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
代码变化，删除降级相关的代码
核心源码
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());
}
}
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);
}
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();
}
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());
}
}
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();
});
}
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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