扩展Spring Cloud Feign 实现自动降级

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自动降级目的

在Spring Cloud 使用feign 的时候,需要明确指定fallback 策略,不然会提示错误
先来看默认的feign service 是要求怎么做的。feign service 定义一个 factory 和 fallback 的类

@FeignClient(value = ServiceNameConstants.UMPS_SERVICE, fallbackFactory = RemoteLogServiceFallbackFactory.class)
public interface RemoteLogService {}
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但是我们大多数情况的feign 降级策略为了保证幂等都会很简单,输出错误日志即可。 类似如下代码,在企业中开发非常不方便

@Slf4j
@Component
public class RemoteLogServiceFallbackImpl implements RemoteLogService {
	@Setter
	private Throwable cause;


	@Override
	public R<Boolean> saveLog(SysLog sysLog, String from) {
		log.error("feign 插入日志失败", cause);
		return null;
	}
}
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自动降级效果

@FeignClient(value = ServiceNameConstants.UMPS_SERVICE)
public interface RemoteLogService {}
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  • Feign Service 完成同样的降级错误输出
  • FeignClient 中无需定义无用的fallbackFactory
  • FallbackFactory 也无需注册到Spring 容器中

代码变化,去掉FeignClient 指定的降级工厂

代码变化,删除降级相关的代码

核心源码

  1. 注入我们个性化后的Feign
@Configuration
@ConditionalOnClass({HystrixCommand.class, HystrixFeign.class})
protected static class HystrixFeignConfiguration {
	@Bean
	@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
	@ConditionalOnProperty("feign.hystrix.enabled")
	public Feign.Builder feignHystrixBuilder(FeignContext feignContext) {
		return PigxHystrixFeign.builder(feignContext)
				.decode404()
				.errorDecoder(new PigxFeignErrorDecoder());
	}
}
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  1. PigxHystrixFeign.target 方法是根据@FeignClient 注解生成代理类的过程,注意注释
@Override
public <T> T target(Target<T> target) {
	Class<T> targetType = target.type();
	FeignClient feignClient = AnnotatedElementUtils.getMergedAnnotation(targetType, FeignClient.class);
	String factoryName = feignClient.name();
	SetterFactory setterFactoryBean = this.getOptional(factoryName, feignContext, SetterFactory.class);
	if (setterFactoryBean != null) {
		this.setterFactory(setterFactoryBean);
	}
	
	// 以下为获取降级策略代码,构建降级,这里去掉了降级非空的非空的校验
	Class<?> fallback = feignClient.fallback();
	if (fallback != void.class) {
		return targetWithFallback(factoryName, feignContext, target, this, fallback);
	}
	Class<?> fallbackFactory = feignClient.fallbackFactory();
	if (fallbackFactory != void.class) {
		return targetWithFallbackFactory(factoryName, feignContext, target, this, fallbackFactory);
	}
	return build().newInstance(target);
}
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  1. 构建feign 客户端执行PigxHystrixInvocationHandler的增强
Feign build(@Nullable final FallbackFactory<?> nullableFallbackFactory) {
		super.invocationHandlerFactory((target, dispatch) ->
				new PigxHystrixInvocationHandler(target, dispatch, setterFactory, nullableFallbackFactory));
		super.contract(new HystrixDelegatingContract(contract));
		return super.build();
	}
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  1. PigxHystrixInvocationHandler.getFallback() 获取降级策略
	@Override
	@Nullable
	@SuppressWarnings("unchecked")
	protected Object getFallback() {
	        // 如果 @FeignClient  没有配置降级策略,使用动态代理创建一个
			if (fallbackFactory == null) {
				fallback = PigxFeignFallbackFactory.INSTANCE.create(target.type(), getExecutionException());
			} else {
			  // 如果 @FeignClient配置降级策略,使用配置的
				fallback = fallbackFactory.create(getExecutionException());
			}
	}
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  1. PigxFeignFallbackFactory.create 动态代理逻辑
	public T create(final Class<?> type, final Throwable cause) {
		return (T) FALLBACK_MAP.computeIfAbsent(type, key -> {
			Enhancer enhancer = new Enhancer();
			enhancer.setSuperclass(key);
			enhancer.setCallback(new PigxFeignFallbackMethod(type, cause));
			return enhancer.create();
		});
	}
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  1. PigxFeignFallbackMethod.intercept, 默认的降级逻辑,输出降级方法信息和错误信息,并且把错误格式
public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) {
	log.error("Fallback class:[{}] method:[{}] message:[{}]",
			type.getName(), method.getName(), cause.getMessage());

	if (R.class == method.getReturnType()) {
		final R result = cause instanceof PigxFeignException ?
				((PigxFeignException) cause).getResult() : R.builder()
				.code(CommonConstants.FAIL)
				.msg(cause.getMessage()).build();
		return result;
	}
	return null;
}
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