概述H2

JSR310的基于@DateTimeFormat注解的转换器实在DefaultFormattingConversionService中注册的

bash
public static void addDefaultFormatters(FormatterRegistry formatterRegistry) {
         ...
         if (jsr310Present) {
            // just handling JSR-310 specific date and time types
            new DateTimeFormatterRegistrar().registerFormatters(formatterRegistry);
        }
}

bash
DateTimeFormatterRegistrar#registerFormatters {
    ...
    registry.addFormatterForFieldAnnotation(new Jsr310DateTimeFormatAnnotationFormatterFactory());
}

分析H2

但是使用@DateTimeFormat并不能说随意的指定格式，这依赖这注解的字段类型，比如说如果像下面这样使用是不行的

bash
@DateTimeFormat(pattern = "yyyy年MM月dd日")
private ZonedDateTime endTime;

因为ZonedDateTime转换的时候还需要时间以及时区信息 下面是获取Parse的逻辑

bash
AnnotationParserConverter#convert
public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
    Annotation ann = targetType.getAnnotation(this.annotationType);
    if (ann == null) {
        throw new IllegalStateException(
                "Expected [" + this.annotationType.getName() + "] to be present on " + targetType);
    }
    AnnotationConverterKey converterKey = new AnnotationConverterKey(ann, targetType.getObjectType());
    GenericConverter converter = cachedParsers.get(converterKey);
    if (converter == null) {
        Parser<?> parser = this.annotationFormatterFactory.getParser(
                converterKey.getAnnotation(), converterKey.getFieldType());
        converter = new ParserConverter(this.fieldType, parser, FormattingConversionService.this);
        cachedParsers.put(converterKey, converter);
    }
    return converter.convert(source, sourceType, targetType);
}

Jsr310DateTimeFormatAnnotationFormatterFactory#getParser
public Parser<?> getParser(DateTimeFormat annotation, Class<?> fieldType) {
    DateTimeFormatter formatter = getFormatter(annotation, fieldType);
    return new TemporalAccessorParser((Class<? extends TemporalAccessor>) fieldType, formatter);
}

/**
 * Factory method used to create a {@link DateTimeFormatter}.
 * @param annotation the format annotation for the field
 * @param fieldType the declared type of the field
 * @return a {@link DateTimeFormatter} instance
 */
protected DateTimeFormatter getFormatter(DateTimeFormat annotation, Class<?> fieldType) {
    DateTimeFormatterFactory factory = new DateTimeFormatterFactory();
    factory.setStylePattern(resolveEmbeddedValue(annotation.style()));
    factory.setIso(annotation.iso());
    factory.setPattern(resolveEmbeddedValue(annotation.pattern()));
    return factory.createDateTimeFormatter();
}

下面是转换的逻辑

bash
ZonedDateTime#parse
public static ZonedDateTime parse(CharSequence text, DateTimeFormatter formatter) {
    Objects.requireNonNull(formatter, "formatter");
    return formatter.parse(text, ZonedDateTime::from);
}


DateTimeFormatter#parse 
public <T> T parse(CharSequence text, TemporalQuery<T> query) {
    Objects.requireNonNull(text, "text");
    Objects.requireNonNull(query, "query");
    try {
        return parseResolved0(text, null).query(query);
    } catch (DateTimeParseException ex) {
        throw ex;
    } catch (RuntimeException ex) {
        throw createError(text, ex);
    }
}

ZonedDateTime#from
public static ZonedDateTime from(TemporalAccessor temporal) {
    if (temporal instanceof ZonedDateTime) {
        return (ZonedDateTime) temporal;
    }
    try {
        ZoneId zone = ZoneId.from(temporal);
        if (temporal.isSupported(INSTANT_SECONDS)) {
            long epochSecond = temporal.getLong(INSTANT_SECONDS);
            int nanoOfSecond = temporal.get(NANO_OF_SECOND);
            return create(epochSecond, nanoOfSecond, zone);
        } else {
            LocalDate date = LocalDate.from(temporal);
            LocalTime time = LocalTime.from(temporal);
            return of(date, time, zone);
        }
    } catch (DateTimeException ex) {
        throw new DateTimeException("Unable to obtain ZonedDateTime from TemporalAccessor: " +
                temporal + " of type " + temporal.getClass().getName(), ex);
    }
}

如果没有时区信息ZoneId zone = ZoneId.from(temporal)会报错，如果没有时间信息LocalTime time = LocalTime.from(temporal);会报错

结论H2

所以上讲，@DateTimeFormat配合ZonedDateTime使用，指定的格式至少是包含时区和时间信息的，例如yyyy-MM-dd HH:mm:ss.SSSZ