【1】格式化后台传给前端的日期

首先第一点需要知道的是springboot默认依赖的json框架是jackson。当使用@ResponseBody注解返回json格式数据时就是该框架在起作用。

在这里插入图片描述

① SpringBoot对Date/DateTime配置

如果字段属性是Date而非LocalDateTime时，通常我们会在application.properties里面配置如下：

spring.mvc.date-format=yyyy-MM-dd HH:mm:ss
spring.jackson.date-format=yyyy-MM-dd HH:mm:ss
spring.jackson.time-zone=GMT+8
spring.jackson.serialization.write-dates-as-timestamps=false

如下图所示，spring.jackson开头的配置会被JacksonProperties类获取进行使用。当返回json格式的时候，Jackson就会根据配置文件中日期格式化的配置对结果进行处理。
在这里插入图片描述

但是如果字段属性为LocalDateTime呢？这种配置就失去了作用。

② 第一种方式：配置localDateTimeSerializer

这时候建议配置如下：

@Configuration
public class LocalDateTimeSerializerConfig {

    @Value("${spring.jackson.date-format:yyyy-MM-dd HH:mm:ss}")
    private String pattern;

    private String datePattern="yyyy-MM-dd";

    // localDateTime 序列化器
    @Bean
    public LocalDateTimeSerializer localDateTimeSerializer() {
        return new LocalDateTimeSerializer(DateTimeFormatter.ofPattern(pattern));
    }
    // localDateTime 反序列化器
    @Bean
    public LocalDateTimeDeserializer localDateTimeDeserializer() {
        return new LocalDateTimeDeserializer(DateTimeFormatter.ofPattern(pattern));
    }
    // localDate 序列化器
    @Bean
    public LocalDateSerializer localDateSerializer() {
        return new LocalDateSerializer(DateTimeFormatter.ofPattern(datePattern));
    }
    // localDate 反序列化器
    @Bean
    public LocalDateDeserializer localDateDeserializer() {
        return new LocalDateDeserializer(DateTimeFormatter.ofPattern(datePattern));
    }

    @Bean
    public Jackson2ObjectMapperBuilderCustomizer jackson2ObjectMapperBuilderCustomizer() {
//        return new Jackson2ObjectMapperBuilderCustomizer() {
//            @Override
//            public void customize(Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder) {
                jacksonObjectMapperBuilder.featuresToDisable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
//                jacksonObjectMapperBuilder.serializerByType(LocalDateTime.class, localDateTimeSerializer());
//                jacksonObjectMapperBuilder.deserializerByType(LocalDateTime.class,localDateTimeDeserializer());
//            }
//        };
        //这种方式同上
        return builder -> {
            builder.serializerByType(LocalDateTime.class, localDateTimeSerializer());
            builder.deserializerByType(LocalDateTime.class,localDateTimeDeserializer());
            builder.serializerByType(LocalDate.class, localDateSerializer());
            builder.deserializerByType(LocalDate.class,localDateDeserializer());
            builder.simpleDateFormat(pattern);
        };
    }

}

其实上文除了序列化器之外还配置了反序列化器也支持了2023-02-19 17:40:26格式向LocalDateTime的转换。也就是支持对象和JSON之间相互转换，但是不支持单个字段比如string转换为LocalDateTime。

//支持
@RequestBody TbSysUser user --> JSON 字符串

//不支持
String --> LocalDateTime createTime

③ 第二种方式：@JsonFormat

这种配置方式自然是全局的，如果想针对某个字段特殊处理，可以在类字段上面添加注解@JsonFormat：

@JsonFormat( pattern = "yyyy-MM-dd HH:mm:ss",timezone="GMT+8")
private Date createdDate;

@JsonFormat( pattern = "yyyy-MM-dd HH:mm:ss")
private LocalDateTime createdTime;

【2】前台传String年月日时分秒格式日期给后台

如下所示，前台传参2020-08-30 11:11:11，后台使用LocalDateTime 接收。通常会报错类似如下：

nested exception is org.springframework.core.convert.ConversionFailedException: 
Failed to convert from type [java.lang.String] to type [java.time.LocalDateTime ]

