import org.apache.commons.dbutils.handlers.BeanListHandler;这个包，BeanListHandler让我发现List<Friend> list = (List<Friend>) runner.query(sql, new BeanHandler<Friend>(Friend.class),username);里的new BeanHandler<Friend>(Friend.class)是错误的，应该：new BeanListHandler<Friend>(Friend.class)

javabean.Friend cannot be cast to java.util.List