spring可以對不同Class型別進行相應的轉換。比如String轉成Boolean. Boolean轉成string類.以下是個簡單的例子

	@Test
	public void testConverter() {
		DefaultConversionService serivce = new DefaultConversionService();
		boolean actual = serivce.canConvert(String.class, Boolean.class);
		Assert.assertEquals(true, actual);
		Object acc = serivce.convert("true", Boolean.class);
 
		Assert.assertEquals(true, ((Boolean)acc).booleanValue());
	}

spring型別轉換涉及到的類圖如下

1、spring是通過ConversionService進行型別轉換，通過ConverterRegistry進行轉換器註冊

其中ConversionService中的方法canConverter判斷是否可以轉失，convert呼叫相應的轉換器進行型別轉換操作

ConverterRegistry中的方法add(Converter)是註冊一個轉換器

2、對於ConversionService和ConverterRegistry的主要實現類是GenericConversionService。GenericConversionService維護了一個內部類的物件Converters.

當註冊一個converter時。會生成一個GenericeConverter的適配類ConverterAdapter，並註冊到內部類Converters中

public void addConverter(Converter<?, ?> converter) {
		GenericConverter.ConvertiblePair typeInfo = getRequiredTypeInfo(converter, Converter.class);
		Assert.notNull(typeInfo, "Unable to the determine sourceType <S
 
> and targetType " +
				"<T> which your Converter<S, T> converts between; declare these generic types.");
		addConverter(new ConverterAdapter(typeInfo, converter));
	}
	public void addConverter(Class<?> sourceType, Class<?> targetType, Converter<?, ?> converter) {
		GenericConverter.ConvertiblePair typeInfo = new GenericConverter.ConvertiblePair(sourceType, targetType);
		addConverter(new ConverterAdapter(typeInfo, converter));
	}
	public void addConverter(GenericConverter converter) {
		this.converters.add(converter);
		invalidateCache();
	}

內部類Converters中維護了一個Map用於存放先前註冊的所有Converter

private static class Converters {
		private final Set<GenericConverter> globalConverters =
				new LinkedHashSet<GenericConverter>();
		private final Map<ConvertiblePair, ConvertersForPair> converters =
			new LinkedHashMap<ConvertiblePair, ConvertersForPair>(36);
		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");
				globalConverters.add(converter);
			}
			else {
				for (ConvertiblePair convertiblePair : convertibleTypes) {
					ConvertersForPair convertersForPair = getMatchableConverters(convertiblePair);
					convertersForPair.add(converter);
				}
			}
		}

3、當呼叫canConvert時，會呼叫內部類的方法find方法查詢轉換的型別sourceType和targetType是否進行了註冊

protected GenericConverter getConverter(TypeDescriptor sourceType, TypeDescriptor targetType) {
		ConverterCacheKey key = new ConverterCacheKey(sourceType, targetType);
		GenericConverter converter = this.converterCache.get(key);
		if (converter != null) {
			return (converter != NO_MATCH ? converter : null);
		}
		converter = this.converters.find(sourceType, targetType);
		if (converter == null) {
			converter = getDefaultConverter(sourceType, targetType);
		}
		if (converter != null) {
			this.converterCache.put(key, converter);
			return converter;
		}
		this.converterCache.put(key, NO_MATCH);
		return null;
	}

其中內部類Converters的find方法根據sourceType和targetType去查詢Converters中維護的Map中是否包括支援的註冊型別，如果存在返回GenericConverter的適配類ConverterAdapter，如果沒有存在返回null

public GenericConverter find(TypeDescriptor sourceType, TypeDescriptor targetType) {
			// Search the full type hierarchy
			List<Class<?>> sourceCandidates = getClassHierarchy(sourceType.getType());
			List<Class<?>> targetCandidates = getClassHierarchy(targetType.getType());
			for (Class<?> sourceCandidate : sourceCandidates) {
				for (Class<?> targetCandidate : targetCandidates) {
					ConvertiblePair convertiblePair = new ConvertiblePair(sourceCandidate, targetCandidate);
					GenericConverter converter = getRegisteredConverter(
							sourceType, targetType, convertiblePair);
					if(converter != null) {
						return converter;
					}
				}
			}
			return null;
		}
		private GenericConverter getRegisteredConverter(TypeDescriptor sourceType,
				TypeDescriptor targetType, ConvertiblePair convertiblePair) {
			// Check specifically registered converters
			ConvertersForPair convertersForPair = converters.get(convertiblePair);
			GenericConverter converter = convertersForPair == null ? null
				: convertersForPair.getConverter(sourceType, targetType);
			if (converter != null) {
				return converter;
			}
			// Check ConditionalGenericConverter that match all types
			for (GenericConverter globalConverter : this.globalConverters) {
				if (((ConditionalConverter)globalConverter).matches(
						sourceType, targetType)) {
					return globalConverter;
				}
			}
			return null;
		}

4、當查詢到註冊的型別轉換器時GenericConverter時。會呼叫ConverterAdapter中的convert方法。ConverterAdapter中的convert方法會呼叫相當維護的converter並轉換

public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
			if (source == null) {
				return convertNullSource(sourceType, targetType);
			}
			return this.converter.convert(source);
		}

5、spring的註冊型別轉換器時，是通過GenericConversionService的子類DefaultConversionService實現的

private static void addScalarConverters(ConverterRegistry converterRegistry) {
		ConversionService conversionService = (ConversionService) converterRegistry;
		converterRegistry.addConverter(new StringToBooleanConverter());
		converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());
		converterRegistry.addConverterFactory(new StringToNumberConverterFactory());
		converterRegistry.addConverter(Number.class, String.class, new ObjectToStringConverter());
		converterRegistry.addConverterFactory(new NumberToNumberConverterFactory());
		converterRegistry.addConverter(new StringToCharacterConverter());
		converterRegistry.addConverter(Character.class, String.class, new ObjectToStringConverter());
		converterRegistry.addConverter(new NumberToCharacterConverter());
		converterRegistry.addConverterFactory(new CharacterToNumberFactory());
		converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
		converterRegistry.addConverter(Enum.class, String.class, new EnumToStringConverter(conversionService));
		converterRegistry.addConverter(new StringToLocaleConverter());
		converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());
		converterRegistry.addConverter(new PropertiesToStringConverter());
		converterRegistry.addConverter(new StringToPropertiesConverter());
		converterRegistry.addConverter(new StringToUUIDConverter());
		converterRegistry.addConverter(UUID.class, String.class, new ObjectToStringConverter());
	}