很显然是在参数绑定的时候没有找到合适的转换器把String转换为对应的格式。

① 配置全局的日期转换器localDateTimeConvert

根据需要进行修改：

@Bean
public Converter<String, LocalDateTime> localDateTimeConvert() {
    return new Converter<String, LocalDateTime>() {
        @Override
        public LocalDateTime convert(String source) {
            DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
            LocalDateTime dateTime = null;
            try {
                //2020-01-01 00:00:00
                switch (source.length()){
                    case 10:
                        logger.debug("传过来的是日期格式：{}",source);
                        source=source+" 00:00:00";
                        break;
                    case 13:
                        logger.debug("传过来的是日期 小时格式：{}",source);
                        source=source+":00:00";
                        break;
                    case 16:
                        logger.debug("传过来的是日期 小时:分钟格式：{}",source);
                        source=source+":00";
                        break;
                }
                dateTime = LocalDateTime.parse(source, df);
            } catch (Exception e) {
               logger.error(e.getMessage(),e);
            }
            return dateTime;
        }
    };
}

实现原理简要描述

在进行参数绑定的时候，会使用WebDataBinder对象。而创建WebDataBinder对象时，会遍历DefaultDataBinderFactory.initializer，使用其WebBindingInitializer initializer对WebDataBinder对象进行初始化。

初始化方法具体可见ConfigurableWebBindingInitializer.initBinder(WebDataBinder binder)，源码如下：

 public void initBinder(WebDataBinder binder) {
        binder.setAutoGrowNestedPaths(this.autoGrowNestedPaths);
        if (this.directFieldAccess) {
            binder.initDirectFieldAccess();
        }
        //设置messageCodesResolver
        if (this.messageCodesResolver != null) {
            binder.setMessageCodesResolver(this.messageCodesResolver);
        }
        //设置bindingErrorProcessor
        if (this.bindingErrorProcessor != null) {
            binder.setBindingErrorProcessor(this.bindingErrorProcessor);
        }
        //设置validator
        if (this.validator != null && binder.getTarget() != null && this.validator.supports(binder.getTarget().getClass())) {
            binder.setValidator(this.validator);
        }
        //设置conversionService
        if (this.conversionService != null) {
            binder.setConversionService(this.conversionService);
        }
        if (this.propertyEditorRegistrars != null) {
            PropertyEditorRegistrar[] var2 = this.propertyEditorRegistrars;
            int var3 = var2.length;
            for(int var4 = 0; var4 < var3; ++var4) {
                PropertyEditorRegistrar propertyEditorRegistrar = var2[var4];
                propertyEditorRegistrar.registerCustomEditors(binder);
            }
        }

    }

而conversionService中包含了许多的convert-类型格式化器。在WebDataBinder进行参数绑定的时候就会使用不同的格式化器即不同的convert进行参数类型转换。

关于参数绑定的过程，有兴趣的可以跟踪DataBinder.doBind方法，在这个过程中会对前台传输的值进行类型转换为目标参数需要的类型。自定义的localDateTimeConvert也是在这里被用到的。

如下所示前台传String格式给后台参数endDate，参数类型为java.time.LocalDateTime。

在这里插入图片描述
找到我们自定义的converter
在这里插入图片描述
在这里插入图片描述
调用convert进行类型转换：
在这里插入图片描述
可以看到转换后的结果为：

在这里插入图片描述

② 配置日期格式化器

/**
  * yyyy-MM-dd HH:mm:ss String-localDateTime
  * @return
  */
 @Bean
 public Formatter<LocalDateTime> localDateTimeFormatter() {
     return new Formatter<LocalDateTime>() {
         @Override
         public LocalDateTime parse(String text, Locale locale) throws ParseException {
             return LocalDateTime.parse(text, DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
         }

         @Override
         public String print(LocalDateTime localDateTime, Locale locale) {
             DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
             return formatter.format(localDateTime);
         }
     };
 }

自定义的格式化器会在SpringBoot启动时自动化配置过程中被加入，具体可以参考如下代码。

WebMvcAutoConfiguration.mvcConversionService：

@Bean
@Override
public FormattingConversionService mvcConversionService() {
	WebConversionService conversionService = new WebConversionService(this.mvcProperties.getDateFormat());
	addFormatters(conversionService);
	return conversionService;
}

③ 配置stringLocalDateConverter

前台传string格式的如 2023-12-20 ，后台使用LocalDate接收。

/**
 * string转为 LocalDate
 * @return
 */
@Bean
public Converter<String, LocalDate> stringLocalDateConverter() {
    //注意这里不要用lambda 表达式，会出现错误  Unable to determine source type <S> and target type <T> for your Converter
    return new Converter<String, LocalDate>() {
        @Override
        public LocalDate convert(String source) {
            if(StringUtils.isEmpty(source)){
                return null;
            }
            DateTimeFormatter df = DateTimeFormatter.ofPattern(datepattern);
            LocalDate dateTime = null;
            try {
                dateTime = LocalDate.parse(source, df);
            } catch (Exception e) {
                logger.error(e.getMessage(),e);
            }
            return dateTime;
        }
    };
}

