spring 之 类型转换 2

spring内置的转换器

在spring xml 文件中，配置属性的时候， 不管实际是 list 还是map ，还是Date， 或者原生的java 类型， 我们只能配置xml 给它们。 那么 spring 在实例化bean 的时候， 这些属性是必须要经过转换的。 那么，spring 具体是怎么实现的呢？

答案就是：

public class DefaultConversionService extends GenericConversionService {
    private static final boolean javaUtilOptionalClassAvailable = ClassUtils.isPresent("java.util.Optional", DefaultConversionService.class.getClassLoader());
    private static final boolean jsr310Available = ClassUtils.isPresent("java.time.ZoneId", DefaultConversionService.class.getClassLoader());
    private static final boolean streamAvailable = ClassUtils.isPresent("java.util.stream.Stream", DefaultConversionService.class.getClassLoader());
    private static volatile DefaultConversionService sharedInstance;

    public static ConversionService getSharedInstance() {
        if(sharedInstance == null) {
            Class var0 = DefaultConversionService.class;
            synchronized(DefaultConversionService.class) {
                if(sharedInstance == null) {
                    sharedInstance = new DefaultConversionService();
                }
            }
        }

        return sharedInstance;
    }

    public DefaultConversionService() {
        addDefaultConverters(this);
    }

    public static void addDefaultConverters(ConverterRegistry converterRegistry) {
        addScalarConverters(converterRegistry); // 注册 标量的转换器， 也就是把xml配置文件中 字面值 转换为 常见的简单的普通类型 
        addCollectionConverters(converterRegistry);  // 转换为 集合
        converterRegistry.addConverter(new ByteBufferConverter((ConversionService)converterRegistry));
        if(jsr310Available) {
            DefaultConversionService.Jsr310ConverterRegistrar.registerJsr310Converters(converterRegistry);
        }

        converterRegistry.addConverter(new ObjectToObjectConverter());
        converterRegistry.addConverter(new IdToEntityConverter((ConversionService)converterRegistry));
        converterRegistry.addConverter(new FallbackObjectToStringConverter());
        if(javaUtilOptionalClassAvailable) {
            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));
        if(streamAvailable) {
            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());
    }

    private static final class Jsr310ConverterRegistrar {
        private Jsr310ConverterRegistrar() {
        }

        public static void registerJsr310Converters(ConverterRegistry converterRegistry) {
            converterRegistry.addConverter(new StringToTimeZoneConverter());
            converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
            converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());
        }
    }
}

大致的堆栈是：

at org.springframework.core.convert.support.StringToNumberConverterFactory$StringToNumber.convert(StringToNumberConverterFactory.java:59)
at org.springframework.core.convert.support.StringToNumberConverterFactory$StringToNumber.convert(StringToNumberConverterFactory.java:49)
at org.springframework.core.convert.support.GenericConversionService$ConverterFactoryAdapter.convert(GenericConversionService.java:436)
at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:37)
at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:203)
at org.springframework.beans.TypeConverterDelegate.convertIfNecessary(TypeConverterDelegate.java:173)
at org.springframework.beans.AbstractNestablePropertyAccessor.convertIfNecessary(AbstractNestablePropertyAccessor.java:576)
at org.springframework.beans.AbstractNestablePropertyAccessor.convertForProperty(AbstractNestablePropertyAccessor.java:603)
at org.springframework.beans.BeanWrapperImpl.convertForProperty(BeanWrapperImpl.java:216)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.convertForProperty(AbstractAutowireCapableBeanFactory.java:1538)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1497)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1237)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:552)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
- locked <0x677> (a java.util.concurrent.ConcurrentHashMap)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)

spring xml中常见属性的注入：

注入array 或者list 有两种方式：

1 直接使用 <value>， 每个 item 使用 逗号分隔， 这个时候使用的是 StringToCollectionConverter

如： 

<property name="strArr">
    <value>aaa,bbb,ccc</value>
</property>

2 使用 <list> 子标签， （别看名字是 list， 但它可以转换list 和 array； spring 并没有单独对 数组即 array 进行转换， 而是把list/array 统一为 list 了） 。但是实际处理的时候是区分开了的， array 使用的是 ArrayToArrayConverter， list 使用的是CollectionToCollectionConverter  。 

<property name="strArr">
    <list>
        <value>　　// list 下面每个 value 是一个item 
            AAA
        </value>
        <value>
            BBB
        </value>
        <value>
            CCC
        </value>
    </list>

</property>

注入Set 有两种方式：

同样， 注入set 也有两种方式： 
1 使用 <value> , 和数组或list相同， 这个时候使用的是 StringToCollectionConverter， 
2 或者使用 <set> 标签 ，对应 set 使用的和数组和list相同： CollectionToCollectionConverter ；

<property name="aSet">
    <set><value>aaa</value><value>bbb</value></set>
</property>

注入Map 或者 Properties：

而，  对于map 或者 properties 的注入， 可以使用<map> 或者 <props> ( 因为他们是可以相互转换的). 对应的 都 是MapToMapConverter 

<property name="strMap">
    <props>
        <prop key="k1">V1</prop>
        <prop key="k2">V2</prop>
    </props>
</property>
<property name="strProp">
    <map>
        <entry key="aa" value="AA"></entry>
        <entry key="bb" value="BB"></entry>
    </map>
</property>

另外， 对于 properties， 我们还可以使用 value 直接注入， 它实际使用的转换器是：StringToPropertiesConverter

格式是， 每行一个 entry，  用 = 分隔 kv ： 

<property name="strProp">
    <value>
        aa = aa
        bb = BB
    </value>
</property>

但是map 不能这样注入。 因为并不存在 StringToMapConverter