源码分析springboot自定义jackson序列化，默认null值个性化处理返回值

最近项目要实现一种需求，对于后端返回给前端的json格式的一种规范，不允许缺少字段和字段值都为null，所以琢磨了一下如何进行将springboot的Jackson序列化自定义一下，先看看如何实现，再去看源码

第一步：写配置类

@Configuration
public class WebConfiguration  extends WebMvcConfigurationSupport {
@Override
    protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.stream().filter(c -> c instanceof MappingJackson2HttpMessageConverter)
                .map(c ->(MappingJackson2HttpMessageConverter)c)
                .forEach(c->{
                    ObjectMapper mapper = c.getObjectMapper();
                    // 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：当序列化类型为array，list、set时，当值为空时，序列化成[]
                    mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
                    c.setObjectMapper(mapper);
                });
    }
}

第二步：编写值为null时的自定义序列化

public class MyBeanSerializerModifier extends BeanSerializerModifier {

    private MyNullStringJsonSerializer myNullStringJsonSerializer;
    private MyNullArrayJsonSerializer MyNullArrayJsonSerializer;
    private MyNullObjectJsonSerializer MyNullObjectJsonSerializer;
    private MyNullJsonSerializer myNullJsonSerializer;

    public MyBeanSerializerModifier(){
        myNullStringJsonSerializer = new MyNullStringJsonSerializer();
        MyNullArrayJsonSerializer = new MyNullArrayJsonSerializer();
        MyNullObjectJsonSerializer =  new MyNullObjectJsonSerializer();
        myNullJsonSerializer = new MyNullJsonSerializer();
    }

    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc,
                                                     List<BeanPropertyWriter> beanProperties) {
        // 循环所有的beanPropertyWriter
        beanProperties.forEach(writer ->{
            // 判断字段的类型
            if (isArrayType(writer)) {
                //给writer注册一个自己的nullSerializer
                writer.assignNullSerializer(MyNullArrayJsonSerializer);
            } else if (isObjectType(writer)) {
                writer.assignNullSerializer(MyNullObjectJsonSerializer);
            } else if (isStringType(writer)) {
                writer.assignNullSerializer(myNullStringJsonSerializer);
            } else if (isPrimitiveType(writer)) {
                writer.assignNullSerializer(myNullJsonSerializer);
            }
        });
        return beanProperties;
    }

    // 判断是否是boolean类型
    private boolean isPrimitiveType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return clazz.isPrimitive();
    }

    // 判断是否是string类型
    private boolean isStringType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return clazz.equals(String.class);
    }

    // 判断是否是对象类型
    private boolean isObjectType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return !clazz.isPrimitive() && !clazz.equals(String.class)
                && clazz.isAssignableFrom(Object.class);
    }
    // 判断是否是集合类型
    protected boolean isArrayType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return clazz.isArray() || clazz.equals(List.class) || clazz.equals(Set.class);
    }

    class MyNullJsonSerializer extends JsonSerializer<Object>{

        @Override
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
            if (value == null) {
                jgen.writeNull();
            }
        }
    }


    class MyNullStringJsonSerializer extends JsonSerializer<Object>{

        @Override
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
            if (value == null) {
                jgen.writeString(StringUtils.EMPTY);
            }
        }
    }

    class MyNullArrayJsonSerializer extends JsonSerializer<Object>{

        @Override
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
            if (value == null) {
                jgen.writeStartArray();
                jgen.writeEndArray();
            }
        }
    }

    class MyNullObjectJsonSerializer extends JsonSerializer<Object>{

        @Override
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
            if (value == null) {
                jgen.writeStartObject();
                jgen.writeEndObject();
            }
        }
    }

}

这样基本配置就完事了，现在可以试试效果了，自己定义一个bean用来返回，定义一个简单的controller去接受访问就行了，博主就不进行写这两个类了。返回结果如下

这是我的项目需求需要实现的，大家可以根据的自己的需求去改写MyBeanSerializerModifier这个类。还有另一种实现方式：不继承

@Configuration
public class WebConfiguration {

@Bean
    public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter(){
        MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper mapper = mappingJackson2HttpMessageConverter.getObjectMapper();
        mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
        mappingJackson2HttpMessageConverter.setObjectMapper(mapper);
        return mappingJackson2HttpMessageConverter;
    }