【3】convert是什么时候添加到ConversionService中的？

① SpringBoot启动的时候运行run方法

其会走到SpringApplication.configureEnvironment方法处：

protected void configureEnvironment(ConfigurableEnvironment environment, String[] args) {
    if (this.addConversionService) {
    //从这里跟踪
        ConversionService conversionService = ApplicationConversionService.getSharedInstance();
        environment.setConversionService((ConfigurableConversionService)conversionService);
    }

    this.configurePropertySources(environment, args);
    this.configureProfiles(environment, args);
}

② 尝试获取ConversionService

ApplicationConversionService.getSharedInstance如下所示，这里可以看到其使用了设计模式中的懒汉式之双重校验锁来获取单例。

public static ConversionService getSharedInstance() {
     ApplicationConversionService sharedInstance = sharedInstance;
     if (sharedInstance == null) {
         Class var1 = ApplicationConversionService.class;
         synchronized(ApplicationConversionService.class) {
             sharedInstance = sharedInstance;
             if (sharedInstance == null) {
                 sharedInstance = new ApplicationConversionService();
                 sharedInstance = sharedInstance;
             }
         }
     }
     return sharedInstance;
 }

③ 获取ApplicationConversionService

继续对象创建过程会发现其走到了configure处：

public ApplicationConversionService(StringValueResolver embeddedValueResolver) {
    if (embeddedValueResolver != null) {
        this.setEmbeddedValueResolver(embeddedValueResolver);
    }
//我们从这里继续跟进
    configure(this);
}

这里我们顺带看一下ApplicationConversionService的类继承示意图（其不只是可以作为ConversionService还可以作为ConverterRegistry与FormatterRegistry）：
在这里插入图片描述

④ ApplicationConversionService.configure

创建ApplicationConversionService时会对其进行配置，这里很重要。其会注入默认的Converter和Formatter：

public static void configure(FormatterRegistry registry) {
      DefaultConversionService.addDefaultConverters(registry);
      DefaultFormattingConversionService.addDefaultFormatters(registry);
      addApplicationFormatters(registry);
      addApplicationConverters(registry);
  }

⑤ DefaultConversionService.addDefaultConverters

该方法执行完，会添加52个类型转换器：

public static void addDefaultConverters(ConverterRegistry converterRegistry) {
	addScalarConverters(converterRegistry);
	addCollectionConverters(converterRegistry);

	converterRegistry.addConverter(new ByteBufferConverter((ConversionService) converterRegistry));
	converterRegistry.addConverter(new StringToTimeZoneConverter());
	converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
	converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());

	converterRegistry.addConverter(new ObjectToObjectConverter());
	converterRegistry.addConverter(new IdToEntityConverter((ConversionService) converterRegistry));
	converterRegistry.addConverter(new FallbackObjectToStringConverter());
	converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService) converterRegistry));
}

addScalarConverters(converterRegistry);如下所示：

private static void addScalarConverters(ConverterRegistry converterRegistry) {
	converterRegistry.addConverterFactory(new NumberToNumberConverterFactory());

	converterRegistry.addConverterFactory(new StringToNumberConverterFactory());
	converterRegistry.addConverter(Number.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverter(new StringToCharacterConverter());
	converterRegistry.addConverter(Character.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverter(new NumberToCharacterConverter());
	converterRegistry.addConverterFactory(new CharacterToNumberFactory());

	converterRegistry.addConverter(new StringToBooleanConverter());
	converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
	converterRegistry.addConverter(new EnumToStringConverter((ConversionService) converterRegistry));

	converterRegistry.addConverterFactory(new IntegerToEnumConverterFactory());
	converterRegistry.addConverter(new EnumToIntegerConverter((ConversionService) converterRegistry));

	converterRegistry.addConverter(new StringToLocaleConverter());
	converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverter(new StringToCharsetConverter());
	converterRegistry.addConverter(Charset.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverter(new StringToCurrencyConverter());
	converterRegistry.addConverter(Currency.class, String.class, new ObjectToStringConverter());

	converterRegistry.addConverter(new StringToPropertiesConverter());
	converterRegistry.addConverter(new PropertiesToStringConverter());

	converterRegistry.addConverter(new StringToUUIDConverter());
	converterRegistry.addConverter(UUID.class, String.class, new ObjectToStringConverter());
}

