spring-boot2.x，使用EnableWebMvc注解导致的自定义HttpMessageConverters不可用

在json对象转换方面，springboot默认使用的是MappingJackson2HttpMessageConverter。常规需求，在工程中使用阿里的FastJson作为json对象的转换器。

FastJson SerializerFeatures

WriteNullListAsEmpty  ：List字段如果为null,输出为[],而非null
WriteNullStringAsEmpty ： 字符类型字段如果为null,输出为"",而非null
DisableCircularReferenceDetect ：消除对同一对象循环引用的问题，默认为false（如果不配置有可能会进入死循环）
WriteNullBooleanAsFalse：Boolean字段如果为null,输出为false,而非null
WriteMapNullValue：是否输出值为null的字段,默认为false。

public enum SerializerFeature {
    QuoteFieldNames,
    UseSingleQuotes,
    WriteMapNullValue,
    WriteEnumUsingToString,
    WriteEnumUsingName,
    UseISO8601DateFormat,
    WriteNullListAsEmpty,
    WriteNullStringAsEmpty,
    WriteNullNumberAsZero,
    WriteNullBooleanAsFalse,
    SkipTransientField,
    SortField,
    /** @deprecated */
    @Deprecated
    WriteTabAsSpecial,
    PrettyFormat,
    WriteClassName,
    DisableCircularReferenceDetect,
    WriteSlashAsSpecial,
    BrowserCompatible,
    WriteDateUseDateFormat,
    NotWriteRootClassName,
    /** @deprecated */
    DisableCheckSpecialChar,
    BeanToArray,
    WriteNonStringKeyAsString,
    NotWriteDefaultValue,
    BrowserSecure,
    IgnoreNonFieldGetter,
    WriteNonStringValueAsString,
    IgnoreErrorGetter,
    WriteBigDecimalAsPlain,
    MapSortField;
}

使用FastJson，有两种常规操作。

一、注入bean的方式，这种方法加入的转换器排序是第一位

package com.gaoshan.verification.config;
 
 
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
import java.util.List;
 
@Configuration
public class WebMvcConfigurerConfig implements WebMvcConfigurer {
 
    @Override
    public void extendMessageConverters(List> converters) {
        for (HttpMessageConverter messageConverter : converters) {
            System.out.println("======================"+messageConverter);
        }
    }
}

package com.gaoshan.verification.config;

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;

import java.util.ArrayList;
import java.util.List;

@Configuration
public class MessageConvertConfig {

    @Bean
    public HttpMessageConverters fastJsonHttpMessageConverters() {
        FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                SerializerFeature.PrettyFormat,
                SerializerFeature.DisableCircularReferenceDetect,
                SerializerFeature.WriteBigDecimalAsPlain,
                SerializerFeature.WriteMapNullValue);
        fastConverter.setFastJsonConfig(fastJsonConfig);
        List supportedMediaTypes = new ArrayList<>();
        supportedMediaTypes.add(MediaType.APPLICATION_JSON);
        fastConverter.setSupportedMediaTypes(supportedMediaTypes);
        return new HttpMessageConverters(fastConverter);
    }
}

======================com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter@2c5708e7
======================org.springframework.http.converter.ByteArrayHttpMessageConverter@4ffa078d
======================org.springframework.http.converter.StringHttpMessageConverter@4e26564d
======================org.springframework.http.converter.ResourceHttpMessageConverter@42238078
======================org.springframework.http.converter.ResourceRegionHttpMessageConverter@5627b8eb
======================org.springframework.http.converter.xml.SourceHttpMessageConverter@49fe0bcd
======================org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter@3516b881
======================org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@6be8ce1b
======================org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter@e3c36d

二、实现WebMvcConfigurer接口，这种方法加入的转换器排序是最后一位

package com.gaoshan.verification.config;

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.ArrayList;
import java.util.List;

@Configuration
public class WebMvcConfigurerConfig implements WebMvcConfigurer {

    @Override
    public void extendMessageConverters(List> converters) {
        for (HttpMessageConverter messageConverter : converters) {
            System.out.println("======================"+messageConverter);
        }
    }

    @Override
    public void configureMessageConverters(List> converters) {
        FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                SerializerFeature.PrettyFormat,
                SerializerFeature.DisableCircularReferenceDetect,
                SerializerFeature.WriteBigDecimalAsPlain,
                SerializerFeature.WriteMapNullValue);
        fastConverter.setFastJsonConfig(fastJsonConfig);
        List supportedMediaTypes = new ArrayList<>();
        supportedMediaTypes.add(MediaType.APPLICATION_JSON);
        fastConverter.setSupportedMediaTypes(supportedMediaTypes);
        converters.add(fastConverter);
    }
}

