClientProperties httpClientProperties) {
  this.httpClient = createClient(httpClientFactory.createBuilder(),
    httpClientConnectionManager, httpClientProperties);
  return this.httpClient;
 }

 private CloseableHttpClient createClient(HttpClientBuilder builder,
   HttpClientConnectionManager httpClientConnectionManager,
   FeignHttpClientProperties httpClientProperties) {
  RequestConfig defaultRequestConfig = RequestConfig.custom()
    .setConnectTimeout(httpClientProperties.getConnectionTimeout())
    .setRedirectsEnabled(httpClientProperties.isFollowRedirects()).build();
  CloseableHttpClient httpClient = builder
    .setDefaultRequestConfig(defaultRequestConfig)
    .setConnectionManager(httpClientConnectionManager).build();
  return httpClient;
 }

 @PreDestroy
 public void destroy() throws Exception {
  this.connectionManagerTimer.cancel();
  if (this.httpClient != null) {
   this.httpClient.close();
  }
 }
}

• HttpClient 配置属性

@ConfigurationProperties(prefix = "feign.httpclient")
public class FeignHttpClientProperties {

 /** * Default value for disabling SSL validation. */
 public static final boolean DEFAULT_DISABLE_SSL_VALIDATION = false;

 /** * Default value for max number od connections. */
 public static final int DEFAULT_MAX_CONNECTIONS = 200;

 /** * Default value for max number od connections per route. */
 public static final int DEFAULT_MAX_CONNECTIONS_PER_ROUTE = 50;

 /** * Default value for time to live. */
 public static final long DEFAULT_TIME_TO_LIVE = 900L;

 /** * Default time to live unit. */
 public static final TimeUnit DEFAULT_TIME_TO_LIVE_UNIT = TimeUnit.SECONDS;

 /** * Default value for following redirects. */
 public static final boolean DEFAULT_FOLLOW_REDIRECTS = true;

 /** * Default value for connection timeout. */
 public static final int DEFAULT_CONNECTION_TIMEOUT = 2000;

 /** * Default value for connection timer repeat. */
 public static final int DEFAULT_CONNECTION_TIMER_REPEAT = 3000;

 private boolean disableSslValidation = DEFAULT_DISABLE_SSL_VALIDATION;

 private int maxConnections = DEFAULT_MAX_CONNECTIONS;

 private int maxConnectionsPerRoute = DEFAULT_MAX_CONNECTIONS_PER_ROUTE;

 private long timeToLive = DEFAULT_TIME_TO_LIVE;

 private TimeUnit timeToLiveUnit = DEFAULT_TIME_TO_LIVE_UNIT;

 private boolean followRedirects = DEFAULT_FOLLOW_REDIRECTS;

 private int connectionTimeout = DEFAULT_CONNECTION_TIMEOUT;

 private int connectionTimerRepeat = DEFAULT_CONNECTION_TIMER_REPEAT;

 //省略 setter 和 getter 方法
}

五、部分注解

• FeignClient 注解源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {

  // 忽略了过时的属性
  
 /** * 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 "";

 /** * 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;
}

六、Feign Client 配置

• FeignClient 配置源码

 /** * Feign client configuration. */
 public static class FeignClientConfiguration {

  private Logger.Level loggerLevel;

  private Integer connectTimeout;

  private Integer readTimeout;

  private Class<Retryer> retryer;

  private Class<ErrorDecoder> errorDecoder;

  private List<Class<RequestInterceptor>> requestInterceptors;

  private Boolean decode404;

  private Class<Decoder> decoder;

  private Class<Encoder> encoder;

  private Class<Contract> contract;

  private ExceptionPropagationPolicy exceptionPropagationPolicy;

    //省略setter 和 getter
 }

七、Spring boot 服务下使用示例

• pom.xml 中引入依赖，部分特性需要额外的依赖扩展(诸如表单提交等)

  <dependencies>
    <!-- spring-cloud-starter-openfeign 支持负载均衡、重试、断路器等 -->
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-openfeign</artifactId>
      <version>2.2.2.RELEASE</version>
    </dependency>
    <!-- Required to use PATCH. feign-okhttp not support PATCH Method -->
    <dependency>
      <groupId>io.github.openfeign</groupId>
      <artifactId>feign-httpclient</artifactId>
      <version>11.0</version>
    </dependency>
  </dependencies>
  

