记一次feignClient导致的启动失败经历

spring cloud starter feign 1.4.4.RELEASE

一、基本排错过程及原因

项目启动报错如下： 

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'xService' defined in class path resource [***/Y.class]: Unsatisfied dependency expressed through method 'xService' parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name '**.zFeignClient': FactoryBean threw exception on object creation; nested exception is java.lang.IllegalStateException: RequestParam.value() was empty on parameter 0
        at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:749)
        at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:467)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1177)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1072)
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:511)           at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:481)             at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:312)
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:308)
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:756)               at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)              at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
        at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:124)
        at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693)

feignClient创建失败，原因描述来看，是RequestParam标注的地方，feignClient的写法如下：

@RequestMapping(value = "/***", method = RequestMethod.POST)
  returnType>> mapByTypeList(@RequestParam List list);

既然报错是RequestParam导致的，那么定位到报错位置：

org.springframework.cloud.netflix.feign.annotation.RequestParamParameterProcessor#processArgument

public boolean processArgument(AnnotatedParameterContext context, Annotation annotation, Method method) {
		int parameterIndex = context.getParameterIndex();
		Class parameterType = method.getParameterTypes()[parameterIndex];
		MethodMetadata data = context.getMethodMetadata();

		if (Map.class.isAssignableFrom(parameterType)) {
			checkState(data.queryMapIndex() == null, "Query map can only be present once.");
			data.queryMapIndex(parameterIndex);

			return true;
		}

		RequestParam requestParam = ANNOTATION.cast(annotation);
        // 获取value进行判断
		String name = requestParam.value();
		checkState(emptyToNull(name) != null,
				"RequestParam.value() was empty on parameter %s",
				parameterIndex);
		context.setParameterName(name);

		Collection query = context.setTemplateParameter(name,
				data.template().queries().get(name));
		data.template().query(name, query);
		return true;
	}

明显，报错位置一目了然，checkState处，原因是获取@RequestParam的value没有值，正好上面我们的写法没有配！！

二、Spring机制探秘

既然位置找到了，那么看看究竟是Spring机制的哪一步导致了该问题

org.springframework.cloud.netflix.feign.support.SpringMvcContract#processAnnotationsOnParameter

protected boolean processAnnotationsOnParameter(MethodMetadata data,
			Annotation[] annotations, int paramIndex) {
		boolean isHttpAnnotation = false;

		AnnotatedParameterProcessor.AnnotatedParameterContext context = new SimpleAnnotatedParameterContext(
				data, paramIndex);
		Method method = this.processedMethods.get(data.configKey());
        // 遍历feign方法上各注解
		for (Annotation parameterAnnotation : annotations) {
            // 找到注解对应的processor，即处理器
			AnnotatedParameterProcessor processor = this.annotatedArgumentProcessors
					.get(parameterAnnotation.annotationType());
			if (processor != null) {
				Annotation processParameterAnnotation;
				// synthesize, handling @AliasFor, while falling back to parameter name on
				// missing String #value():
				processParameterAnnotation = synthesizeWithMethodParameterNameAsFallbackValue(
						parameterAnnotation, method, paramIndex);
                // 进行参数校验等过程（我们的出错点就是这里）
				isHttpAnnotation |= processor.processArgument(context,
						processParameterAnnotation, method);
			}
		}
		if (isHttpAnnotation && data.indexToExpander().get(paramIndex) == null
				&& this.conversionService.canConvert(
						method.getParameterTypes()[paramIndex], String.class)) {
			data.indexToExpander().put(paramIndex, this.expander);
		}
		return isHttpAnnotation;
	}

再往上找，feign.Contract.BaseContract#parseAndValidateMetadata【针对method的解析和校验】

protected MethodMetadata parseAndValidateMetadata(Class targetType, Method method) {
      MethodMetadata data = new MethodMetadata();
      data.returnType(Types.resolve(targetType, targetType, method.getGenericReturnType()));
      data.configKey(Feign.configKey(targetType, method));

      if(targetType.getInterfaces().length == 1) {
        processAnnotationOnClass(data, targetType.getInterfaces()[0]);
      }
      processAnnotationOnClass(data, targetType);


      for (Annotation methodAnnotation : method.getAnnotations()) {
        processAnnotationOnMethod(data, methodAnnotation, method);
      }
      checkState(data.template().method() != null,
                 "Method %s not annotated with HTTP method type (ex. GET, POST)",
                 method.getName());
      Class[] parameterTypes = method.getParameterTypes();

