詳解Feign的實現原理

public abstract class Feign {
    
  ...  

  public static Builder builder() {
    return new Builder();
  }

  public abstract <T> T newInstance(Target<T> target);

  public static class Builder {

    ...

    private final List<RequestInterceptor> requestInterceptors = new ArrayList<RequestInterceptor>();
    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();
    private InvocationHandlerFactory invocationHandlerFactory =
        new InvocationHandlerFactory.Default();
    private boolean decode404;
    private boolean closeAfterDecode = true;
    private ExceptionPropagationPolicy propagationPolicy = NONE;
    private boolean forceDecoding = false;
    private List<Capability> capabilities = new ArrayList<>();

    // 設置輸入打印日志級別
    public Builder logLevel(Logger.Level logLevel) {
      this.logLevel = logLevel;
      return this;
    }

    // 設置接口方法註解處理器（契約） 
    public Builder contract(Contract contract) {
      this.contract = contract;
      return this;
    }

    // 設置使用的 Client（默認使用 JDK 的 HttpURLConnection）
    public Builder client(Client client) {
      this.client = client;
      return this;
    }

    // 設置重試器
    public Builder retryer(Retryer retryer) {
      this.retryer = retryer;
      return this;
    }

    // 設置請求編碼器 
    public Builder encoder(Encoder encoder) {
      this.encoder = encoder;
      return this;
    }

    // 設置響應解碼器
    public Builder decoder(Decoder decoder) {
      this.decoder = decoder;
      return this;
    }

    // 設置 404 返回結果解碼器
    public Builder decode404() {
      this.decode404 = true;
      return this;
    }

    // 設置錯誤解碼器
    public Builder errorDecoder(ErrorDecoder errorDecoder) {
      this.errorDecoder = errorDecoder;
      return this;
    }

    // 設置請求攔截器
    public Builder requestInterceptors(Iterable<RequestInterceptor> requestInterceptors) {
      this.requestInterceptors.clear();
      for (RequestInterceptor requestInterceptor : requestInterceptors) {
        this.requestInterceptors.add(requestInterceptor);
      }
      return this;
    }

    public <T> T target(Class<T> apiType, String url) {
      return target(new HardCodedTarget<T>(apiType, url));
    }

    public <T> T target(Target<T> target) {
      return build().newInstance(target);
    }

  }

  ...

}

public class ReflectiveFeign extends Feign {

  ...  

  private final ParseHandlersByName targetToHandlersByName;
  private final InvocationHandlerFactory factory;
  private final QueryMapEncoder queryMapEncoder;

  ReflectiveFeign(ParseHandlersByName targetToHandlersByName, InvocationHandlerFactory factory,
      QueryMapEncoder queryMapEncoder) {
    this.targetToHandlersByName = targetToHandlersByName;
    this.factory = factory;
    this.queryMapEncoder = queryMapEncoder;
  }

  @SuppressWarnings("unchecked")
  @Override
  public <T> T newInstance(Target<T> target) {
    // <類名#方法簽名, MethodHandler>，key 是通過 feign.Feign.configKey(Class targetType, Method method) 生成的
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    // 將 Map<String, MethodHandler> 轉換為  Map<Method, MethodHandler> 方便調用
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    // 默認方法處理器
    List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

    for (Method method : target.type().getMethods()) {
      // 跳過 Object 類定於的方法  
      if (method.getDeclaringClass() == Object.class) {
        continue;
      } else if (Util.isDefault(method)) {
        // 默認方法（接口聲明的默認方法）使用默認的方法處理器  
        DefaultMethodHandler handler = new DefaultMethodHandler(method);
        defaultMethodHandlers.add(handler);
        methodToHandler.put(method, handler);
      } else {
        // 接口正常聲明的方法（e.g. GitHub.listContributors(String, String)）  
        methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
      }
    }

    // 生成 Feign 封裝的 InvocationHandler
    InvocationHandler handler = factory.create(target, methodToHandler);
    // 基於 JDK 動態代理生成接口的代理類（e.g. Github 接口）
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
        new Class<?>[] {target.type()}, handler);

    for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
      defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
  }

...

}

final class SynchronousMethodHandler implements MethodHandler {

  ...

  @Override
  public Object invoke(Object[] argv) throws Throwable {
    // 獲取 RequestTemplate 將請求參數封裝成請求模板  
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Options options = findOptions(argv);
    // 請求重試器
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        // 執行請求並解碼後返回  
        return executeAndDecode(template, options);
      } catch (RetryableException e) {
        try {
          // 發生重試異常則進行重試處理  
          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;
      }
    }
  }

  Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
    // 從請求模板 RequestTemplate 構造請求參數對象 Request  
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
      logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
      // 通過 client（Apache HttpComponnets、HttpURLConnection OkHttp 等）執行 HTTP 請求調用，默認是 HttpURLConnection 
      response = client.execute(request, options);
      // ensure the request is set. TODO: remove in Feign 12
      response = response.toBuilder()
          .request(request)
          .requestTemplate(template)
          .build();
    } catch (IOException e) {
      if (logLevel != Logger.Level.NONE) {
        logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
      }
      throw errorExecuting(request, e);
    }
    long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);

    if (decoder != null)
      // 對返回結果進行解碼操作
      return decoder.decode(response, metadata.returnType());

    CompletableFuture<Object> resultFuture = new CompletableFuture<>();
    asyncResponseHandler.handleResponse(resultFuture, metadata.configKey(), response,
        metadata.returnType(),
        elapsedTime);

    try {
      if (!resultFuture.isDone())
        throw new IllegalStateException("Response handling not done");

      return resultFuture.join();
    } catch (CompletionException e) {
      Throwable cause = e.getCause();
      if (cause != null)
        throw cause;
      throw e;
    }
  }

...

}