@Converter
public class StringSetConverter implements AttributeConverter<Set<String>, String> {

	public static final String SEPARATOR = ",";

	@Override
	public String convertToDatabaseColumn(Set<String> set) {
		if (set == null)
			return null;
		if (set.isEmpty())
			return "";
		return StringUtils.join(set.iterator(), SEPARATOR);
	}

	@Override
	public Set<String> convertToEntityAttribute(String string) {
		if (string == null)
			return null;
		Set<String> set = new LinkedHashSet<>();
		if (StringUtils.isNotBlank(string))
			set.addAll(Arrays.asList(string.split(SEPARATOR)));
		return set;
	}

}

@Entity
public class Post implements Serializable {

        ...

	@Convert(converter=StringSetConverter.class)
	private Set<String> tags = new LinkedHashSet<>(0);

	public Set<String> getTags() {
		return tags;
	}

	public void setTags(Set<String> tags) {
		this.tags = tags;
	}

        ...

}

Restrictions.eq("tags", Collections.singleton(tag));

Restrictions.eq("tags", tag); //tag is target type already, shouldn't be converted 

Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.util.Set
        at org.ironrhino.core.hibernate.convert.StringSetConverter.convertToDatabaseColumn(StringSetConverter.java:1) ~[classes/:?]
        at org.hibernate.type.descriptor.converter.AttributeConverterSqlTypeDescriptorAdapter$1.bind(AttributeConverterSqlTypeDescriptorAdapter.java:78) ~[hibernate-core-5.1.0.
jar:5.1.0.Final]