Interface Grouper<T>

  • Type Parameters:
    T -

    public interface Grouper<T>

    User applications may implement this interface in order to customize the compution of groups in cases when the modifying the key is not possible, or when the value determined by the Group annotation needs customizing.

    Grouper acts as an interceptor, passing the previously computed value in. The group passed to the first Grouper will be that determined by @Group (if @Group is defined).

    For example:

     public class KXGrouper implements Grouper<String> {
         // A pattern that can extract from a "kX" (e.g. k1, k2) style key
         private static Pattern kPattern = Pattern.compile("(ˆk)(\\d)$");
         public String computeGroup(String key, String group) {
             Matcher matcher = kPattern.matcher(key);
             if (matcher.matches()) {
                 String g = Integer.parseInt( % 2 + "";
                 return g;
             } else
                 return null;
         public Class<String> getKeyType() {
             return String.class;

    You must set the groupsEnabled property to true in your configuration in order to use groups. You can specify an order list of groupers there.

    Pete Muir
    See Also:
    • Method Detail

      • computeGroup

        default Object computeGroup​(T key,
                                    Object group)
        Compute the group for a given key
        key - the key to compute the group for
        group - the group as currently computed, or null if no group has been determined yet
        the group, or null if no group is defined
      • getKeyType

        Class<T> getKeyType()