@RestController
public class LocalDateTimeController {
    @GetMapping("/resolveRequestParamDateTime")
    public void resolveRequestParamDateTime(@RequestParam LocalDateTime dateTime) {
        System.out.println("ok");
    }
?
    @GetMapping("/resolvePathVariableDateTime/{dateTime}")
    public void resolvePathVariableDateTime(@PathVariable LocalDateTime dateTime) {
        System.out.println("ok");
    }
}

@RestController
public class LocalDateTimeController {
?
    @PostMapping("/resolveBodyDateTime")
    public void resolveBodyDateTime(@RequestBody ResolveBody body) {
        System.out.println("ok");
    }
?
    @Data
    private static class ResolveBody {
        private LocalDateTime dateTime;
    }
}

GenericConversionService.java類源碼：
@Nullable
public Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType) {
    Assert.notNull(targetType, "Target type to convert to cannot be null");
    if (sourceType == null) {
        Assert.isTrue(source == null, "Source must be [null] if source type == [null]");
        return this.handleResult((TypeDescriptor)null, targetType, this.convertNullSource((TypeDescriptor)null, targetType));
    } else if (source != null && !sourceType.getObjectType().isInstance(source)) {
        throw new IllegalArgumentException("Source to convert from must be an instance of [" + sourceType + "]; instead it was a [" + source.getClass().getName() + "]");
    } else {
        GenericConverter converter = this.getConverter(sourceType, targetType);
        if (converter != null) {
            # 這里去做類型轉(zhuǎn)換
            Object result = ConversionUtils.invokeConverter(converter, source, sourceType, targetType);
            return this.handleResult(sourceType, targetType, result);
        } else {
            return this.handleConverterNotFound(source, sourceType, targetType);
        }
    }
}

AbstractMessageConverterMethodArgumentResolver類源碼：
for (HttpMessageConverter<?> converter : this.messageConverters) {
    Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
    GenericHttpMessageConverter<?> genericConverter =
    (converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
    if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
        (targetClass != null && converter.canRead(targetClass, contentType))) {
        if (message.hasBody()) {
            HttpInputMessage msgToUse =
            getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
            # 這里去做類型轉(zhuǎn)換
            body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
                    ((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
            body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
        }
        else {
            body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
        }
        break;
    }
}

AbstractJackson2HttpMessageConverter類源碼：
@Override
  public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
      throws IOException, HttpMessageNotReadableException {
?
    JavaType javaType = getJavaType(type, contextClass);
    return readJavaType(javaType, inputMessage);
  }
?
  private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
    try {
      if (inputMessage instanceof MappingJacksonInputMessage) {
        Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
        if (deserializationView != null) {
          return this.objectMapper.readerWithView(deserializationView).forType(javaType).
              readValue(inputMessage.getBody());
        }
      }
            # jackson實(shí)際解析
      return this.objectMapper.readValue(inputMessage.getBody(), javaType);
    }
    catch (InvalidDefinitionException ex) {
      throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
    }
    catch (JsonProcessingException ex) {
      throw new HttpMessageNotReadableException("JSON parse error: " + ex.getOriginalMessage(), ex, inputMessage);
    }
  }

WebMvcAutoConfiguration#EnableWebMvcConfiguration類源碼
// springboot自動(dòng)裝配WebConversionService對(duì)象
@Bean
public FormattingConversionService mvcConversionService() {
    WebMvcProperties.Format format = this.mvcProperties.getFormat();
    WebConversionService conversionService = new WebConversionService((new DateTimeFormatters()).dateFormat(format.getDate()).timeFormat(format.getTime()).dateTimeFormat(format.getDateTime()));
    // 這里允許用戶自定往容器中添加Converter
    this.addFormatters(conversionService);
    return conversionService;
}
WebConversionService類源碼：
private void addFormatters(DateTimeFormatters dateTimeFormatters) {
    // 這是默認(rèn)情況下springboot自動(dòng)注入的日期格式解析器
    this.registerJsr310(dateTimeFormatters);
    this.registerJavaDate(dateTimeFormatters);
}

