Spring data JPA

最近使用了spring data jpa来完成数据访问层的实现。感觉比较强大,也比较复杂,中间还有不少限制。

话说数据库sql的应用有点像打怪升级,一点一点不断增加难度。

1. 对于一般应用中各类简单的增删查改,spring data提供了根据名字直接查询的代理方法,啥都不需要做,唯一需要编写接口,命名方法,这部分实在是太方便了,而且简单查询解决了差不多80%的问题。这部分相对简单,不再赘述,参考用法大全部分。

2. 对于复杂但又相对固定的查询,可以使用JPQL和Native Sql,即@Query直接写JPQL语句的方式。这部分也不难,简单给个例子,需要注意的是返回结果如果是多个值并且返回多组,那应该以Object[][]表示

@Query(value = "SELECT su.id, su.name_CN, avg(s.rate), count(b.id), count(concat(b.key, '@', s.admin)) "  
            + "FROM  " + CommonConstants.SCHEMA_PREFIX + "Submarket su,  " + CommonConstants.SCHEMA_PREFIX + "Building b,  " + CommonConstants.SCHEMA_PREFIX + "Space s,  " + CommonConstants.SCHEMA_PREFIX + "Market m,  " + CommonConstants.SCHEMA_PREFIX + "City c "  
            + "where b.submarket_id = su.id and s.building_id = b.id and su.market_id = m.id and m.city_id = c.id and c.country_id = ?1 group by su.id", nativeQuery=true)  
    Object[][] findAreaInfoByCountryId(int parentId);

3. 对于复杂且动态的查询,使用Criteria。对于criteria的用法,那就有相当多的内容了。需要使用到criteria的场景通常是界面上有各种过滤和条件选项。

Criteria由于需要使用类的方式将整个复杂sql语句组织起来,因此有不少类,我们先来理解下这些类的含义。

3.1大多数情况下,搜索返回的结果是一个数据库中的实体对象,对于这种情况,实际上可以直接使用spring data jpa提供的toPredicate方法,该方法定义如下

public interface Specification<T> {  

    Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder cb);  
}

实际使用时只需要把预定义好的Repository对象继承JpaSpecificationExecutor对象即可

@Repository    
public interface CityDao extends JpaSpecificationExecutor<City>{    
}

真正调用时只需要传递如下回调方法,spring会自动帮你完成分页

Page<City> page = cityDao.findAll(new Specification<City>() {    
            @Override    
            public Predicate toPredicate(Root<City> root, CriteriaQuery<?> query, CriteriaBuilder cb) {    
                root = query.from(City.class);    
                Path<String> nameExp = root.get("name");    
                return cb.like(nameExp, "%北京%");    
            }    

        }, new PageRequest(1, 5, new Sort(Direction.DESC, new String[] { "id" })));

对于这种情况,虽然动态查询比较复杂,但是要庆幸是其中相当简单的类型了。

3.2 我们来看boss升级难度以后的情况。假如此时你的查询中不是一个简单实体类型,而是一个复杂的聚合对象,有一堆聚合查询,有一堆a对象的属性,一堆b对象的属性。可能你还试图用toPredicate方法继续,但实际上Page只允许你传递已定义好的数据库中的实体对象,因为其root中定义的泛型实际上限制了后续的行为,比如想在root上join,如果root不是一个数据库实体则编译就报错了。另外由此引发的query.multiselect自定义查询结果无效,因为结果默认就是你定义好的那个实体。这时候的解决办法就是自定义dao实现类。首先,定义一个自定义实现接口

@NoRepositoryBean  
public interface SearchJpaRepositoryCustom {  

    public Page<Tuple> searchListing(final ListingSearchContext searchContext, Pageable pageable);  

}

其次,dao接口得继承该自定义接口

public interface BuildingRepository extends PagingAndSortingRepository<Building, Integer>, SearchJpaRepositoryCustom

然后,真正的dao模型实现类如下,需要注意,自定义的实现类必须实现自定义接口,并且,名字是BuildingRepository+impl,注意这里踩过坑

public class BuildingRepositoryImpl extends PagableRepository implements SearchJpaRepositoryCustom {  

