Spring Type Conversion(Spring类型转换)

1:概述:

Spring3引入了core.convert包,提供了通用类型转换系统,定义了实现类型转换和运行时执行类型的SPI。

在Spring3.0之前,提供的PropertyEditor来将外部化bean属性值字符串转换成必需的实现类型。

2:Converter SPI

/**
 * A converter converts a source object of type {@code S} to a target of type {@code T}.
 *
 * 
Implementations of this interface are thread-safe and can be shared.
 *
 * 
Implementations may additionally implement {@link ConditionalConverter}.
 *
 * @author Keith Donald
 * @since 3.0
 * @param  the source type
 * @param  the target type
 */
@FunctionalInterface
public interface Converter {

	/**
	 * Convert the source object of type {@code S} to target type {@code T}.
	 * @param source the source object to convert, which must be an instance of {@code S} (never {@code null})
	 * @return the converted object, which must be an instance of {@code T} (potentially {@code null})
	 * @throws IllegalArgumentException if the source cannot be converted to the desired target type
	 */
	@Nullable
	T convert(S source);

}

实现自定义的类型转换可以实现Converter接口。但是如果S是集合或者数组转换为T的集合或者数组,

建议参考诸如ArrayToCollectionConverter实现。前提是已经注册了委托数组或集合转换器。例如,

DefaultConversionService实现。

Converter.convert(S source)中source确保不能为null,否则转换器可能抛出异常如果转换失败。具体

说,应该会抛出IllegalArgumentException报告不合理的转换源。确保Converter实现是线程安全。

在core.convert.support包下,注册了常见了类型转换器。例如:

/**
 * Converts from a String any JDK-standard Number implementation.
 *
 * 
Support Number classes including Byte, Short, Integer, Float, Double, Long, BigInteger, BigDecimal. This class
 * delegates to {@link NumberUtils#parseNumber(String, Class)} to perform the conversion.
 *
 * @author Keith Donald
 * @since 3.0
 * @see java.lang.Byte
 * @see java.lang.Short
 * @see java.lang.Integer
 * @see java.lang.Long
 * @see java.math.BigInteger
 * @see java.lang.Float
 * @see java.lang.Double
 * @see java.math.BigDecimal
 * @see NumberUtils
 */
final class StringToNumberConverterFactory implements ConverterFactory {

	@Override
	public  Converter getConverter(Class targetType) {
		return new StringToNumber<>(targetType);
	}


	private static final class StringToNumber implements Converter {

		private final Class targetType;

		public StringToNumber(Class targetType) {
			this.targetType = targetType;
		}

		@Override
		public T convert(String source) {
			if (source.isEmpty()) {
				return null;
			}
			return NumberUtils.parseNumber(source, this.targetType);
		}
	}

}

3:ConverterFactory

当你需要集中整理类层次结构的类型转换器,可以使用ConverterFactory。例如StringToNumberConverterFactory,

该接口定义如下,当你需要范围转换器,可以转换这些对象从S类型转换成R的子类型。使用该接口。

/**
 * A factory for "ranged" converters that can convert objects from S to subtypes of R.
 *
 * 
Implementations may additionally implement {@link ConditionalConverter}.
 *
 * @author Keith Donald
 * @since 3.0
 * @see ConditionalConverter
 * @param  the source type converters created by this factory can convert from
 * @param  the target range (or base) type converters created by this factory can convert to;
 * for example {@link Number} for a set of number subtypes.
 */
public interface ConverterFactory {

	/**
	 * Get the converter to convert from S to target type T, where T is also an instance of R.
	 * @param  the target type
	 * @param targetType the target type to convert to
	 * @return a converter from S to T
	 */
	 Converter getConverter(Class targetType);

}

/**
 * Converts from a String any JDK-standard Number implementation.
 *
 * 
Support Number classes including Byte, Short, Integer, Float, Double, Long, BigInteger, BigDecimal. This class
 * delegates to {@link NumberUtils#parseNumber(String, Class)} to perform the conversion.
 *
 * @author Keith Donald
 * @since 3.0
 * @see java.lang.Byte
 * @see java.lang.Short
 * @see java.lang.Integer
 * @see java.lang.Long
 * @see java.math.BigInteger
 * @see java.lang.Float
 * @see java.lang.Double
 * @see java.math.BigDecimal
 * @see NumberUtils
 */
final class StringToNumberConverterFactory implements ConverterFactory {

	@Override
	public  Converter getConverter(Class targetType) {
		return new StringToNumber<>(targetType);
	}


	private static final class StringToNumber implements Converter {

		private final Class targetType;

		public StringToNumber(Class targetType) {
			this.targetType = targetType;
		}

		@Override
		public T convert(String source) {
			if (source.isEmpty()) {
				return null;
			}
			return NumberUtils.parseNumber(source, this.targetType);
		}
	}

}

4:GenericConverter

GenericConverter提供多种源和目标类型之间转换,比Converter更灵活但是对类型要求不高。它提供了实现

转换逻辑的源和目标上下文。 这样的上下文允许类型转换由字段注释或在字段签名上声明的通用信息驱动。接口

如下:

package org.springframework.core.convert.converter;

public interface GenericConverter {

    public Set getConvertibleTypes();

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}

ConvertiblePair持有转换源和目标类型对。convert(Object, TypeDescriptor, TypeDescriptor)。

源TypeDescriptor提供对保存正在转换的值的源字段的访问。 目标TypeDescriptor提供对要设置转换值的目标字段的访问。TypeDescriptor类是关于要转换类型的上下文。

一个好的实例是GenericConverter在Java数组和集合之间转换。例如ArrayToCollectionConverter。

注意

因为GenericConverter是一个更复杂的SPI接口,所以只有在需要时才应该使用它.喜欢Converter或ConverterFactory以满足基本的类型转换需求。

5:ConditionalGenericConverter

该接口是一个带有判断条件的类型转换器。该接口是GenericConverter和ConditionalConverter的组合。

/**
 * A {@link GenericConverter} that may conditionally execute based on attributes
 * of the {@code source} and {@code target} {@link TypeDescriptor}.
 *
 * 
See {@link ConditionalConverter} for details.
 *
 * @author Keith Donald
 * @author Phillip Webb
 * @since 3.0
 * @see GenericConverter
 * @see ConditionalConverter
 */
public interface ConditionalGenericConverter extends GenericConverter, ConditionalConverter {

}

ConditionalGenericConverter 的一个好示例是StringToCollectionConverter

/**
 * Converts a comma-delimited String to a Collection.
 * If the target collection element type is declared, only matches if
 * {@code String.class} can be converted to it.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @since 3.0
 */
final class StringToCollectionConverter implements ConditionalGenericConverter {

	private final ConversionService conversionService;


	public StringToCollectionConverter(ConversionService conversionService) {
		this.conversionService = conversionService;
	}


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

	@Override
	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
		return (targetType.getElementTypeDescriptor() == null ||
				this.conversionService.canConvert(sourceType, targetType.getElementTypeDescriptor()));
	}

	@Override
	@Nullable
	public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
		if (source == null) {
			return null;
		}
		String string = (String) source;

		String[] fields = StringUtils.commaDelimitedListToStringArray(string);
		TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();
		Collection