}

这种方法也是可以设置成功的，主要是不是继承了WebMvcConfigurationSupport类，毕竟这个类有很多可以自定义的方法，用起来顺手而已。

第一个问题：为什么继承WebMvcConfigurationSupport后，要重写extendMessageConverters方法；

第二个问题：为什么继承WebMvcConfigurationSupport后，再去生成@Bean的MappingJackson2HttpMessageConverter，却不生效；

第三个问题：为什么不继承WebMvcConfigurationSupport时，生成@Bean的MappingJackson2HttpMessageConverter是生效的；

这几个问题，都需要我们进入源码观察，废活不多说，我们来进入源码的世界。解决问题之前必须搞清楚在哪里进行了序列化。

第一步：我们要弄清楚在哪里进行的Jackson序列化，看这里https://www.processon.com/embed/5f5c6464f346fb7afd55448b，从返回请求开始的序列化基本流程就在这里了，虽然图有点low，但是清楚的记录的每一步，我们主要看一下下面的源码

/*
    /**********************************************************
    /* Field serialization methods
    /**********************************************************
     */
    //序列化每一个字段
    protected void serializeFields(Object bean, JsonGenerator gen, SerializerProvider provider)
            throws IOException
    {
        final BeanPropertyWriter[] props;
        if (_filteredProps != null && provider.getActiveView() != null) {
            props = _filteredProps;
        } else {
            props = _props;
        }
        int i = 0;
        try {
            for (final int len = props.length; i < len; ++i) {
                BeanPropertyWriter prop = props[i];
                if (prop != null) { // can have nulls in filtered list
                    //关键就在这一步进行的序列化，而为什么BeanPropertyWriter是数组，我们一会解释
                    prop.serializeAsField(bean, gen, provider);
                }
            }
            if (_anyGetterWriter != null) {
                _anyGetterWriter.getAndSerialize(bean, gen, provider);
            }
        } catch (Exception e) {
            String name = (i == props.length) ? "[anySetter]" : props[i].getName();
            wrapAndThrow(provider, e, bean, name);
        } catch (StackOverflowError e) {
            // 04-Sep-2009, tatu: Dealing with this is tricky, since we don't have many
            //   stack frames to spare... just one or two; can't make many calls.

            // 10-Dec-2015, tatu: and due to above, avoid "from" method, call ctor directly:
            //JsonMappingException mapE = JsonMappingException.from(gen, "Infinite recursion (StackOverflowError)", e);
            JsonMappingException mapE = new JsonMappingException(gen, "Infinite recursion (StackOverflowError)", e);

            String name = (i == props.length) ? "[anySetter]" : props[i].getName();
            mapE.prependPath(new JsonMappingException.Reference(bean, name));
            throw mapE;
        }
    }

既然已经找到了在哪里要进行序列化，那我们看看是如何实现的：

/**
     * Method called to access property that this bean stands for, from within
     * given bean, and to serialize it as a JSON Object field using appropriate
     * serializer.
     */
    @Override
    public void serializeAsField(Object bean, JsonGenerator gen,
                                 SerializerProvider prov) throws Exception {
        // inlined 'get()'
        final Object value = (_accessorMethod == null) ? _field.get(bean)
                : _accessorMethod.invoke(bean, (Object[]) null);

        // Null handling is bit different, check that first
        if (value == null) {
            //看到这里大家应该就知道null值是如何进行序列化 的了，如果不配置的话，默认是返回null
            //因为_nullSerializer是有默认值的，大家看一看这个类的初始化
            //那我们要是改一下_nullSerializer的这个默认类，让每一个字段调用我们自己的_nullSerializer不就可以了吗，
            //yes、我们就这么干
            if (_nullSerializer != null) {
                gen.writeFieldName(_name);
                _nullSerializer.serialize(null, gen, prov);
            }
            return;
        }
        // then find serializer to use
        JsonSerializer<Object> ser = _serializer;
        if (ser == null) {
            Class<?> cls = value.getClass();
            PropertySerializerMap m = _dynamicSerializers;
            ser = m.serializerFor(cls);
            if (ser == null) {
                ser = _findAndAddDynamic(m, cls, prov);
            }
        }
        // and then see if we must suppress certain values (default, empty)
        if (_suppressableValue != null) {
            if (MARKER_FOR_EMPTY == _suppressableValue) {
                if (ser.isEmpty(prov, value)) {
                    return;
                }
            } else if (_suppressableValue.equals(value)) {
                return;
            }
        }
        // For non-nulls: simple check for direct cycles
        if (value == bean) {
            // three choices: exception; handled by call; or pass-through
            if (_handleSelfReference(bean, gen, prov, ser)) {
                return;
            }
        }
        gen.writeFieldName(_name);
        if (_typeSerializer == null) {
            ser.serialize(value, gen, prov);
        } else {
            ser.serializeWithType(value, gen, prov, _typeSerializer);
        }
    }

