@Configuration
public class WebConfigurer extends WebMvcConfigurerAdapter {

  @Override
  public void addInterceptors(InterceptorRegistry interceptorRegistry) {
    interceptorRegistry.addInterceptor(new SignInterceptor()).addPathPatterns("/v1/**");
  }

  public HttpMessageConverter<String> stringConverter() {
    StringHttpMessageConverter converter = new StringHttpMessageConverter(
        Charset.forName("UTF-8"));
    return converter;
  }

  public HttpMessageConverter fastConverter() {
    //1、定義一個convert轉換訊息的物件
    FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
    //2、新增fastjson的配置資訊
    FastJsonConfig fastJsonConfig = new FastJsonConfig();
    fastJsonConfig.setSerializerFeatures(SerializerFeature.PrettyFormat);
    fastJsonConfig.setCharset(Charset.forName("UTF-8"));
    //2-1 處理中文亂碼問題
    List<MediaType> fastMediaTypes = new ArrayList<>();
    fastMediaTypes.add(MediaType.APPLICATION_JSON_UTF8);
    fastConverter.setSupportedMediaTypes(fastMediaTypes);
    //3、在convert中新增配置資訊
    fastConverter.setFastJsonConfig(fastJsonConfig);
    return fastConverter;
  }

  @Override
  public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
    converters.clear();
    converters.add(stringConverter());
    converters.add(fastConverter());
  }

}

問題原因是FastJsonHttpMessageConverter  前面必須有一個StringHttpMessageConverter，converters裡可以檢視具體的順序