JPA使用CriteriaQuery实现动态分组查询

JPA中实现动态分组查询，即输入几个筛选参数就按照几个参数进行分组查询，但是不知道输入的是几个参数，要实现动态的分组查询，用CriteriaQuery实现。

Repository：

@Repository
public interface TestCostRepository extends JpaRepository<TestCost, Integer>, JpaSpecificationExecutor<TestCost> {
}

TestCostService：

public interface TestCostService {
    List<Map<String,String>> dynamicGroupBy(List<String> groupByParams,String sumParam);
}

TestCostServiceImpl：

@Service
public class TestCostServiceImpl implements TestCostService {

    @Autowired
    TestCostRepository repository;

    @Autowired
    private EntityManagerFactory entityManagerFactory;

    @Override
    public List<Map<String, String>> dynamicGroupBy(List<String> groupByParams, String sumParam) {
        EntityManager entityManager = entityManagerFactory.createEntityManager();
        CriteriaBuilder cb = entityManager.getCriteriaBuilder();
        CriteriaQuery<Object[]> query = cb.createQuery(Object[].class);
        Root<TestCost> root = query.from(TestCost.class);

        List<Expression<?>> expressions = new ArrayList<>();
        for (String field : groupByParams) {
            expressions.add(root.get(field));
        }
        query.groupBy(expressions.toArray(new Expression[0]));

        Expression<BigDecimal> sumExpression = cb.sum(root.get(sumParam));
        expressions.add(sumExpression);
        query.multiselect(expressions.toArray(new Expression[0]));

        TypedQuery<Object[]> typedQuery = entityManager.createQuery(query);
        List<Object[]> resultList = typedQuery.getResultList();

        List<Map<String, String>> returnList = new ArrayList<>();
        for (Object[] result : resultList) {
            Map<String, String> returnMap = new LinkedHashMap<>();
            returnMap.put("name", (String) result[0]);
            returnMap.put("gender", (String) result[1]);
            returnMap.put("cost", result[2].toString());
            returnList.add(returnMap);
        }

        return returnList;
    }

}