那我们来解决第一个问题：为什么继承WebMvcConfigurationSupport后，要重写extendMessageConverters方法？

不知道大家记得不记得我们请求过来的时候，如果我们配置类集成了WebMvcConfigurationSupport类，dispatchservlet处理handle请求的ha，其实就是RequestMappingHandlerAdapter类，这个类是在WebMvcConfigurationSupport配置的，看源码：

@Bean
    public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
            @Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
            @Qualifier("mvcConversionService") FormattingConversionService conversionService,
            @Qualifier("mvcValidator") Validator validator) {

        RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
        adapter.setContentNegotiationManager(contentNegotiationManager);
        adapter.setMessageConverters(getMessageConverters());
        adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer(conversionService, validator));
        adapter.setCustomArgumentResolvers(getArgumentResolvers());
        adapter.setCustomReturnValueHandlers(getReturnValueHandlers());

        if (jackson2Present) {
            adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
            adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
        }

        AsyncSupportConfigurer configurer = new AsyncSupportConfigurer();
        configureAsyncSupport(configurer);
        if (configurer.getTaskExecutor() != null) {
            adapter.setTaskExecutor(configurer.getTaskExecutor());
        }
        if (configurer.getTimeout() != null) {
            adapter.setAsyncRequestTimeout(configurer.getTimeout());
        }
        adapter.setCallableInterceptors(configurer.getCallableInterceptors());
        adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());

        return adapter;
    }

adapter.setMessageConverters(getMessageConverters());当大家看到这个方法的时候，应该就会想到我们的默认jackson转换器：MappingJackson2HttpMessageConverter,我们看看这个getMessageConverters()有什么幺蛾子：

protected final List<HttpMessageConverter<?>> getMessageConverters() {
        if (this.messageConverters == null) {
            this.messageConverters = new ArrayList<>();
            configureMessageConverters(this.messageConverters);
            if (this.messageConverters.isEmpty()) {
                addDefaultHttpMessageConverters(this.messageConverters);
            }
            extendMessageConverters(this.messageConverters);
        }
        return this.messageConverters;
    }

 protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
  
              //这些都不用管，跟我们的需求没啥作用，我们只看关键的部分，在下面
              messageConverters.add(new ByteArrayHttpMessageConverter());
              messageConverters.add(new StringHttpMessageConverter());
              messageConverters.add(new ResourceHttpMessageConverter());
              messageConverters.add(new ResourceRegionHttpMessageConverter());
              try {
                  messageConverters.add(new SourceHttpMessageConverter<>());
             }
             catch (Throwable ex) {
                 // Ignore when no TransformerFactory implementation is available...
             }
             messageConverters.add(new AllEncompassingFormHttpMessageConverter());
 
             if (romePresent) {
                 messageConverters.add(new AtomFeedHttpMessageConverter());
                 messageConverters.add(new RssChannelHttpMessageConverter());
             }
 
             if (jackson2XmlPresent) {
                 Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
                 if (this.applicationContext != null) {
                     builder.applicationContext(this.applicationContext);
                 }
                 messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
             }
             else if (jaxb2Present) {
                 messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
             }
 
if (jackson2Present) {                 Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
                 if (this.applicationContext != null) {
                     builder.applicationContext(this.applicationContext);
                 }
                 //解析我们返回值的转换器就是在这里生成的
                 messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
             }
             else if (gsonPresent) {
                 messageConverters.add(new GsonHttpMessageConverter());
             }
             else if (jsonbPresent) {
                 messageConverters.add(new JsonbHttpMessageConverter());
             }
 
             if (jackson2SmilePresent) {
                 Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
                 if (this.applicationContext != null) {
                     builder.applicationContext(this.applicationContext);
                 }
                 messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
             }
             if (jackson2CborPresent) {
                 Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
                 if (this.applicationContext != null) {
                     builder.applicationContext(this.applicationContext);
                 }
                 messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
             }
        }

我们的MappingJackson2HttpMessageConverter类就是这里初始化的，初始化的时候默认的_nullSerializer也会被初始化，大家肯定说这已经初始化完了，该咋办，大家应该看到了extendMessageConverters(this.messageConverters);这个方法就是用来重写实现的了，这回知道我们继承WebMvcConfigurationSupport后，为什么要重写extendMessageConverters，我们的配置类遍历已经获取到的convert，然后对我们想要的转换器进行修改添加，那修改完了，是在哪里起作用的呢，我们再来看一看源码：

在序列化之前有一些方法是可以进行修改操作的，在调用writeWithMessageConverters方法的时候：

 protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
                                                        ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
                  throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
  
              .......
  
