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);
}
@Component
public class NacosFeignImpl implements NacosFeignClient{@Overridepublic 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> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,Target.HardCodedTarget<T> 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<Method, MethodHandler> 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> T newInstance(Target<T> 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.FeignInvocationHandlerif ("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<Object> hystrixCommand = new HystrixCommand<Object>(setterMethodMap.get(method)) {@Overrideprotected Object run() throws Exception {try {return HystrixInvocationHandler.this.dispatch.get(method).invoke(args);} catch (Exception e) {throw e;} catch (Throwable t) {throw (Error) t;}}@Overrideprotected Object getFallback() {if (fallbackFactory == null) {return super.getFallback();}try {Object fallback = fallbackFactory.create(getExecutionException());Object result = fallbackMethodMap.get(method).invoke(fallback, args);if (isReturnsHystrixCommand(method)) {return ((HystrixCommand) result).execute();} else if (isReturnsObservable(method)) {// Create a cold Observablereturn ((Observable) result).toBlocking().first();} else if (isReturnsSingle(method)) {// Create a cold Observable as a Singlereturn ((Single) result).toObservable().toBlocking().first();} else if (isReturnsCompletable(method)) {((Completable) result).await();return null;} else {return result;}} catch (IllegalAccessException e) {// shouldn't happen as method is public due to being an interfacethrow new AssertionError(e);} catch (InvocationTargetException e) {// Exceptions on fallback are tossed by Hystrixthrow new AssertionError(e.getCause());}}};if (isReturnsHystrixCommand(method)) {return hystrixCommand;} else if (isReturnsObservable(method)) {// Create a cold Observablereturn hystrixCommand.toObservable();} else if (isReturnsSingle(method)) {// Create a cold Observable as a Singlereturn hystrixCommand.toObservable().toSingle();} else if (isReturnsCompletable(method)) {return hystrixCommand.toObservable().toCompletable();}return hystrixCommand.execute();}

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