• 开启支持-使用 EnableFeignClients 注解

  @SpringBootApplication
  @EnableFeignClients
  public class TyaleApplication {
  
   public static void main(String[] args) {
    SpringApplication.run(TyaleApplication.class, args);
   }
  
  }
  

• 接口注解-标记请求地址、请求header、请求方式、参数(是否必填)等

  //如果是微服务内部调用则 value 可以直接指定对方服务在服务发现中的服务名，不需要 url
  @FeignClient(value = "tyale", url = "${base.uri}")
  public interface TyaleFeignClient {
  
      @PostMapping(value = "/token", consumes ="application/x-www-form-urlencoded")
      Map<String, Object> obtainToken(Map<String, ?> queryParam);
    
      @GetMapping(value = Constants.STATION_URI)
      StationPage stations(@RequestHeader("Accept-Language") String acceptLanguage,
                           @RequestParam(name = "country") String country,
                           @RequestParam(name = "order") String order,
                           @RequestParam(name = "page", required = false) Integer page,
                           @RequestParam(name = "pageSize") Integer pageSize);
  
      @PostMapping(value = Constants.PAYMENT_URI)
      PaymentDTO payment(@RequestHeader("Accept-Language") String acceptLanguage,
                         @RequestBody PaymentRQ paymentRq);
  }
  

• FormEncoder 支持

  @Configuration
  public class FeignFormConfiguration {
  
      @Autowired
      private ObjectFactory<HttpMessageConverters> messageConverters;
  
      @Bean
      @Primary
      public Encoder feignFormEncoder() {
          return new FormEncoder(new SpringEncoder(this.messageConverters));
      }
  }
  

• 拦截器-自动添加header 或者 token 等

  @Configuration
  public class FeignInterceptor implements RequestInterceptor {
  
      @Override
      public void apply(RequestTemplate requestTemplate) {
          requestTemplate.header(Constants.TOKEN_STR, "Bearer xxx");
      }
  }
  

• ErrorCode-可以自定义错误响应码的处理

  @Configuration
  public class TyaleErrorDecoder implements ErrorDecoder {
  
      @Override
      public Exception decode(String methodKey, Response response) {
          TyaleErrorException errorException = null;
          try {
              if (response.body() != null) {
                 Charset utf8 = StandardCharsets.UTF_8;
                  var body = Util.toString(response.body().asReader(utf8));
                  errorException = GsonUtils.fromJson(body, TyaleErrorException.class);
              } else {
                  errorException = new TyaleErrorException();
              }
          } catch (IOException ignored) {
  
          }
          return errorException;
      }
  }
  

• TyaleErrorException 类示例-处理返回失败响应码时的数据，不同的服务端可能需要不同的处理

  @EqualsAndHashCode(callSuper = true)
  @Data
  @AllArgsConstructor
  @NoArgsConstructor
  public class TyaleErrorException extends Exception {
  
      /** * example: "./api/{service-name}/{problem-id}" */
      private String type;
  
      /** * example: {title} */
      private String title;
  
      /** * example: https://api/docs/index.html#error-handling */
      private String documentation;
  
      /** * example: {code} */
      private String status;
  }
  

• FeignClient 使用示例

  @RestController
  @RequestMapping(value = "/rest/tyale")
  public class TyaleController {
  
      @Autowired
      private TyaleFeignClient feignClient;
  
      @GetMapping(value="/stations")
      public BaseResponseDTO<StationPage> stations() {
          try {
              String acceptLanguage = "en";
              String country = "DE";
              String order = "NAME";
              Integer page = 0;
              Integer pageSize = 20;
              StationPage stationPage = feignClient.stations(acceptLanguage,
                      country, order, page, pageSize);
              return ResponseBuilder.buildSuccessRS(stationPage);
          } catch (TyaleErrorException tyaleError) {
              System.out.println(tyaleError);
              //todo 处理异常返回时的响应
          }
          return ResponseBuilder.buildSuccessRS();
      }
  }
  

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