DateTimeFormatterRegistrar類源碼：
    public void registerFormatters(FormatterRegistry registry) {
        // 這里可以拿到application.yml配置文件指定的格式
        DateTimeFormatter df = this.getFormatter(DateTimeFormatterRegistrar.Type.DATE);
        DateTimeFormatter tf = this.getFormatter(DateTimeFormatterRegistrar.Type.TIME);
        DateTimeFormatter dtf = this.getFormatter(DateTimeFormatterRegistrar.Type.DATE_TIME);
        registry.addFormatterForFieldType(LocalDate.class, new TemporalAccessorPrinter(df == DateTimeFormatter.ISO_DATE ? DateTimeFormatter.ISO_LOCAL_DATE : df), new TemporalAccessorParser(LocalDate.class, df));
        registry.addFormatterForFieldType(LocalTime.class, new TemporalAccessorPrinter(tf == DateTimeFormatter.ISO_TIME ? DateTimeFormatter.ISO_LOCAL_TIME : tf), new TemporalAccessorParser(LocalTime.class, tf));
        registry.addFormatterForFieldType(LocalDateTime.class, new TemporalAccessorPrinter(dtf == DateTimeFormatter.ISO_DATE_TIME ? DateTimeFormatter.ISO_LOCAL_DATE_TIME : dtf), new TemporalAccessorParser(LocalDateTime.class, dtf));
        // 使用@DateTimeFormat注解來幫忙完成LocalDateTime類型的解析
        registry.addFormatterForFieldAnnotation(new Jsr310DateTimeFormatAnnotationFormatterFactory());
    }

// WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter#addFormatters()方法
public void addFormatters(FormatterRegistry registry) {
    ApplicationConversionService.addBeans(registry, this.beanFactory);
}
// ApplicationConversionService#addBeans()方法
public static void addBeans(FormatterRegistry registry, ListableBeanFactory beanFactory) {
    Set<Object> beans = new LinkedHashSet();
    beans.addAll(beanFactory.getBeansOfType(GenericConverter.class).values());
    beans.addAll(beanFactory.getBeansOfType(Converter.class).values());
    beans.addAll(beanFactory.getBeansOfType(Printer.class).values());
    beans.addAll(beanFactory.getBeansOfType(Parser.class).values());
    Iterator var3 = beans.iterator();
    // spirngboot會(huì)掃描ioc容器中以下所有類型的bean，并添加到WebConversionService中
    while(var3.hasNext()) {
        Object bean = var3.next();
        if (bean instanceof GenericConverter) {
            registry.addConverter((GenericConverter)bean);
        } else if (bean instanceof Converter) {
            registry.addConverter((Converter)bean);
        } else if (bean instanceof Formatter) {
            registry.addFormatter((Formatter)bean);
        } else if (bean instanceof Printer) {
            registry.addPrinter((Printer)bean);
        } else if (bean instanceof Parser) {
            registry.addParser((Parser)bean);
        }
    }
}

JacksonAutoConfiguration#JacksonObjectMapperBuilderConfiguration類源碼：
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(Jackson2ObjectMapperBuilder.class)
static class JacksonObjectMapperBuilderConfiguration {
    @Bean
    @Scope("prototype")
    @ConditionalOnMissingBean
    Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder(ApplicationContext applicationContext,
            List<Jackson2ObjectMapperBuilderCustomizer> customizers) {
        Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
        builder.applicationContext(applicationContext);
        // ioc容器中獲取所有Jackson2ObjectMapperBuilderCustomizer類型的bean, 調(diào)用customize方法配置Jackson2ObjectMapperBuilder
        customize(builder, customizers);
        return builder;
    }
    private void customize(Jackson2ObjectMapperBuilder builder,
            List<Jackson2ObjectMapperBuilderCustomizer> customizers) {
        for (Jackson2ObjectMapperBuilderCustomizer customizer : customizers) {
            customizer.customize(builder);
        }
    }
}

