Feign 客户端源码解析

Feign的使用非常简单，增加如下配置之后，便可以使用Feign进行调用。非常简单是不是。主要的工作由Feign框架完成。业务代码只提供了一个Interface, 然后由Feign动态生成代理类来实现整个的调用过程。

@Configuration
public class FeignConfig {

  @Bean(name = "myCient")
  public MyClient myClient() {
    return Feign.builder().requestInterceptor(new MyRequestInterceptor()).retryer(Retryer.NEVER_RETRY).logLevel(Logger.Level.BASIC).logger(new Slf4jLogger())
                .encoder(new JacksonEncoder()).decoder(new JacksonDecoder()).
        errorDecoder(new MyResultDecoder()).target(MyClient.class, "serviceUrl");
  }
}

　　

框架相关代码：

Feign.java

public Feign build() {
      SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
          new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
                                               logLevel, decode404);
      ParseHandlersByName handlersByName =
          new ParseHandlersByName(contract, options, encoder, decoder,
                                  errorDecoder, synchronousMethodHandlerFactory);
      return new ReflectiveFeign(handlersByName, invocationHandlerFactory);
    }

ReflectiveFeign.java

@SuppressWarnings("unchecked")
  @Override
  public <T> T newInstance(Target<T> target) {
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target); // 这个方法是重点，生成了代理类具体实现。具体参考SynchronousMethodHandler。
    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;
      } else if(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);
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);

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

　　

SynchronousMethodHandler.java

final class SynchronousMethodHandler implements MethodHandler{

...
//具体实现逻辑，代理类的InvokeHandler的invoke方法会调用这个方法。

@Override
  public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        return executeAndDecode(template);
      } catch (RetryableException e) {
        retryer.continueOrPropagate(e);
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }
...

}