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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
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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>
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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();
  }
 }
}
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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();
  }
 }
}
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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
}
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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;
}

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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
 }
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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>
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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);
 }

}
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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);
}
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FormEncoder 支持

@Configuration
public class FeignFormConfiguration {

    @Autowired
    private ObjectFactory<HttpMessageConverters> messageConverters;

    @Bean
    @Primary
    public Encoder feignFormEncoder() {
        return new FormEncoder(new SpringEncoder(this.messageConverters));
    }
}
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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");
    }
}
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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;
    }
}
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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;
}
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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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