JacksonAutoConfiguration#JacksonObjectMapperConfiguration類源碼
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(Jackson2ObjectMapperBuilder.class)
static class JacksonObjectMapperConfiguration {
    @Bean
    @Primary
    @ConditionalOnMissingBean
    ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        // 通過Jackson2ObjectMapperBuilder構(gòu)建出ObjectMapper
        return builder.createXmlMapper(false).build();
    }
}

spring:
  mvc:
    date: yyyy-MM-dd
    time: HH:mm:ss
    date-time: yyyy-MM-dd HH:mm:ss

@RestController
public class LocalDateTimeController {
    @GetMapping("/resolveRequestParamDateTime")
    public void resolveRequestParamDateTime(@RequestParam @DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss") LocalDateTime dateTime) {
        System.out.println("ok");
    }
    @GetMapping("/resolvePathVariableDateTime/{dateTime}")
    public void resolvePathVariableDateTime(@PathVariable @DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss") LocalDateTime dateTime) {
        System.out.println("ok");
    }
}

@Configuration
public class LocalDateTimeConfig {
    @Bean
    public Converter<String, LocalDateTime> stringToLocalDateTimeConverter() {
        return new Converter<String, LocalDateTime>() {
            @Override
            public LocalDateTime convert(String source) {
                return LocalDateTime.parse(source, DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
            }
        };
    }
    @Bean
    public Converter<String, LocalDate> stringToLocalDateConverter() {
        return new Converter<String, LocalDate>() {
            @Override
            public LocalDate convert(String source) {
                return LocalDate.parse(source, DateTimeFormatter.ofPattern("yyyy-MM-dd"));
            }
        };
    }
    @Bean
    public Converter<String, LocalTime> stringToLocalTimeConverter() {
        return new Converter<String, LocalTime>() {
            @Override
            public LocalTime convert(String source) {
                return LocalTime.parse(source, DateTimeFormatter.ofPattern("HH:mm:ss"));
            }
        };
    }
}

@RestController
public class LocalDateTimeController {
    @PostMapping("/resolveBodyDateTime")
    public void resolveBodyDateTime(@RequestBody ResolveBody body) {
        System.out.println("ok");
    }
    @Data
    private static class ResolveBody {
        @JsonSerialize(using = LocalDateTimeSerializer.class)
        @JsonDeserialize(using = LocalDateTimeDeserializer.class)
        @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss")
        private LocalDateTime dateTime;
    }
}

@Configuration
public class LocalDateTimeConfig {
    @Bean
    public Jackson2ObjectMapperBuilderCustomizer jackson2ObjectMapperBuilderCustomizer() {
        return new Jackson2ObjectMapperBuilderCustomizer() {
            @Override
            public void customize(Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder) {
                final JavaTimeModule javaTimeModule = new JavaTimeModule();
                javaTimeModule.addSerializer(LocalDateTime.class, new LocalDateTimeSerializer(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")));
                javaTimeModule.addSerializer(LocalDate.class, new LocalDateSerializer(DateTimeFormatter.ofPattern("yyyy-MM-dd")));
                javaTimeModule.addSerializer(LocalTime.class, new LocalTimeSerializer(DateTimeFormatter.ofPattern("HH:mm:ss")));
                javaTimeModule.addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")));
                javaTimeModule.addDeserializer(LocalDate.class, new LocalDateDeserializer(DateTimeFormatter.ofPattern("yyyy-MM-dd")));
                javaTimeModule.addDeserializer(LocalTime.class, new LocalTimeDeserializer(DateTimeFormatter.ofPattern("HH:mm:ss")));
                jacksonObjectMapperBuilder
                        .modules(javaTimeModule)
                        .featuresToDisable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
                        .featuresToDisable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
                        .simpleDateFormat("yyyy-MM-dd HH:mm:ss")
                        .timeZone(TimeZone.getTimeZone("GMT+8"))
                        .locale(Locale.CHINA);
            }
        };
    }
}