18. Spring类型转换之ConversionService

简介

前面我们讲了PropertyEditor，是jdk提供的类型转换，spring也有自己的类型转换器，比PropertyEditor稍微强大一点

快速使用

public class User {
	private String name;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
}

@Component
public class UserBean {

	@Value("xx")
	private User user;

	public User getUser() {
		return user;
	}
}


public class String2UserConverter implements ConditionalGenericConverter {
	@Override
	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
		return sourceType.getType().equals(String.class) && targetType.getType().equals(User.class);
	}

	@Override
	public Set<ConvertiblePair> getConvertibleTypes() {
		return Collections.singleton(new ConvertiblePair(String.class, User.class));
	}

	@Override
	public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
		User user = new User();
		user.setName((String) source);
		return user;
	}
}


@Configuration
public class ConvertConfig {
	@Bean
	public ConversionServiceFactoryBean conversionService() {
		ConversionServiceFactoryBean factoryBean = new ConversionServiceFactoryBean();
		factoryBean.setConverters(Collections.singleton(new String2UserConverter()));
		return factoryBean;
	}
}

定义如上类，值得注意的是ConversionServiceFactoryBean的方法名一定要是 conversionService

测试

public static void main(String[] args) {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
    UserBean userBean = context.getBean("userBean", UserBean.class);
    System.out.println(userBean);
    System.out.println(userBean.getUser());
    System.out.println(userBean.getUser().getName());
}

输出 

com.shura.beans.UserBean@7225790e
com.shura.convert.User@54a097cc
xx

从上面的结果来看 ConversionService 也能做到类型转换

实现的地方跟PropertyEditor也差不多，下面看源码分析

源码分析

与PropertyEditor一样，也是在BeanWrapper中实现

// ConstructorResolver类
public BeanWrapper instantiateUsingFactoryMethod(）{
    BeanWrapperImpl bw = new BeanWrapperImpl();
	this.beanFactory.initBeanWrapper(bw);
	...
}

// AbstractBeanFactory类
protected void initBeanWrapper(BeanWrapper bw) {
    bw.setConversionService(getConversionService()); // 下节分析
    registerCustomEditors(bw); // 注册PropertyEditor 
}

上面可以看出可以从beanFactory获取ConversionService到BeanWrapper

同样也是在 TypeConverterDelegate 判断是否有ConversionService

public <T> T convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue,
			@Nullable Class<T> requiredType, @Nullable TypeDescriptor typeDescriptor) throws IllegalArgumentException {

    ConversionService conversionService = this.propertyEditorRegistry.getConversionService();
    if (editor == null && conversionService != null && newValue != null && typeDescriptor != null) {
        TypeDescriptor sourceTypeDesc = TypeDescriptor.forObject(newValue);
        if (conversionService.canConvert(sourceTypeDesc, typeDescriptor)) {
            try {
                return (T) conversionService.convert(newValue, sourceTypeDesc, typeDescriptor);
            }
            catch (ConversionFailedException ex) {
                // fallback to default conversion logic below
                conversionAttemptEx = ex;
            }
        }
    }

}

上面就是ConversionService的处理逻辑，那么ConversionService应该也有默认的类型转换器，在哪里设置的呢

进入ConversionServiceFactoryBean，它是一个FactoryBean，真正的对象是getObject返回的GenericConversionService，该属性由createConversionService方法创建，实际上就是实例化了一个DefaultConversionService

public class ConversionServiceFactoryBean implements FactoryBean<ConversionService>, InitializingBean {

	@Nullable
	private Set<?> converters;

	@Nullable
	private GenericConversionService conversionService;

	public void setConverters(Set<?> converters) {
		this.converters = converters;
	}

	@Override
	public void afterPropertiesSet() {
		this.conversionService = createConversionService();
		ConversionServiceFactory.registerConverters(this.converters, this.conversionService);
	}

	protected GenericConversionService createConversionService() {
		return new DefaultConversionService();
	}

	@Override
	@Nullable
	public ConversionService getObject() {
		return this.conversionService;
	}

	@Override
	public Class<? extends ConversionService> getObjectType() {
		return GenericConversionService.class;
	}

}

那么DefaultConversionService做了什么呢

public class DefaultConversionService extends GenericConversionService {

	@Nullable
	private static volatile DefaultConversionService sharedInstance;


	public DefaultConversionService() {
		addDefaultConverters(this);
	}


	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());
	}

}

就是添加了一堆的默认转换器，到这基本上就能知道spring的转换机制了，以及如何实现自己的转换器

还有个小问题就是前面说了注册ConversionServiceFactoryBean的方法名一定要是 conversionService，Why?

请看如下代码，在实例化非懒加载单例Bean之前就会获取ConversionService设置到beanFactory,而这个名字就只能叫conversionService

org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    String CONVERSION_SERVICE_BEAN_NAME = "conversionService";
    
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // 实例化非懒加载的单例Bean
    beanFactory.preInstantiateSingletons();
}

ConversionService就介绍到这

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