feign的Fallback机制

对接口使用@FeignClient后声明feign客户端后，可以使用属性fallback指定异常处理类，这个类必须实现@FeignClient作用的接口，且被注入到容器中。

@FeignClient(name = "service-provider1",fallback = NacosFeignImpl.class)public interface NacosFeignClient {    @RequestMapping(value = "/echo/{str}",method = RequestMethod.GET)    String echo(@PathVariable("str") String str);}@Componentpublic class NacosFeignImpl implements NacosFeignClient{    @Override    public String echo(String str) {        System.out.println("NacosFeignImpl#echo called");        return "echo error";    }}

添加配置项feign.hystrix.enabled=true，当配置了HystrixFeign时，即会创建HystrixFeign.Builder。

// FeignClientsConfiguration.java@Configuration@ConditionalOnClass({ HystrixCommand.class, HystrixFeign.class })protected static class HystrixFeignConfiguration {@Bean@Scope("prototype")@ConditionalOnMissingBean@ConditionalOnProperty(name = "feign.hystrix.enabled")public Feign.Builder feignHystrixBuilder() {return HystrixFeign.builder();}}

在HystrixTargeter#target()发现使用了HystrixFeign.Builder并且属性fallback存在时，就会通过targetWithFallback()创建feign客户端。这里会到容器中获取实现了@FeignClient接口的Bean。

public  T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,Target.HardCodedTarget target) {if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {return feign.target(target);}feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;SetterFactory setterFactory = getOptional(factory.getName(), context,SetterFactory.class);if (setterFactory != null) {builder.setterFactory(setterFactory);}Class fallback = factory.getFallback();if (fallback != void.class) {return targetWithFallback(factory.getName(), context, target, builder, fallback);}Class fallbackFactory = factory.getFallbackFactory();if (fallbackFactory != void.class) {return targetWithFallbackFactory(factory.getName(), context, target, builder, fallbackFactory);}return feign.target(target);}

此时创建InvocationHandlerFactory，注入到属性invocationHandlerFactory，invocationHandlerFactory默认是InvocationHandlerFactory.Default，这里重写create()方法，创建了HystrixInvocationHandler的实例对象。

    Feign build(final FallbackFactory nullableFallbackFactory) {      super.invocationHandlerFactory(new InvocationHandlerFactory() {        @Override public InvocationHandler create(Target target,            Map dispatch) {          return new HystrixInvocationHandler(target, dispatch, setterFactory, nullableFallbackFactory);        }      });      super.contract(new HystrixDelegatingContract(contract));      return super.build();    }

生成代理对象时，就会调用create()对HystrixInvocationHandler实例化，这样就会调用HystrixInvocationHandler#invoke()。

  public  T newInstance(Target target) {    ......    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;  }

invoke()中run()的逻辑还是调用SynchronousMethodHandler#invoke()处理请求，发生异常时就会到getFallback()调用接口的实现类对象的方法。

  public Object invoke(final Object proxy, final Method method, final Object[] args)      throws Throwable {    // early exit if the invoked method is from java.lang.Object    // code is the same as ReflectiveFeign.FeignInvocationHandler    if ("equals".equals(method.getName())) {      try {        Object otherHandler =            args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;        return equals(otherHandler);      } catch (IllegalArgumentException e) {        return false;      }    } else if ("hashCode".equals(method.getName())) {      return hashCode();    } else if ("toString".equals(method.getName())) {      return toString();    }    HystrixCommand