              MediaType selectedMediaType = null;
              MediaType contentType = outputMessage.getHeaders().getContentType();
             boolean isContentTypePreset = contentType != null && contentType.isConcrete();
             if (isContentTypePreset) {
                 if (logger.isDebugEnabled()) {
                     logger.debug("Found 'Content-Type:" + contentType + "' in response");
                 }
                 selectedMediaType = contentType;
             }
             else {
                 HttpServletRequest request = inputMessage.getServletRequest();
                 List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
                 //这里进行自定义操作修改MappingJackson2HttpMessageConverter
                 List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
 
                 .......
 
             if (selectedMediaType != null) {
                 selectedMediaType = selectedMediaType.removeQualityValue();
                 //这这里进行选择我们的MappingJackson2HttpMessageConverter去自定义序列化
                 for (HttpMessageConverter<?> converter : this.messageConverters) {
                     GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
                             (GenericHttpMessageConverter<?>) converter : null);
                     if (genericConverter != null ?
                             ((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
                             converter.canWrite(valueType, selectedMediaType)) {
                         body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
                                 (Class<? extends HttpMessageConverter<?>>) converter.getClass(),
                                 inputMessage, outputMessage);
                         if (body != null) {
                             Object theBody = body;
                             LogFormatUtils.traceDebug(logger, traceOn ->
                                     "Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
                             addContentDispositionHeader(inputMessage, outputMessage);
                             if (genericConverter != null) {
                                 genericConverter.write(body, targetType, selectedMediaType, outputMessage);
                             }
                             else {
                                 ((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
                             }
                         }
                         else {
                             if (logger.isDebugEnabled()) {
                                 logger.debug("Nothing to write: null body");
                             }
                         }
                         return;
                     }
                 }
             }
 
             if (body != null) {
                 Set<MediaType> producibleMediaTypes =
                         (Set<MediaType>) inputMessage.getServletRequest()
                                 .getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
 
                 if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {
                     throw new HttpMessageNotWritableException(
                             "No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
                 }
                 throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
             }
         }

我们一直追踪getProducibleMediaTypes后，最终发现会调用BeanSerializerFactory的constructBeanOrAddOnSerializer，就是这里进行修改操作的。

protected JsonSerializer<Object> constructBeanOrAddOnSerializer(SerializerProvider prov,
            JavaType type, BeanDescription beanDesc, boolean staticTyping)
        throws JsonMappingException
    {
        // 13-Oct-2010, tatu: quick sanity check: never try to create bean serializer for plain Object
        // 05-Jul-2012, tatu: ... but we should be able to just return "unknown type" serializer, right?
        if (beanDesc.getBeanClass() == Object.class) {
            return prov.getUnknownTypeSerializer(Object.class);
//            throw new IllegalArgumentException("Cannot create bean serializer for Object.class");
        }
        final SerializationConfig config = prov.getConfig();
        BeanSerializerBuilder builder = constructBeanSerializerBuilder(beanDesc);
        builder.setConfig(config);

        // First: any detectable (auto-detect, annotations) properties to serialize?
        List<BeanPropertyWriter> props = findBeanProperties(prov, beanDesc, builder);
        if (props == null) {
            props = new ArrayList<BeanPropertyWriter>();
        } else {
            props = removeOverlappingTypeIds(prov, beanDesc, builder, props);
        }

        // [databind#638]: Allow injection of "virtual" properties:
        prov.getAnnotationIntrospector().findAndAddVirtualProperties(config, beanDesc.getClassInfo(), props);

        // [JACKSON-440] Need to allow modification bean properties to serialize:
        if (_factoryConfig.hasSerializerModifiers()) {
            for (BeanSerializerModifier mod : _factoryConfig.serializerModifiers()) {
                props = mod.changeProperties(config, beanDesc, props);
            }
        }
.......//此处省略
}