    @PersistenceContext  
    private EntityManager em;  

    private Join<Space, ?> getSearchExpression(final CriteriaBuilder cb, final ListingSearchContext searchContext,  
            final Root<Space> root, final Predicate predicate) {  
        List<Expression<Boolean>> expressions = predicate.getExpressions();  
        // 只搜索版本为0的(即当前版本)  
        expressions.add(cb.equal(root.<String> get("ver"), 0));  
        if (searchContext.getSpaceId() > 0) {  
            expressions.add(cb.equal(root.<Integer> get("id"), searchContext.getSpaceId())); // id  
        }  
        if (null != searchContext.getMinRate()) {  
            expressions.add(cb.greaterThanOrEqualTo(root.<BigDecimal> get("rate"), searchContext.getMinRate())); // 价格  
        }  
        if (null != searchContext.getMaxRate()) {  
            expressions.add(cb.lessThanOrEqualTo(root.<BigDecimal> get("rate"), searchContext.getMaxRate())); // 价格  
        }  
        if (null != searchContext.getLCD()) {  
            expressions.add(cb.lessThanOrEqualTo(root.<Date> get("dateAvailable"), searchContext.getLCD())); // 可用日期  
        }  
        // spaceTypeId  
        if (searchContext.getSpaceTypeId() > 0) {  
            expressions.add(cb.equal(root.<String> get("spaceType").get("id"), searchContext.getSpaceTypeId()));  
        }  
        // buildingGrade&submarket  
        Join<Space, ?> buildingJoin = root.join(root.getModel().getSingularAttribute("building"), JoinType.INNER);  
        if (searchContext.getBuildingGradeId() > 0) {  
            expressions.add(cb.equal(buildingJoin.get("buildingGrade").get("id"), searchContext.getBuildingGradeId()));  
        }  
        if (searchContext.getSubmarketId() > 0) {  
            expressions.add(cb.equal(buildingJoin.get("submarket").get("id"), searchContext.getSubmarketId()));  
        }  
        if (StringUtils.isNotEmpty(searchContext.getKeyword())) {  
            Predicate like1 = cb.like(buildingJoin.<String> get("buildingNameCn"),  
                    "%" + searchContext.getKeyword() + "%");  
            Predicate like2 = cb.like(buildingJoin.<String> get("addressCn"), "%" + searchContext.getKeyword() + "%");  
            expressions.add(cb.or(like1, like2)); // 关键字  
        }  
        return buildingJoin;  
    }  

    @Override  
    public Page<Tuple> searchListing(final ListingSearchContext searchContext, Pageable pageable) {  
        final CriteriaBuilder cb = em.getCriteriaBuilder();  
        final CriteriaQuery<Tuple> query = cb.createTupleQuery();  
        final Root<Space> root = query.from(Space.class);  

        Predicate predicate = cb.conjunction();  

        Join<Space, ?> buildingJoin = getSearchExpression(cb, searchContext, root, predicate);  

        Join<Space, ?> spaceTypeJoin = root.join(root.getModel().getSingularAttribute("spaceType"), JoinType.INNER);  

        Join<Space, ?> contiguousJoin = root.join(root.getModel().getSingularAttribute("contiguous"), JoinType.INNER);  

        Join<Building, ?> assetJoin = buildingJoin.join("asset", JoinType.INNER);  

        Join<BuildingGrade, ?> buildingGradeJoin = buildingJoin.join("buildingGrade", JoinType.INNER);  

        SetJoin<Asset, ?> mediaJoin = assetJoin.joinSet("medias");  

        mediaJoin.on(cb.and(cb.equal(mediaJoin.get("type"), "photo"), cb.equal(mediaJoin.get("subtype"), "main")));  

        Expression<BigDecimal> maxConExp = cb.max(contiguousJoin.<BigDecimal> get("maxContiguous"));  
        Expression<BigDecimal> totalConExp = cb.sum(root.<BigDecimal> get("size"));  
        query.multiselect(cb.count(root.<Integer> get("id")), root.<Integer> get("userByAdmin").get("id"), totalConExp,  
                maxConExp, cb.min(root.<BigDecimal> get("minDivisible")), root.<Integer> get("building"),  
                cb.max(root.<Integer> get("stage")), cb.min(root.<Integer> get("lcd")),  
                cb.min(root.<Integer> get("led")), cb.min(root.<Integer> get("floor")),  
                cb.max(root.<Integer> get("floor")), mediaJoin.get("path"), spaceTypeJoin.get("nameEn"),  
                buildingGradeJoin.get("nameEn"));  

        query.where(predicate);  
        query.orderBy(cb.desc(root.get("gmtCreate").as(Date.class)));  
        query.groupBy(root.<Integer> get("building").get("id"), root.<String> get("userByAdmin").get("id"));  

        Predicate minExp = null;  
        Predicate maxExp = null;  
        Predicate minMaxResultExp = null;  
        if (null != searchContext.getMinSize()) {  
            minExp = cb.greaterThanOrEqualTo(cb.min(root.<BigDecimal> get("minDivisible")), searchContext.getMinSize()); // 最小面积  
            minMaxResultExp = minExp;  
        }  
        if (null != searchContext.getMaxSize()) {  
            maxExp = cb.lessThanOrEqualTo(searchContext.isContiguous() ? maxConExp : totalConExp,  
                    searchContext.getMaxSize()); // 最大面积  
            minMaxResultExp = maxExp;  
        }  
        if (null != searchContext.getMinSize() && null != searchContext.getMaxSize()) {  
            minMaxResultExp = cb.or(minExp, maxExp);  
        }  
        if (null != minMaxResultExp) {  
            query.having(minMaxResultExp);  
        }  
        TypedQuery<Tuple> pagableQuery = em.createQuery(query);  
        return pageable == null ? new PageImpl<Tuple>(pagableQuery.getResultList())  
                : readPage(pagableQuery, pageable);  
    }  

