扩展Spring Cloud Feign 实现自动降级

男娘i 2022-02-13 01:37 167阅读 0赞

![20190222163215_Y3eyGU_E5_86_85_E9_83_A8_E8_B0_83_E7_94_A8.jpeg][]

### 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 {}

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;
        }
    }

### 自动降级效果 ###

@FeignClient(value = ServiceNameConstants.UMPS_SERVICE)
    public interface RemoteLogService {}

*  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 ####

![007xHoylly1g068r6192aj32c21fetha.jpg][]

#### 代码变化，删除降级相关的代码 ####

![007xHoylly1g068pg8ws0j30sg0riwhm.jpg][]

### 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());
        }
    }

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);
    }

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();
        }

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());
                }
        }

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();
            });
        }

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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[20190222163215_Y3eyGU_E5_86_85_E9_83_A8_E8_B0_83_E7_94_A8.jpeg]: /images/20220213/a539559550bf4754bef6d768a851462a.png
[007xHoylly1g068r6192aj32c21fetha.jpg]: https://ws1.sinaimg.cn/large/007xHoylly1g068r6192aj32c21fetha.jpg
[007xHoylly1g068pg8ws0j30sg0riwhm.jpg]: https://ws1.sinaimg.cn/large/007xHoylly1g068pg8ws0j30sg0riwhm.jpg
[Spring Cloud _mica]: https://github.com/lets-mica/mica
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