Spring Converter

@FunctionalInterface
public interface Converter<S, T> {

	/**
	 * Convert the source object of type {@code S} to target type {@code T}.
	 * 从 source 转换成 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);

}

final class ObjectToStringConverter implements Converter<Object, String> {

   @Override
   public String convert(Object source) {
      return source.toString();
   }

}

class StringToCharsetConverter implements Converter<String, Charset> {

   @Override
   public Charset convert(String source) {
      return Charset.forName(source);
   }

}