}

3.3 难度又升级了,解决了上面的问题以后,面临新的问题:分页。使用toPredicate时,spring已经实现了分页,而自定义实现类后,分页也需要自己实现。

首当其冲的问题就是count,如果在3.2中用了group by这种操作,而此处分页时候又想统计分页结果,那么实际上就是想count 分组数。常见做法是:

select count(*) from  
(select ... group by ...)

但是实际上jpa不支持这种类型的子查询,本人这里试了很久各种方法,实际上都行不通。参考文末“特殊情况的count写法”

最终使用了将 criteria还原为hql,通过translator组装成native的sql,附加上参数,最终执行的方式,通过后的参考代码如下

protected Long executeCountQuery(final ListingSearchContext searchContext, TypedQuery<Tuple> query) {  

        String hqlString = query.unwrap(Query.class).getQueryString();  
        QueryTranslatorFactory translatorFactory = new ASTQueryTranslatorFactory();  
        Query hibernateQuery = ((HibernateQuery)query).getHibernateQuery();  
        QueryImpl hImpl = (QueryImpl)hibernateQuery;  
        Map<String, TypedValue> paramMap = (Map<String, TypedValue>)ReflectionUtils.getFieldValue(hImpl,"namedParameters");  
        QueryTranslator translator = translatorFactory.createQueryTranslator(hqlString, hqlString, paramMap,  
                (SessionFactoryImplementor) emf.unwrap(SessionFactory.class), null);  
        translator.compile(paramMap, false);  

        javax.persistence.Query nativeQuery = em  
                .createNativeQuery("select count(*) from (" + translator.getSQLString() + ") x");  
        ParameterTranslations parameterTranslations = translator.getParameterTranslations();  

        for(String name : paramMap.keySet()){  
            for (int position : parameterTranslations.getNamedParameterSqlLocations(name)) {  
                nativeQuery.setParameter(position + 1, paramMap.get(name).getValue());  
            }  
        }  
        Long cnt = ((Number) nativeQuery.getSingleResult()).longValue();  
        return cnt;  
    }

通过利用已有的分页类,实现自定义的分页

protected Page<Tuple> readPage(final ListingSearchContext searchContext, TypedQuery<Tuple> query, Pageable pageable) {  

        query.setFirstResult(pageable.getOffset());  
        query.setMaxResults(pageable.getPageSize());  
        Long total = executeCountQuery(searchContext, query);  
        List<Tuple> content = total > pageable.getOffset() ? query.getResultList() : Collections.<Tuple> emptyList();  

        return new PageImpl<Tuple>(content, pageable, total);  
    }

4. 复杂sql的简化。对于一些明显复杂的sql(比如需要经过多次嵌套子查询的sql),建议将该查询简化,简化的方式无非是修改模型,增加适应简化查询的的新方法。

 

用法大全

spring data reference http://docs.spring.io/spring-data/data-jpa/docs/current/reference/html/#specifications
jpa api用法汇总 http://www.cnblogs.com/xingqi/p/3929386.html

http://www.objectdb.com/Java/jpa/query/jpql/structure

jpa subquery例子大全 http://www.programcreek.com/java-api-examples/index.PHP?api=javax.persistence.criteria.Subquery
specification例子http://www.programcreek.com/java-api-examples/index.php?api=org.springframework.data.jpa.domain.Specification
spring hql ASTQueryTranslatorFactory例子 http://www.programcreek.com/java-api-examples/index.php?api=org.hibernate.hql.internal.ast.ASTQueryTranslatorFactory
spring Query 例子 http://www.programcreek.com/java-api-examples/org.hibernate.Query

JPA动态类型安全查询 https://www.ibm.com/developerworks/cn/java/j-typesafejpa/

mybatis问题集 http://www.diguage.com/archives/47.html

http://m.oschina.NET/blog/133500

 

join用法

http://www.altuure.com/2010/09/23/jpa-criteria-api-by-samples-%E2%80%93-part-ii/

http://info.michael-simons.eu/2012/09/25/jpa-criteria-query-or-plain-sql/

 

子查询用法

http://stackoverflow.com/questions/4483576/jpa-2-0-criteria-api-subqueries-in-expressions

 

特殊情况的count写法

http://jdevelopment.nl/counting-rows-returned-jpa-query/



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