客户端负载均衡Feign设计原理详解

interface GitHub {
  @RequestLine("GET /repos/{owner}/{repo}/contributors")
  List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
}publicstaticclass Contributor {
  String login;int contributions;
}publicclass MyApp {publicstaticvoid main(String... args) {
    GitHub github= Feign.builder()
                         .decoder(new GsonDecoder())
                         .target(GitHub.class, "https://api.github.com");// Fetch and print a list of the contributors to this library.
    List<Contributor> contributors = github.contributors("OpenFeign", "feign");for (Contributor contributor : contributors) {
      System.out.println(contributor.login+ " (" + contributor.contributions + ")");
    }
  }
}

publicclass ReflectiveFeignextends Feign{///省略部分代码 @Overridepublic <T> T newInstance(Target<T> target) {//根据接口类和Contract协议解析方式，解析接口类上的方法和注解，转换成内部的MethodHandler处理方式 Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target); Map<Method, MethodHandler> methodToHandler =new LinkedHashMap<Method, MethodHandler>(); List<DefaultMethodHandler> defaultMethodHandlers =new LinkedList<DefaultMethodHandler>();for (Method method : target.type().getMethods()) {if (method.getDeclaringClass() == Object.class) {continue; }elseif(Util.isDefault(method)) { DefaultMethodHandler handler =new DefaultMethodHandler(method); defaultMethodHandlers.add(handler); methodToHandler.put(method, handler); }else { methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method))); } } InvocationHandler handler = factory.create(target, methodToHandler);// 基于Proxy.newProxyInstance 为接口类创建动态实现，将所有的请求转换给InvocationHandler 处理。 T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),new Class<?>[]{target.type()}, handler);for(DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) { defaultMethodHandler.bindTo(proxy); }return proxy; }//省略部分代码

/**
 * Defines what annotations and values are valid on interfaces.*/publicinterface Contract {/**
   * Called to parse the methods in the class that are linked to HTTP requests.
   * 传入接口定义，解析成相应的方法内部元数据表示
   *@param targetType {@link feign.Target#type() type} of the Feign interface.*/// TODO: break this and correct spelling at some point
  List<MethodMetadata> parseAndValidatateMetadata(Class<?> targetType);
}

publicinterface GitHub {
  
  @RequestLine("GET /repos/{owner}/{repo}/contributors")
  List<Contributor> getContributors(@Param("owner") String owner, @Param("repo") String repository);class Contributor {
    String login;int contributions;
  }
}

publicinterface Encoder {/** Type literal for {@code Map<String, ?>}, indicating the object to encode is a form.*/
  Type MAP_STRING_WILDCARD= Util.MAP_STRING_WILDCARD;/**
   * Converts objects to an appropriate representation in the template.
   *  将实体对象转换成Http请求的消息正文中
   *@param object   what to encode as the request body.
   *@param bodyType the type the object should be encoded as. {@link #MAP_STRING_WILDCARD}
   *                 indicates form encoding.
   *@param template the request template to populate.
   *@throws EncodeException when encoding failed due to a checked exception.*/void encode(Object object, Type bodyType, RequestTemplate template)throws EncodeException;/**
   * Default implementation of {@code Encoder}.*/class Defaultimplements Encoder {

    @Overridepublicvoid encode(Object object, Type bodyType, RequestTemplate template) {if (bodyType == String.class) {
        template.body(object.toString());
      }elseif (bodyType ==byte[].class) {
        template.body((byte[]) object,null);
      }elseif (object !=null) {thrownew EncodeException(
            format("%s is not a type supported by this encoder.", object.getClass()));
      }
    }
  }
}publicinterface Decoder {/**
   * Decodes an http response into an object corresponding to its {@link
   * java.lang.reflect.Method#getGenericReturnType() generic return type}. If you need to wrap
   * exceptions, please do so via {@link DecodeException}.
   *  从Response 中提取Http消息正文，通过接口类声明的返回类型，消息自动装配
   *@param response the response to decode 
   *@param type     {@link java.lang.reflect.Method#getGenericReturnType() generic return type} of
   *                 the method corresponding to this {@code response}.
   *@return instance of {@code type}
   *@throws IOException     will be propagated safely to the caller.
   *@throws DecodeException when decoding failed due to a checked exception besides IOException.
   *@throws FeignException  when decoding succeeds, but conveys the operation failed.*/
  Object decode(Response response, Type type)throws IOException, DecodeException, FeignException;/** Default implementation of {@code Decoder}.*/publicclass Defaultextends StringDecoder {

