Spring Cloud Feign Jackson自定义配置

Spring Cloud Feign 默认支持Spring MVC的注解 使用相同的HttpMessageConverters类转换

官方文档说明：

Spring Cloud adds support for Spring MVC annotations and for using the same HttpMessageConverters used by default in Spring Web.

但是我们一般在返回给前端JSON格式的时候 都会把相应的 NULL值转为相应的""，这使得Spring Cloud Feign也使用相同的Jackson配置，例如我们项目的配置

	@Bean
	public ObjectMapper jacksonObjectMapper() {
		ObjectMapper objectMapper = new ObjectMapper();
		// objectMapper.setSerializationInclusion(Include.NON_NULL);
		objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
			@Override
			public void serialize(Object value, JsonGenerator jg, SerializerProvider sp)
				throws IOException, JsonProcessingException {
				jg.writeString("");
				sp.getDefaultNullKeySerializer();
			}
		});
		objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
		return objectMapper;
	}

出现的问题：
在服务通过Feign进行请求的时候，传NULL值引用类型值时，会出现类型转换异常，由于HttpMessageConverters直接把NULL转为了""。

解决思路：
自定义配置 Spring Cloud Feign Encoder与Decoder

官方文档说明：

Spring Cloud Netflix provides the following beans by default for feign (BeanType beanName: ClassName):
Decoder feignDecoder: ResponseEntityDecoder (which wraps a SpringDecoder)
Encoder feignEncoder: SpringEncoder
Logger feignLogger: Slf4jLogger
Contract feignContract: SpringMvcContract
Feign.Builder feignBuilder: HystrixFeign.Builder
Client feignClient: if Ribbon is enabled it is a LoadBalancerFeignClient, otherwise the default feign client is used.

解决方法：
统一配置Feign 的Encoder和Decoder的Jackson转换方式

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.cloud.openfeign.support.ResponseEntityDecoder;
import org.springframework.cloud.openfeign.support.SpringDecoder;
import org.springframework.cloud.openfeign.support.SpringEncoder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import feign.codec.Decoder;
import feign.codec.Encoder;
@Configuration
public class CustomFeignConfig {
    @Bean
    public Decoder feignDecoder() {
        HttpMessageConverter jacksonConverter = new MappingJackson2HttpMessageConverter(customObjectMapper());
        ObjectFactory<HttpMessageConverters> objectFactory = () -> new HttpMessageConverters(jacksonConverter);
        return new ResponseEntityDecoder(new SpringDecoder(objectFactory));
    }
    @Bean
    public Encoder feignEncoder(){
        HttpMessageConverter jacksonConverter = new MappingJackson2HttpMessageConverter(customObjectMapper());
        ObjectFactory<HttpMessageConverters> objectFactory = () -> new HttpMessageConverters(jacksonConverter);
        return new SpringEncoder(objectFactory);
    }

    public ObjectMapper customObjectMapper(){
        ObjectMapper objectMapper = new ObjectMapper();
        //Customize as much as you want
        objectMapper.configure(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT, true);
        return objectMapper;
    }
}

如果配置DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT 不起作用可以试试这种方式

public ObjectMapper customObjectMapper(){
        ObjectMapper objectMapper = new ObjectMapper();
        //Customize as much as you want
		objectMapper.registerModule(new StringSanitizerModule());
        return objectMapper;
}

public class StringSanitizerModule extends SimpleModule {
    public StringSanitizerModule() {
        addDeserializer(String.class, new StdScalarDeserializer<String>(String.class) {
            @Override
            public String deserialize(JsonParser jsonParser, DeserializationContext ctx) throws IOException {
               return StringUtils.trimToNull(jsonParser.getValueAsString());
            }
        });
    }
}