 ======================org.springframework.http.converter.ByteArrayHttpMessageConverter@71f29d91
======================org.springframework.http.converter.StringHttpMessageConverter@6785df10
======================org.springframework.http.converter.StringHttpMessageConverter@6143b2b1
======================org.springframework.http.converter.ResourceHttpMessageConverter@a63643e
======================org.springframework.http.converter.ResourceRegionHttpMessageConverter@43294e9b
======================org.springframework.http.converter.xml.SourceHttpMessageConverter@26d24d7a
======================org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter@5a78b52b
======================org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@144440f5
======================org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@4bab78ce
======================org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter@42ffbab6
======================com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter@7672960e

注意：

1、可以两种方式同时使用，这样可以达到目的，在转换器列表的头尾，都会出现FastJsonHttpMessageConverter

======================com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter@2c5708e7
======================org.springframework.http.converter.ByteArrayHttpMessageConverter@4ffa078d
======================org.springframework.http.converter.StringHttpMessageConverter@4e26564d
======================org.springframework.http.converter.ResourceHttpMessageConverter@42238078
======================org.springframework.http.converter.ResourceRegionHttpMessageConverter@5627b8eb
======================org.springframework.http.converter.xml.SourceHttpMessageConverter@49fe0bcd
======================org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter@3516b881
======================org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@6be8ce1b
======================org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter@e3c36d
======================com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter@397a10df
 

2、不要乱加 @EnableWebMvc标签，这个标签会导致添加自定义消息转换器失败。因为时间问题，目前还不清楚具体原因

• 针对方案一，启动类或任意配置类，加了@EnableWebMvc后，导致自定义的转换器没有出现在集合内，即添加自定义转换器失败

package com.gaoshan.verification.config;


import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.List;

@Configuration
@EnableWebMvc
public class WebMvcConfigurerConfig implements WebMvcConfigurer {

    @Override
    public void extendMessageConverters(List> converters) {
        for (HttpMessageConverter messageConverter : converters) {
            System.out.println("======================"+messageConverter);
        }
    }
}

package com.gaoshan.verification.config;

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;

import java.util.ArrayList;
import java.util.List;

@Configuration
public class MessageConvertConfig {

    @Bean
    public HttpMessageConverters fastJsonHttpMessageConverters() {
        FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                SerializerFeature.PrettyFormat,
                SerializerFeature.DisableCircularReferenceDetect,
                SerializerFeature.WriteBigDecimalAsPlain,
                SerializerFeature.WriteMapNullValue);
        fastConverter.setFastJsonConfig(fastJsonConfig);
        List supportedMediaTypes = new ArrayList<>();
        supportedMediaTypes.add(MediaType.APPLICATION_JSON);
        fastConverter.setSupportedMediaTypes(supportedMediaTypes);
        return new HttpMessageConverters(fastConverter);
    }
}

 ======================org.springframework.http.converter.ByteArrayHttpMessageConverter@42238078
======================org.springframework.http.converter.StringHttpMessageConverter@5627b8eb
======================org.springframework.http.converter.ResourceHttpMessageConverter@49fe0bcd
======================org.springframework.http.converter.ResourceRegionHttpMessageConverter@3516b881
======================org.springframework.http.converter.xml.SourceHttpMessageConverter@6be8ce1b
======================org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter@e3c36d
======================org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter@397a10df
======================org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@39a865c1

• 针对方案二，启动类或任意配置类，加了@EnableWebMvc后，导致集合内仅有自定义转换器

package com.gaoshan.verification.config;

import com.alibaba.fastjson.serializer.SerializerFeature;
import com.alibaba.fastjson.support.config.FastJsonConfig;
import com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.ArrayList;
import java.util.List;

@Configuration
@EnableWebMvc
public class WebMvcConfigurerConfig implements WebMvcConfigurer {

    @Override
    public void extendMessageConverters(List> converters) {
        for (HttpMessageConverter messageConverter : converters) {
            System.out.println("======================"+messageConverter);
        }
    }

    @Override
    public void configureMessageConverters(List> converters) {
        FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                SerializerFeature.PrettyFormat,
                SerializerFeature.DisableCircularReferenceDetect,
                SerializerFeature.WriteBigDecimalAsPlain,
                SerializerFeature.WriteMapNullValue);
        fastConverter.setFastJsonConfig(fastJsonConfig);
        List supportedMediaTypes = new ArrayList<>();
        supportedMediaTypes.add(MediaType.APPLICATION_JSON);
        fastConverter.setSupportedMediaTypes(supportedMediaTypes);
        converters.add(fastConverter);
    }
}

======================com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter@1df06ecd

• 启动类代码

  package com;
  
  
  import org.springframework.boot.SpringApplication;
  import org.springframework.boot.autoconfigure.SpringBootApplication;
  
  @SpringBootApplication
  public class VerificationApplication {
  
      public static void main(String[] args) {
          SpringApplication.run(VerificationApplication.class, args);
      }
  
  }