    @Overridepublic Object decode(Response response, Type type)throws IOException {if (response.status() == 404)return Util.emptyValueOf(type);if (response.body() ==null)returnnull;if (byte[].class.equals(type)) {return Util.toByteArray(response.body().asInputStream());
      }returnsuper.decode(response, type);
    }
  }
}

publicinterface RequestInterceptor {/**
   * 可以在构造RequestTemplate 请求时，增加或者修改Header, Method, Body 等信息
   * Called for every request. Add data using methods on the supplied {@link RequestTemplate}.*/void apply(RequestTemplate template);
}

publicclass FeignAcceptGzipEncodingInterceptorextends BaseRequestInterceptor {/**
     * Creates new instance of {@link FeignAcceptGzipEncodingInterceptor}.
     *
     *@param properties the encoding properties*/protected FeignAcceptGzipEncodingInterceptor(FeignClientEncodingProperties properties) {super(properties);
    }/**
     * {@inheritDoc}*/
    @Overridepublicvoid apply(RequestTemplate template) {//  在Header 头部添加相应的数据信息        addHeader(template, HttpEncoding.ACCEPT_ENCODING_HEADER, HttpEncoding.GZIP_ENCODING,
                HttpEncoding.DEFLATE_ENCODING);
    }
}

publicabstractclass Logger {protectedstatic String methodTag(String configKey) {returnnew StringBuilder().append('[').append(configKey.substring(0, configKey.indexOf('(')))
        .append("] ").toString();
  }/**
   * Override to log requests and responses using your own implementation. Messages will be http
   * request and response text.
   *
   *@param configKey value of {@link Feign#configKey(Class, java.lang.reflect.Method)}
   *@param format    {@link java.util.Formatter format string}
   *@param args      arguments applied to {@code format}*/protectedabstractvoid log(String configKey, String format, Object... args);protectedvoid logRequest(String configKey, Level logLevel, Request request) {
    log(configKey,"---> %s %s HTTP/1.1", request.method(), request.url());if (logLevel.ordinal() >= Level.HEADERS.ordinal()) {for (String field : request.headers().keySet()) {for (String value : valuesOrEmpty(request.headers(), field)) {
          log(configKey,"%s: %s", field, value);
        }
      }int bodyLength = 0;if (request.body() !=null) {
        bodyLength= request.body().length;if (logLevel.ordinal() >= Level.FULL.ordinal()) {
          String
              bodyText=
              request.charset()!=null ?new String(request.body(), request.charset()) :null;
          log(configKey,"");// CRLF
          log(configKey, "%s", bodyText !=null ? bodyText : "Binary data");
        }
      }
      log(configKey,"---> END HTTP (%s-byte body)", bodyLength);
    }
  }protectedvoid logRetry(String configKey, Level logLevel) {
    log(configKey,"---> RETRYING");
  }protected Response logAndRebufferResponse(String configKey, Level logLevel, Response response,long elapsedTime)throws IOException {
    String reason= response.reason() !=null && logLevel.compareTo(Level.NONE) > 0 ?
        " " + response.reason() : "";int status = response.status();
    log(configKey,"<--- HTTP/1.1 %s%s (%sms)", status, reason, elapsedTime);if (logLevel.ordinal() >= Level.HEADERS.ordinal()) {for (String field : response.headers().keySet()) {for (String value : valuesOrEmpty(response.headers(), field)) {
          log(configKey,"%s: %s", field, value);
        }
      }int bodyLength = 0;if (response.body() !=null && !(status == 204 || status == 205)) {// HTTP 204 No Content "...response MUST NOT include a message-body"// HTTP 205 Reset Content "...response MUST NOT include an entity"if (logLevel.ordinal() >= Level.FULL.ordinal()) {
          log(configKey,"");// CRLF        }byte[] bodyData = Util.toByteArray(response.body().asInputStream());
        bodyLength= bodyData.length;if (logLevel.ordinal() >= Level.FULL.ordinal() && bodyLength > 0) {
          log(configKey,"%s", decodeOrDefault(bodyData, UTF_8, "Binary data"));
        }
        log(configKey,"<--- END HTTP (%s-byte body)", bodyLength);return response.toBuilder().body(bodyData).build();
      }else {
        log(configKey,"<--- END HTTP (%s-byte body)", bodyLength);
      }
    }return response;
  }protected IOException logIOException(String configKey, Level logLevel, IOException ioe,long elapsedTime) {
    log(configKey,"<--- ERROR %s: %s (%sms)", ioe.getClass().getSimpleName(), ioe.getMessage(),
        elapsedTime);if (logLevel.ordinal() >= Level.FULL.ordinal()) {
      StringWriter sw=new StringWriter();
      ioe.printStackTrace(new PrintWriter(sw));
      log(configKey, sw.toString());
      log(configKey,"<--- END ERROR");
    }return ioe;
  }

finalclass SynchronousMethodHandlerimplements MethodHandler {// 省略部分代码
  @Overridepublic Object invoke(Object[] argv)throws Throwable {//根据输入参数，构造Http 请求。
    RequestTemplate template = buildTemplateFromArgs.create(argv);// 克隆出一份重试器
    Retryer retryer =this.retryer.clone();// 尝试最大次数，如果中间有结果，直接返回while (true) {try {return executeAndDecode(template);
      }catch (RetryableException e) {
        retryer.continueOrPropagate(e);if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }continue;
      }
    }
  }

publicinterface Client {/**
   * Executes a request against its {@link Request#url() url} and returns a response.
   *  执行Http请求，并返回Response
   *@param request safe to replay.
   *@param options options to apply to this request.
   *@return connected response, {@link Response.Body} is absent or unread.
   *@throws IOException on a network error connecting to {@link Request#url()}.*/
  Response execute(Request request, Options options)throws IOException;
  }

publicstaticclass Defaultimplements Client {privatefinal SSLSocketFactory sslContextFactory;privatefinal HostnameVerifier hostnameVerifier;/**
     * Null parameters imply platform defaults.*/public Default(SSLSocketFactory sslContextFactory, HostnameVerifier hostnameVerifier) {this.sslContextFactory = sslContextFactory;this.hostnameVerifier = hostnameVerifier;
    }

