spring data jpa将@ManyToOne查询从cross join变成left outer join

在多对一关系中，当多端的表作为主表，并且需要连接一端的表时，默认的连接方式为cross join。在某些业务场景下，查询的数据是不完整的。因为cross join会过滤掉关联字段为null的行数据。所以需要将连接方式从cross join变成其他方式，如left outer join。

见QueryDslJpaRepository类，有如下对于QueryDsl的定义

private final EntityPath<T> path;private final PathBuilder<T> builder;private final Querydsl querydsl;public QueryDslJpaRepository(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager) {this(entityInformation, entityManager, DEFAULT_ENTITY_PATH_RESOLVER);}public QueryDslJpaRepository(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager,EntityPathResolver resolver) {super(entityInformation, entityManager);this.path = resolver.createPath(entityInformation.getJavaType());this.builder = new PathBuilder<T>(path.getType(), path.getMetadata());this.querydsl = new Querydsl(entityManager, builder);}

实现连接方式转换的关键就是要定义自己的QueryDsl，并覆写其中的createQuery方法

public class CustomQuerydsl extends Querydsl {public CustomQuerydsl(EntityManager em, PathBuilder<?> builder) {super(em, builder);}@Overridepublic AbstractJPAQuery<Object, JPAQuery<Object>> createQuery(EntityPath<?>... paths) {AbstractJPAQuery<Object, JPAQuery<Object>> jpaQuery = createQuery();for(EntityPath<?> path:paths) {jpaQuery = jpaQuery.leftJoin(path);Class<?> clazz = path.getClass();Field[] fields = clazz.getDeclaredFields();for(Field field:fields) {Class<?> type = field.getType();if(EntityPath.class.isAssignableFrom(type)&&!type.equals(path.getClass())) {try {EntityPath<?> newPath = (EntityPath<?>)field.get(path);jpaQuery = jpaQuery.leftJoin(newPath);} catch (IllegalArgumentException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();}}}}return jpaQuery;}}

然后我们需要定义一个QueryDslJpaRepository的实现类去定义我们自己的QueryDsl并让我们的Querydsl去创建查询

public class BaseDaoImpl<T, ID extends Serializable> extends QueryDslJpaRepository<T, ID> implements BaseDao<T, ID> {final Log log = LogFactory.getLog(getClass());protected final EntityManager entityManager;protected final JpaEntityInformation<T, ID> entityInformation;private static final EntityPathResolver DEFAULT_ENTITY_PATH_RESOLVER = SimpleEntityPathResolver.INSTANCE;    private final EntityPath<T> path;    private final PathBuilder<T> builder;    private final Querydsl querydsl;public BaseDaoImpl(JpaEntityInformation<T, ID> entityInformation, EntityManager entityManager,EntityPathResolver resolver) {super(entityInformation, entityManager, resolver);this.entityManager = entityManager;this.entityInformation = entityInformation;this.path = resolver.createPath(entityInformation.getJavaType());        this.builder = new PathBuilder<>(path.getType(), path.getMetadata());        this.querydsl = new CustomQuerydsl(entityManager, builder);}

同时，我们需要覆写createQuery方法，因为Querydsl在QueryDslJpaRepository中被定义为final，如果我们不覆写该方法，创建查询时用的还是QueryDslJpaRepository的QueryDsl

@Overrideprotected JPQLQuery<?> createQuery(Predicate... predicate) {AbstractJPAQuery<?, ?> query = querydsl.createQuery(path).where(predicate);CrudMethodMetadata metadata = getRepositoryMethodMetadata();if (metadata == null) {return query;}LockModeType type = metadata.getLockModeType();query = type == null ? query : query.setLockMode(type);for (Entry<String, Object> hint : getQueryHints().entrySet()) {query.setHint(hint.getKey(), hint.getValue());}return query;}

这样ManyToOne的查询就从cross join转换成left outer join