这里会添加23个类型转换器：
在这里插入图片描述

添加集合处理的类型转换器(这里会添加17个类型转换器)：

public static void addCollectionConverters(ConverterRegistry converterRegistry) {
	ConversionService conversionService = (ConversionService) converterRegistry;

	converterRegistry.addConverter(new ArrayToCollectionConverter(conversionService));
	converterRegistry.addConverter(new CollectionToArrayConverter(conversionService));

	converterRegistry.addConverter(new ArrayToArrayConverter(conversionService));
	converterRegistry.addConverter(new CollectionToCollectionConverter(conversionService));
	converterRegistry.addConverter(new MapToMapConverter(conversionService));

	converterRegistry.addConverter(new ArrayToStringConverter(conversionService));
	converterRegistry.addConverter(new StringToArrayConverter(conversionService));

	converterRegistry.addConverter(new ArrayToObjectConverter(conversionService));
	converterRegistry.addConverter(new ObjectToArrayConverter(conversionService));

	converterRegistry.addConverter(new CollectionToStringConverter(conversionService));
	converterRegistry.addConverter(new StringToCollectionConverter(conversionService));

	converterRegistry.addConverter(new CollectionToObjectConverter(conversionService));
	converterRegistry.addConverter(new ObjectToCollectionConverter(conversionService));

	converterRegistry.addConverter(new StreamConverter(conversionService));
}

⑥ addDefaultFormatters添加格式化器

/**
 * Add formatters appropriate for most environments: including number formatters,
 * JSR-354 Money & Currency formatters, JSR-310 Date-Time and/or Joda-Time formatters,
 * depending on the presence of the corresponding API on the classpath.
 * @param formatterRegistry the service to register default formatters with
 */
public static void addDefaultFormatters(FormatterRegistry formatterRegistry) {
	// Default handling of number values
	formatterRegistry.addFormatterForFieldAnnotation(new NumberFormatAnnotationFormatterFactory());

	// Default handling of monetary values
	if (jsr354Present) {
		formatterRegistry.addFormatter(new CurrencyUnitFormatter());
		formatterRegistry.addFormatter(new MonetaryAmountFormatter());
		formatterRegistry.addFormatterForFieldAnnotation(new Jsr354NumberFormatAnnotationFormatterFactory());
	}

	// Default handling of date-time values

	// just handling JSR-310 specific date and time types
	new DateTimeFormatterRegistrar().registerFormatters(formatterRegistry);

	if (jodaTimePresent) {
		// handles Joda-specific types as well as Date, Calendar, Long
		new JodaTimeFormatterRegistrar().registerFormatters(formatterRegistry);
	}
	else {
		// regular DateFormat-based Date, Calendar, Long converters
		new DateFormatterRegistrar().registerFormatters(formatterRegistry);
	}
}

DateTimeFormatterRegistrar.registerFormatters

@Override
public void registerFormatters(FormatterRegistry registry) {
	DateTimeConverters.registerConverters(registry);

	DateTimeFormatter df = getFormatter(Type.DATE);
	DateTimeFormatter tf = getFormatter(Type.TIME);
	DateTimeFormatter dtf = getFormatter(Type.DATE_TIME);

	// Efficient ISO_LOCAL_* variants for printing since they are twice as fast...

	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));

	registry.addFormatterForFieldType(ZonedDateTime.class,
			new TemporalAccessorPrinter(dtf),
			new TemporalAccessorParser(ZonedDateTime.class, dtf));

	registry.addFormatterForFieldType(OffsetDateTime.class,
			new TemporalAccessorPrinter(dtf),
			new TemporalAccessorParser(OffsetDateTime.class, dtf));

	registry.addFormatterForFieldType(OffsetTime.class,
			new TemporalAccessorPrinter(tf),
			new TemporalAccessorParser(OffsetTime.class, tf));

	registry.addFormatterForFieldType(Instant.class, new InstantFormatter());
	registry.addFormatterForFieldType(Period.class, new PeriodFormatter());
	registry.addFormatterForFieldType(Duration.class, new DurationFormatter());
	registry.addFormatterForFieldType(Year.class, new YearFormatter());
	registry.addFormatterForFieldType(Month.class, new MonthFormatter());
	registry.addFormatterForFieldType(YearMonth.class, new YearMonthFormatter());
	registry.addFormatterForFieldType(MonthDay.class, new MonthDayFormatter());

	registry.addFormatterForFieldAnnotation(new Jsr310DateTimeFormatAnnotationFormatterFactory());
}

