DefaultConversionService
DefaultConversionService 提供方法为 ConverterRegistry 增加一些常用的 Converter
public class DefaultConversionService extends GenericConversionService { @Nullable private static volatile DefaultConversionService sharedInstance; public DefaultConversionService() { addDefaultConverters(this); } public static ConversionService getSharedInstance() { DefaultConversionService cs = sharedInstance; if (cs == null) { synchronized (DefaultConversionService.class) { cs = sharedInstance; if (cs == null) { cs = new DefaultConversionService(); sharedInstance = cs; } } } return cs; } 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)); } 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)); } 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()); } } 复制代码
FormattingConversionService
FormattingConversionService 则实现了 FormatterRetistry 接口
网络异常，图片无法展示
|
网络异常，图片无法展示
|
/** * Prints objects of type T for display. * * @author Keith Donald * @since 3.0 * @param <T> the type of object this Printer prints */ @FunctionalInterface public interface Printer<T> { /** * Print the object of type T for display. * @param object the instance to print * @param locale the current user locale * @return the printed text string */ String print(T object, Locale locale); } 复制代码
Printer 主要是为了打印展示某个类型对象的、根据 locale
/** * Parses text strings to produce instances of T. * * @author Keith Donald * @since 3.0 * @param <T> the type of object this Parser produces */ @FunctionalInterface public interface Parser<T> { /** * Parse a text String to produce a T. * @param text the text string * @param locale the current user locale * @return an instance of T * @throws ParseException when a parse exception occurs in a java.text parsing library * @throws IllegalArgumentException when a parse exception occurs */ T parse(String text, Locale locale) throws ParseException; } 复制代码
Parser 则是从 String 根据 locale 解释为 T、这个跟 Converter 优点类似、但是是根据 Locale 的不用来解释转换的
/** * Formats objects of type T. * A Formatter is both a Printer <i>and</i> a Parser for an object type. * * @author Keith Donald * @since 3.0 * @param <T> the type of object this Formatter formats */ public interface Formatter<T> extends Printer<T>, Parser<T> { } 复制代码
Formatter 则直接整合这两个接口
网络异常，图片无法展示
|
网络异常，图片无法展示
|
无论是 Printer 或者是 Parser 最终都会转换为 GenericConverter、在 convert 方法中在调用对应的方法进行转换解释。
ApplicationConversionService
Spring Boot 默认使用的 ConversionService 。
但是在 Spring 中管理的却是 WebConversionService。它最终也会调用 addBeans 方法
网络异常，图片无法展示
|
public class ApplicationConversionService extends FormattingConversionService { private static volatile ApplicationConversionService sharedInstance; public ApplicationConversionService() { this(null); } public ApplicationConversionService(StringValueResolver embeddedValueResolver) { if (embeddedValueResolver != null) { setEmbeddedValueResolver(embeddedValueResolver); } configure(this); } public static ConversionService getSharedInstance() { ApplicationConversionService sharedInstance = ApplicationConversionService.sharedInstance; if (sharedInstance == null) { synchronized (ApplicationConversionService.class) { sharedInstance = ApplicationConversionService.sharedInstance; if (sharedInstance == null) { sharedInstance = new ApplicationConversionService(); ApplicationConversionService.sharedInstance = sharedInstance; } } } return sharedInstance; } public static void configure(FormatterRegistry registry) { DefaultConversionService.addDefaultConverters(registry); DefaultFormattingConversionService.addDefaultFormatters(registry); addApplicationFormatters(registry); addApplicationConverters(registry); } 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 StringToPeriodConverter()); registry.addConverter(new PeriodToStringConverter()); registry.addConverter(new