    @Overridepublic Response execute(Request request, Options options)throws IOException {
      HttpURLConnection connection= convertAndSend(request, options);return convertResponse(connection).toBuilder().request(request).build();
    }

    HttpURLConnection convertAndSend(Request request, Options options)throws IOException {final HttpURLConnection
          connection=
          (HttpURLConnection)new URL(request.url()).openConnection();if (connectioninstanceof HttpsURLConnection) {
        HttpsURLConnection sslCon= (HttpsURLConnection) connection;if (sslContextFactory !=null) {
          sslCon.setSSLSocketFactory(sslContextFactory);
        }if (hostnameVerifier !=null) {
          sslCon.setHostnameVerifier(hostnameVerifier);
        }
      }
      connection.setConnectTimeout(options.connectTimeoutMillis());
      connection.setReadTimeout(options.readTimeoutMillis());
      connection.setAllowUserInteraction(false);
      connection.setInstanceFollowRedirects(true);
      connection.setRequestMethod(request.method());

      Collection<String> contentEncodingValues = request.headers().get(CONTENT_ENCODING);boolean
          gzipEncodedRequest=
          contentEncodingValues!=null && contentEncodingValues.contains(ENCODING_GZIP);boolean
          deflateEncodedRequest=
          contentEncodingValues!=null && contentEncodingValues.contains(ENCODING_DEFLATE);boolean hasAcceptHeader =false;
      Integer contentLength=null;for (String field : request.headers().keySet()) {if (field.equalsIgnoreCase("Accept")) {
          hasAcceptHeader=true;
        }for (String value : request.headers().get(field)) {if (field.equals(CONTENT_LENGTH)) {if (!gzipEncodedRequest && !deflateEncodedRequest) {
              contentLength= Integer.valueOf(value);
              connection.addRequestProperty(field, value);
            }
          }else {
            connection.addRequestProperty(field, value);
          }
        }
      }// Some servers choke on the default accept string.if (!hasAcceptHeader) {
        connection.addRequestProperty("Accept", "*/*");
      }if (request.body() !=null) {if (contentLength !=null) {
          connection.setFixedLengthStreamingMode(contentLength);
        }else {
          connection.setChunkedStreamingMode(8196);
        }
        connection.setDoOutput(true);
        OutputStream out= connection.getOutputStream();if (gzipEncodedRequest) {
          out=new GZIPOutputStream(out);
        }elseif (deflateEncodedRequest) {
          out=new DeflaterOutputStream(out);
        }try {
          out.write(request.body());
        }finally {try {
            out.close();
          }catch (IOException suppressed) {// NOPMD          }
        }
      }return connection;
    }

    Response convertResponse(HttpURLConnection connection)throws IOException {int status = connection.getResponseCode();
      String reason= connection.getResponseMessage();if (status < 0) {thrownew IOException(format("Invalid status(%s) executing %s %s", status,
            connection.getRequestMethod(), connection.getURL()));
      }

      Map<String, Collection<String>> headers =new LinkedHashMap<String, Collection<String>>();for (Map.Entry<String, List<String>> field : connection.getHeaderFields().entrySet()) {// response messageif (field.getKey() !=null) {
          headers.put(field.getKey(), field.getValue());
        }
      }

      Integer length= connection.getContentLength();if (length == -1) {
        length=null;
      }
      InputStream stream;if (status >= 400) {
        stream= connection.getErrorStream();
      }else {
        stream= connection.getInputStream();
      }return Response.builder()
              .status(status)
              .reason(reason)
              .headers(headers)
              .body(stream, length)
              .build();
    }
  }

    @Configuration@ConditionalOnClass(ApacheHttpClient.class)
    @ConditionalOnMissingClass("com.netflix.loadbalancer.ILoadBalancer")
    @ConditionalOnMissingBean(CloseableHttpClient.class)
    @ConditionalOnProperty(value= "feign.httpclient.enabled", matchIfMissing =true)protectedstaticclass HttpClientFeignConfiguration {//...
}

<dependency>
<groupId>io.github.openfeign</groupId>
<artifactId>feign-httpclient</artifactId>
<version>9.4.0</version>
</dependency>

    @Configuration
    @ConditionalOnClass(OkHttpClient.class)
    @ConditionalOnMissingClass("com.netflix.loadbalancer.ILoadBalancer")
    @ConditionalOnMissingBean(okhttp3.OkHttpClient.class)
    @ConditionalOnProperty(value= "feign.okhttp.enabled")protectedstaticclass OkHttpFeignConfiguration {private okhttp3.OkHttpClient okHttpClient;//...
}