Jackson全面解析--SpringBoot集成Jackson二（HttpMessageConverter）

上一章讲解了sb中如何的对Jackson进行细粒度的配置，那么sb是如何在框架中嵌入Jackson呢，针对@ResponseBody注解的type或是method，sb会使用RequestResponseBodyMethodProcessor类来进行处理。其中的handleReturnValue方法，会将方法返回的对象进行Converter成我们需要的类型，不如json或是xml的类型。

@Override
                                                          //返回的值
    public void handleReturnValue(@Nullable Object returnValue , 
                                                        //返回的类型
                                                        MethodParameter returnType,
                                                        //spring的modeView容器
                                           ModelAndViewContainer 
 //当前请求的request
 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.
        writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    }

核心的方法就是最后writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);

那这些HttpMessageConverter是如何的注册到sb中的呢，肯定也是configure的配置了，在sb中提供了多处的configure来注册HttpMessageConverter。一个是
HttpMessageConvertersAutoConfiguration，在改配置文件中注册了一个
StringHttpMessageConverter的bean，由于import

@Import({ JacksonHttpMessageConvertersConfiguration.class,
        GsonHttpMessageConvertersConfiguration.class,
        JsonbHttpMessageConvertersConfiguration.class })

由于我的classpath是有jackson的，所以跟到JacksonHttpMessageConvertersConfiguration，其中注册了一个MappingJackson2HttpMessageConverter的bean，

而在生成HttpMessageConverters bean的时候会调用WebMvcConfigurationSupport类的addDefaultHttpMessageConverters方法，在这个里面会继续的注册一些默认使用到的消息转换器。
如下

protected final void addDefaultHttpMessageConverters(List> messageConverters) {
        StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316

        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(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);
            }
            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()));
        }
    }

通过这样的方式，sb在启动成功之后，在容器里面就有了这些默认的消息转化器，当然我们可以自定义消息转换器，然后注册进去，也可以实现我们的自定义逻辑