NumberToPeriodConverter()); registry.addConverter(new StringToDataSizeConverter()); registry.addConverter(new NumberToDataSizeConverter()); registry.addConverter(new StringToFileConverter()); registry.addConverter(new InputStreamSourceToByteArrayConverter()); 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)); } public static void addApplicationFormatters(FormatterRegistry registry) { registry.addFormatter(new CharArrayFormatter()); registry.addFormatter(new InetAddressFormatter()); registry.addFormatter(new IsoOffsetFormatter()); } 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()); for (Object bean : beans) { 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); } } } } 复制代码
并且会将所有的 GenericConverter、Converter、Printer、Parser bean 注册到 FormatterRegistry 中
Converter VS PropertyEditor
- PropertyEditor 从 String 转为其他类型
- Converter 从各种类型转为各种类型
Spring 同时支持两者。
BeanWrapper 实现了 TypeConverter、而 TypeConverter 则 先使用PropertyEditor转换器器转换，如果没找到对应的转换器器，会⽤ConversionService来进⾏行行对象转换。将两者整合起来做转换。
ConditionalConverter
public interface ConditionalConverter { boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType); } public interface ConditionalGenericConverter extends GenericConverter, ConditionalConverter { } 复制代码
网络异常，图片无法展示
|
当你实现了 Converter 接口和 ConditionalConverter 接口的时候、在 ConverterAdapter 适配类中会回调你的 matchs 方法
还有一种情况是
当存在多个一样的 sourceType 和 targetType 的转换器时、怎么选择出一个合适的 Converter
private static class ConvertersForPair { private final Deque<GenericConverter> converters = new ConcurrentLinkedDeque<>(); public void add(GenericConverter converter) { this.converters.addFirst(converter); } @Nullable public GenericConverter getConverter(TypeDescriptor sourceType, TypeDescriptor targetType) { for (GenericConverter converter : this.converters) { if (!(converter instanceof ConditionalGenericConverter) || ((ConditionalGenericConverter) converter).matches(sourceType, targetType)) { return converter; } } return null; } @Override public String toString() { return StringUtils.collectionToCommaDelimitedString(this.converters); } } 复制代码
实际问题
封装 RPC 模块过程中、涉及到各种类型的转换、有 json和 PoJo 的转换、也有 xml 和 PoJo 的转换、更有一些自定义协议报文到 PoJo 的转换、而这些 PoJo 具体是什么类型、在 RPC 的底层模块中是不知道的、只有在运行时通过泛型相关的信息获取的
比如 Json 和 PoJo 之间的转换、可能需要两个转换器、一个是 String 到 PoJo 的、一个是 PoJo 到 String
PoJo 到 String 是比较简单的、这里讨论下 String 到 PoJo。
String 到 PoJo 、这个关系貌似就是 1:N 的关系、貌似使用 ConverterFactory 就可以解决
网络异常，图片无法展示
|
第一个是泛型问题、而且作为 ConverterFactory 需要手动注册到 ConverterRegistry 中、
最终选定 GenericConverter、
@Component public class JsonStringToObjectConverter implements GenericConverter , ConditionalConverter { @Autowired private ObjectMapper objectMapper; @Override public Set<ConvertiblePair> getConvertibleTypes() { return null; } @Override public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) { try { if (source instanceof JsonStringMessage) { return objectMapper.readValue(((JsonStringMessage) source).getSource(), targetType.getType()); }else { // impossible throw new IllegalArgumentException("illegal argument"); } } catch (JsonProcessingException e) { throw new IllegalStateException(e); } } @Override public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) { return sourceType.getType().equals(JsonStringMessage.class); } 复制代码
getConvertibleTypes 返回 null 的话、必须实现接口 ConditionalConverter
此 Converter 作为 globalConverter
public void add(GenericConverter converter) { Set<ConvertiblePair> convertibleTypes = converter.getConvertibleTypes(); if (convertibleTypes == null) { Assert.state(converter instanceof ConditionalConverter, "Only conditional converters may return null convertible types"); this.globalConverters.add(converter); } else { for (ConvertiblePair convertiblePair : convertibleTypes) { getMatchableConverters(convertiblePair).add(converter); } } }