      Annotation[][] parameterAnnotations = method.getParameterAnnotations();
      int count = parameterAnnotations.length;
      for (int i = 0; i < count; i++) {
        boolean isHttpAnnotation = false;
        if (parameterAnnotations[i] != null) {
          // 调用上步的方法，作为注解和参数双重校验
          isHttpAnnotation = processAnnotationsOnParameter(data, parameterAnnotations[i], i);
        }
        if (parameterTypes[i] == URI.class) {
          data.urlIndex(i);
        } else if (!isHttpAnnotation) {
          checkState(data.formParams().isEmpty(),
                     "Body parameters cannot be used with form parameters.");
          checkState(data.bodyIndex() == null, "Method has too many Body parameters: %s", method);
          data.bodyIndex(i);
          data.bodyType(Types.resolve(targetType, targetType, method.getGenericParameterTypes()[i]));
        }
      }

      if (data.headerMapIndex() != null) {
        checkState(Map.class.isAssignableFrom(parameterTypes[data.headerMapIndex()]),
                "HeaderMap parameter must be a Map: %s", parameterTypes[data.headerMapIndex()]);
      }

      if (data.queryMapIndex() != null) {
        checkState(Map.class.isAssignableFrom(parameterTypes[data.queryMapIndex()]),
                "QueryMap parameter must be a Map: %s", parameterTypes[data.queryMapIndex()]);
      }

      return data;
    }

上级调用feign.Contract.BaseContract#parseAndValidatateMetadata(java.lang.Class)【上一步的重载方法：针对class】

public List parseAndValidatateMetadata(Class targetType) {
      checkState(targetType.getTypeParameters().length == 0, "Parameterized types unsupported: %s",
                 targetType.getSimpleName());
      checkState(targetType.getInterfaces().length <= 1, "Only single inheritance supported: %s",
                 targetType.getSimpleName());
      if (targetType.getInterfaces().length == 1) {
        checkState(targetType.getInterfaces()[0].getInterfaces().length == 0,
                   "Only single-level inheritance supported: %s",
                   targetType.getSimpleName());
      }
      Map result = new LinkedHashMap();
      for (Method method : targetType.getMethods()) {
        if (method.getDeclaringClass() == Object.class ||
            (method.getModifiers() & Modifier.STATIC) != 0 ||
            Util.isDefault(method)) {
          continue;
        }
        // 调用自己的重载方法，针对单个method的元数据解析和校验
        MethodMetadata metadata = parseAndValidateMetadata(targetType, method);
        checkState(!result.containsKey(metadata.configKey()), "Overrides unsupported: %s",
                   metadata.configKey());
        result.put(metadata.configKey(), metadata);
      }
      return new ArrayList(result.values());
    }

接下来，我们找到了最初的起点：feign.ReflectiveFeign.ParseHandlersByName#apply【feign的生成构成之解析、校验元数据】

public Map apply(Target key) {
        // 解析所处理feign的元数据
      List metadata = contract.parseAndValidatateMetadata(key.type());
      Map result = new LinkedHashMap();
      for (MethodMetadata md : metadata) {
        BuildTemplateByResolvingArgs buildTemplate;
        if (!md.formParams().isEmpty() && md.template().bodyTemplate() == null) {
          buildTemplate = new BuildFormEncodedTemplateFromArgs(md, encoder);
        } else if (md.bodyIndex() != null) {
          buildTemplate = new BuildEncodedTemplateFromArgs(md, encoder);
        } else {
          buildTemplate = new BuildTemplateByResolvingArgs(md);
        }
        result.put(md.configKey(),
                   factory.create(key, md, buildTemplate, options, decoder, errorDecoder));
      }
      return result;
    }

最终，跟到了ReflectiveFeign，也就是feing的生成器，由此看，feign的生成过程一目了然！！