简介
ConversionService 是 Spring 框架中的一个接口，它提供了一种机制来执行类型之间的转换。ConversionService 允许开发者定义自己的转换逻辑，并注册到服务中，从而可以在运行时动态地转换对象。这种机制在数据绑定、服务层方法参数转换以及任何需要类型转换的场景中都非常有用。
ConversionService 接口定义了一个简单的方法 convert(Object source, Class<?> targetType)，它尝试将源对象转换为目标类型的实例。如果转换不可能或未定义，该方法通常会抛出一个异常。
Spring 提供了 ConversionService 的多个实现，其中 DefaultConversionService 是最常用的一个。DefaultConversionService 提供了对许多内置类型转换的支持，包括基本数据类型、字符串、集合和数组之间的转换等。此外，开发者还可以通过实现 Converter 或 ConditionalConverter 接口，并将自定义的转换器注册到 ConversionService 中，来扩展转换功能。
源码
public interface ConversionService {
/**
* Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
* <p>If this method returns {@code true}, it means {@link #convert(Object, Class)} is capable
* of converting an instance of {@code sourceType} to {@code targetType}.
* <p>Special note on collections, arrays, and maps types:
* For conversion between collection, array, and map types, this method will return {@code true}
* even though a convert invocation may still generate a {@link ConversionException} if the
* underlying elements are not convertible. Callers are expected to handle this exceptional case
* when working with collections and maps.
* @param sourceType the source type to convert from (may be {@code null} if source is {@code null})
* @param targetType the target type to convert to (required)
* @return {@code true} if a conversion can be performed, {@code false} if not
* @throws IllegalArgumentException if {@code targetType} is {@code null}
* 是否可以从sourceType转换为targetType
*/
boolean canConvert(@Nullable Class<?> sourceType, Class<?> targetType);
/**
* Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
* The TypeDescriptors provide additional context about the source and target locations
* where conversion would occur, often object fields or property locations.
* <p>If this method returns {@code true}, it means {@link #convert(Object, TypeDescriptor, TypeDescriptor)}
* is capable of converting an instance of {@code sourceType} to {@code targetType}.
* <p>Special note on collections, arrays, and maps types:
* For conversion between collection, array, and map types, this method will return {@code true}
* even though a convert invocation may still generate a {@link ConversionException} if the
* underlying elements are not convertible. Callers are expected to handle this exceptional case
* when working with collections and maps.
* @param sourceType context about the source type to convert from
* (may be {@code null} if source is {@code null})
* @param targetType context about the target type to convert to (required)
* @return {@code true} if a conversion can be performed between the source and target types,
* {@code false} if not
* @throws IllegalArgumentException if {@code targetType} is {@code null}
* 同上
*/
boolean canConvert(@Nullable TypeDescriptor sourceType, TypeDescriptor targetType);
/**
* Convert the given {@code source} to the specified {@code targetType}.
* @param source the source object to convert (may be {@code null})
* @param targetType the target type to convert to (required)
* @return the converted object, an instance of targetType
* @throws ConversionException if a conversion exception occurred
* @throws IllegalArgumentException if targetType is {@code null}
* 将source转换为targetType
*/
@Nullable
<T> T convert(@Nullable Object source, Class<T> targetType);
/**
* Convert the given {@code source} to the specified {@code targetType}.
* The TypeDescriptors provide additional context about the source and target locations
* where conversion will occur, often object fields or property locations.
* @param source the source object to convert (may be {@code null})
* @param sourceType context about the source type to convert from
* (may be {@code null} if source is {@code null})
* @param targetType context about the target type to convert to (required)
* @return the converted object, an instance of {@link TypeDescriptor#getObjectType() targetType}
* @throws ConversionException if a conversion exception occurred
* @throws IllegalArgumentException if targetType is {@code null},
* or {@code sourceType} is {@code null} but source is not {@code null}
* 同上
*/
@Nullable
Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType);
}
示例
import org.springframework.context.support.ConversionServiceFactoryBean;
import org.springframework.format.support.DefaultFormattingConversionService;
public class ConversionServiceExample {
public static void main(String[] args) {
// 创建一个 ConversionService 实例
ConversionService conversionService = new DefaultFormattingConversionService();
// 注册自定义转换器
conversionService.addConverter(new MyCustomConverter());
// 执行类型转换
String sourceValue = "123";
Integer targetValue = conversionService.convert(sourceValue, Integer.class);
System.out.println(targetValue); // 输出：123
}
// 自定义转换器
static class MyCustomConverter implements Converter<String, Integer> {
@Override
public Integer convert(String source) {
// 自定义转换逻辑
return Integer.parseInt(source);
}
}
}
在这个例子中，我们首先创建了一个 DefaultFormattingConversionService 实例。然后，我们注册了一个自定义的转换器 MyCustomConverter，它将字符串转换为整数。最后，我们使用 ConversionService 的 convert 方法将字符串 “123” 转换为 Integer 对象。
除了 Converter，Spring 还提供了 GenericConverter 接口，它允许更复杂的转换逻辑，包括处理源类型和目标类型都是泛型参数的情况。
ConversionService 在 Spring MVC 中也扮演着重要角色，它用于将请求参数绑定到控制器方法的参数上时进行类型转换。通过配置 Spring MVC 的 FormattingConversionService 或自定义 ConversionService，你可以控制请求参数的转换行为。
总之，ConversionService 是 Spring 框架中用于类型转换的强大工具，它提供了灵活且可扩展的转换机制，适用于各种需要类型转换的场景。