大家看一下props = mod.changeProperties(config, beanDesc, props);我们在配置类里面可是为我们的MappingJackson2HttpMessageConverter配置了 withSerializerModifier方法，并且设置了我们的MyBeanSerializerModifier并且继承BeanSerializerModifier重写了 changeProperties，所以会调用我们的changeProperties方法，进行修改null值的序列化类，我们也返回了一个list类型的BeanPropertyWriter，所以知道为什么那个BeanPropertyWriter在解析时，会是个数组类型的了吧，因为不同字段解析是不一样的。

剩下的解释一下为什么单独配置并设置实例化@bean的MappingJackson2HttpMessageConverter也是好使的呢，大家可以看一下JacksonHttpMessageConvertersConfiguration类的源码，里面明确写了@ConditionalOnMissingBean注解，如果我们自己进行配置了，这个注入后就是一个备胎，以我们的为准，这个不多说

我们再来解析一下第二个问题：为什么继承WebMvcConfigurationSupport后，再去生成@Bean的MappingJackson2HttpMessageConverter，却不生效，这需要跟第三个问题一起解决：为什么不继承WebMvcConfigurationSupport时，生成@Bean的MappingJackson2HttpMessageConverter是生效的。

我们知道当我们继承WebMvcConfigurationSupport后，有一个配置会自动失效，就是自动注入的一个mvc配置，可以看看@SpringBootApplication注解里面有个@EnableAutoConfiguration注解，会引入一个AutoConfigurationImportSelector类。这个类就会扫描org.springframework.boot:spring-boot-autoconfigure下的spring.factories文件，这里面有一个我们默认的mvn配置也是继承了WebMvcConfigurationSupport，叫WebMvcAutoConfiguration，我们来看一下源码：

@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
//注意此处有一个ConditionalOnMissingBean注解，所以如果我们自己继承后，就相当于已经存在WebMvcConfigurationSupport类，
//就会走我们自己的配置类，此配置会失效
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
        ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
    .....

    @Configuration(proxyBeanMethods = false)
    public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {

        private final ResourceProperties resourceProperties;

        private final WebMvcProperties mvcProperties;

        private final ListableBeanFactory beanFactory;

        private final WebMvcRegistrations mvcRegistrations;

        private ResourceLoader resourceLoader;

        public EnableWebMvcConfiguration(ResourceProperties resourceProperties,
                ObjectProvider<WebMvcProperties> mvcPropertiesProvider,
                ObjectProvider<WebMvcRegistrations> mvcRegistrationsProvider, ListableBeanFactory beanFactory) {
            this.resourceProperties = resourceProperties;
            this.mvcProperties = mvcPropertiesProvider.getIfAvailable();
            this.mvcRegistrations = mvcRegistrationsProvider.getIfUnique();
            this.beanFactory = beanFactory;
        }
        //如果我们不继承的话，处理请求的RequestMappingHandlerAdapter就会在这里生成
        //会调用DelegatingWebMvcConfiguration里面的 requestMappingHandlerAdapter方法，
        @Bean
        @Override
        public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
                @Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
                @Qualifier("mvcConversionService") FormattingConversionService conversionService,
                @Qualifier("mvcValidator") Validator validator) {
            RequestMappingHandlerAdapter adapter = super.requestMappingHandlerAdapter(contentNegotiationManager,
                    conversionService, validator);
            adapter.setIgnoreDefaultModelOnRedirect(
                    this.mvcProperties == null || this.mvcProperties.isIgnoreDefaultModelOnRedirect());
            return adapter;
        }

.....
}

不知道大家是否还记得getMessageConverters()方法里面添加默认messageConverters的时候之前，会调用一个configureMessageConverters(this.messageConverters);方法，我们的DelegatingWebMvcConfiguration 就已经重写了这个方法，所以我们如果不继承WebMvcConfigurationSupport，将会把我们的@bean形式存在的MappingJackson2HttpMessageConverter将会被扫描到

@Override
    protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        this.configurers.configureMessageConverters(converters);
    }

//会添加我们的convert
@Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        for (WebMvcConfigurer delegate : this.delegates) {
            delegate.configureMessageConverters(converters);
        }
    }

现在我们配置的自定义jackson序列化已经生效了，但是，你仔细看我的流程图会发现，其实调用序列化的时候走的是RequestResponseBodyMethodProcessor的handleReturnValue方法

@Override
    public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

        mavContainer.setRequestHandled(true);
        ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
        ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

        // Try even with null return value. ResponseBodyAdvice could get involved.
        //这里进入序列化流程
        writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    }