DateTimeConverters.registerConverters

public static void registerConverters(ConverterRegistry registry) {
	DateFormatterRegistrar.addDateConverters(registry);

	registry.addConverter(new LocalDateTimeToLocalDateConverter());
	registry.addConverter(new LocalDateTimeToLocalTimeConverter());
	registry.addConverter(new ZonedDateTimeToLocalDateConverter());
	registry.addConverter(new ZonedDateTimeToLocalTimeConverter());
	registry.addConverter(new ZonedDateTimeToLocalDateTimeConverter());
	registry.addConverter(new ZonedDateTimeToOffsetDateTimeConverter());
	registry.addConverter(new ZonedDateTimeToInstantConverter());
	registry.addConverter(new OffsetDateTimeToLocalDateConverter());
	registry.addConverter(new OffsetDateTimeToLocalTimeConverter());
	registry.addConverter(new OffsetDateTimeToLocalDateTimeConverter());
	registry.addConverter(new OffsetDateTimeToZonedDateTimeConverter());
	registry.addConverter(new OffsetDateTimeToInstantConverter());
	registry.addConverter(new CalendarToZonedDateTimeConverter());
	registry.addConverter(new CalendarToOffsetDateTimeConverter());
	registry.addConverter(new CalendarToLocalDateConverter());
	registry.addConverter(new CalendarToLocalTimeConverter());
	registry.addConverter(new CalendarToLocalDateTimeConverter());
	registry.addConverter(new CalendarToInstantConverter());
	registry.addConverter(new LongToInstantConverter());
	registry.addConverter(new InstantToLongConverter());
}

DateFormatterRegistrar.addDateConverters

public static void addDateConverters(ConverterRegistry converterRegistry) {
	converterRegistry.addConverter(new DateToLongConverter());
	converterRegistry.addConverter(new DateToCalendarConverter());
	converterRegistry.addConverter(new CalendarToDateConverter());
	converterRegistry.addConverter(new CalendarToLongConverter());
	converterRegistry.addConverter(new LongToDateConverter());
	converterRegistry.addConverter(new LongToCalendarConverter());
}

⑦ addApplicationFormatters(registry)

添加全局格式化器：

   public static void addApplicationFormatters(FormatterRegistry registry) {
        registry.addFormatter(new CharArrayFormatter());
        registry.addFormatter(new InetAddressFormatter());
        registry.addFormatter(new IsoOffsetFormatter());
    }

⑧ addApplicationConverters(registry)

添加全局类型转换器：

public static void addApplicationConverters(ConverterRegistry registry) {
       addDelimitedStringConverters(registry);
       registry.addConverter(new StringToDurationConverter());
       registry.addConverter(new DurationToStringConverter());
       registry.addConverter(new NumberToDurationConverter());
       registry.addConverter(new DurationToNumberConverter());
       registry.addConverter(new StringToDataSizeConverter());
       registry.addConverter(new NumberToDataSizeConverter());
       registry.addConverter(new StringToFileConverter());
       registry.addConverterFactory(new LenientStringToEnumConverterFactory());
       registry.addConverterFactory(new LenientBooleanToEnumConverterFactory());
   }

   public static void addDelimitedStringConverters(ConverterRegistry registry) {
       ConversionService service = (ConversionService)registry;
       registry.addConverter(new ArrayToDelimitedStringConverter(service));
       registry.addConverter(new CollectionToDelimitedStringConverter(service));
       registry.addConverter(new DelimitedStringToArrayConverter(service));
       registry.addConverter(new DelimitedStringToCollectionConverter(service));
   }

【4】前端传String格式年月日给后端LocalDate

如客户端传2024-05-13 给后端使用LocalDate接收。

① 注册日期格式化器

@Bean
public Formatter<LocalDate> localDateFormatter() {
    return new Formatter<LocalDate>() {
        @Override
        public LocalDate parse(String text, Locale locale)  {
            return LocalDate.parse(text, DateTimeFormatter.ofPattern("yyyy-MM-dd"));
        }

        @Override
        public String print(LocalDate localDate, Locale locale) {
            DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
            return formatter.format(localDate);
        }
    };
}

② 注册转换器

@Configuration
public class WebConfig implements WebMvcConfigurer {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(String.class, LocalDate.class, new Converter<String, LocalDate>() {
            @Override
            public LocalDate convert(String source) {
                return LocalDate.parse(source, DateTimeFormatter.ofPattern("yyyy-MM-dd"));
            }
        });
    }
}
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