Struts2 报Conversion registration error

import org.apache.commons.lang.StringUtils;
import org.apache.struts2.util.StrutsTypeConverter;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import java.util.Map;

public class DateConverter extends StrutsTypeConverter {
    private SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");

    public DateConverter(String formatPattern) {
        if (StringUtils.isNotBlank(formatPattern)) {
            format = new SimpleDateFormat(formatPattern);
        }
    }

    @Override
    public Object convertFromString(Map ctx, String[] value, Class arg2) {
        if (value[0] == null || value[0].trim().equals("")) {
            return null;
        }

        try {
            return format.parse(value[0]);
        } catch (ParseException pe) {
            pe.printStackTrace();
        }
        return null;
    }

    @Override
    public String convertToString(Map ctx, Object data) {
        return format.format(data);
    }
}