最后在序列化的时候也是从这个类或则父类里面的一个属性：messageConverters

public abstract class AbstractMessageConverterMethodArgumentResolver implements HandlerMethodArgumentResolver {

    private static final Set<HttpMethod> SUPPORTED_METHODS =
            EnumSet.of(HttpMethod.POST, HttpMethod.PUT, HttpMethod.PATCH);

    private static final Object NO_VALUE = new Object();


    protected final Log logger = LogFactory.getLog(getClass());
    //这个属性取值的
    protected final List<HttpMessageConverter<?>> messageConverters;

    protected final List<MediaType> allSupportedMediaTypes;

    private final RequestResponseBodyAdviceChain advice;

...
}

于是，小伙伴们就疑惑了，这我们自定义的在RequestMappingHandlerAdapter里面呢，跟这个类也没关系啊，属性是咋设置进来的呢？我们再看看RequestMappingHandlerAdapter的源码，你会发现，RequestMappingHandlerAdapter这个类实现了InitializingBean类，也就说明，创建RequestMappingHandlerAdapter的时候会调用afterPropertiesSet方法，至于为啥，看源码吧：（不是主要流程）

 //在createBean的时候会调用这个方法，看看是否实现了InitializingBean
  protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
              throws Throwable {
  
          boolean isInitializingBean = (bean instanceof InitializingBean);
          if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
              if (logger.isTraceEnabled()) {
                  logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
              }
             if (System.getSecurityManager() != null) {
                 try {
                     AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                         ((InitializingBean) bean).afterPropertiesSet();
                         return null;
                     }, getAccessControlContext());
                 }
                 catch (PrivilegedActionException pae) {
                     throw pae.getException();
                 }
             }
             else {
                 //在这里进行调用的，
                 ((InitializingBean) bean).afterPropertiesSet();
             }
         }
 
         if (mbd != null && bean.getClass() != NullBean.class) {
             String initMethodName = mbd.getInitMethodName();
             if (StringUtils.hasLength(initMethodName) &&
                     !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                     !mbd.isExternallyManagedInitMethod(initMethodName)) {
                 invokeCustomInitMethod(beanName, bean, mbd);
             }
         }
     }

那我们看看RequestMappingHandlerAdapter的afterPropertiesSet方法都干了些啥吧。

@Override
    public void afterPropertiesSet() {
        // Do this first, it may add ResponseBody advice beans
        initControllerAdviceCache();

        if (this.argumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
            this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.initBinderArgumentResolvers == null) {
            List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
            this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
        }
        if (this.returnValueHandlers == null) {
            //是在这里生成的类
            List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
            this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
        }
    }

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
        List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>();

        // Single-purpose return value types
        handlers.add(new ModelAndViewMethodReturnValueHandler());
        handlers.add(new ModelMethodProcessor());
        handlers.add(new ViewMethodReturnValueHandler());
        handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),
                this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));
        handlers.add(new StreamingResponseBodyReturnValueHandler());
        handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
                this.contentNegotiationManager, this.requestResponseBodyAdvice));
        handlers.add(new HttpHeadersReturnValueHandler());
        handlers.add(new CallableMethodReturnValueHandler());
        handlers.add(new DeferredResultMethodReturnValueHandler());
        handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));

        // Annotation-based return value types
        handlers.add(new ModelAttributeMethodProcessor(false));
        //看到这个类了吗？生成的时候将RequestMappingHandlerAdapter里面的转换器设置进去了
        handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
                this.contentNegotiationManager, this.requestResponseBodyAdvice));

        // Multi-purpose return value types
        handlers.add(new ViewNameMethodReturnValueHandler());
        handlers.add(new MapMethodProcessor());

        // Custom return value types
        if (getCustomReturnValueHandlers() != null) {
            handlers.addAll(getCustomReturnValueHandlers());
        }

        // Catch-all
        if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
            handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
        }
        else {
            handlers.add(new ModelAttributeMethodProcessor(true));
        }

        return handlers;
    }

讲到这里，不知道大家理解了多少，这些都是博主遇到需求后，自己问自己的问题，自己通过源码回答问题的，也希望大家能理解源码。还有一篇源码文章在路上：为什么我们的项目里出现两个配置类继承WebMvcConfigurationSupport时，只有一个会生效。我在网上找了半天都是说结果的，没有人分析源码到底是为啥，博主准备讲解一下，希望可以帮到大家！