Spring Data JPA：解析JpaSpecificationExecutor & Specification
- 2021 年 8 月 28 日
- 筆記
- Java--Spring
源碼
在前面關於SimpleJpaRepository的文章[地址]中可以得知，SimpleJpaRepository間接實現了JpaSpecificationExecutor接口，本文就詳細探究一下該接口。
JpaSpecificationExecutor的定義如下：
/** * Interface to allow execution of {@link Specification}s based on the JPA criteria API. * * @author Oliver Gierke * @author Christoph Strobl */ public interface JpaSpecificationExecutor<T> { /** * Returns a single entity matching the given {@link Specification} or {@link Optional#empty()} if none found. * * @param spec can be {@literal null}. * @return never {@literal null}. * @throws org.springframework.dao.IncorrectResultSizeDataAccessException if more than one entity found. */ Optional<T> findOne(@Nullable Specification<T> spec); /** * Returns all entities matching the given {@link Specification}. * * @param spec can be {@literal null}. * @return never {@literal null}. */ List<T> findAll(@Nullable Specification<T> spec); /** * Returns a {@link Page} of entities matching the given {@link Specification}. * * @param spec can be {@literal null}. * @param pageable must not be {@literal null}. * @return never {@literal null}. */ Page<T> findAll(@Nullable Specification<T> spec, Pageable pageable); /** * Returns all entities matching the given {@link Specification} and {@link Sort}. * * @param spec can be {@literal null}. * @param sort must not be {@literal null}. * @return never {@literal null}. */ List<T> findAll(@Nullable Specification<T> spec, Sort sort); /** * Returns the number of instances that the given {@link Specification} will return. * * @param spec the {@link Specification} to count instances for. Can be {@literal null}. * @return the number of instances. */ long count(@Nullable Specification<T> spec); }
解讀：
上述接口提供了一個findOne方法以及三個接受不同參數的findAll方法，這幾個方法接受Specification類型的參數
示例
在實際開發中，通常按如下示例中展示的方式使用JpaSpecificationExecutor接口
Repository層：
@Repository public interface UserRepository extends JpaRepository<User, Integer>, JpaSpecificationExecutor<User> { }
Service層：
public Page<User> getUsers(Integer id, Integer pageNum, Integer pageSize) { Sort sort = Sort.by(Sort.Direction.DESC, "id"); Pageable pageable = PageRequest.of(pageNum, pageSize, sort); Specification<User> specification = (Specification<User>) (root, query, cb) -> { Path<Integer> path = root.get("id"); return cb.lt(path, id); }; return userRepository.findAll(specification, pageable); }
解讀：
此處Service調用了userRepository的findAll方法，參數為Specification的實例以及Pageable的實例，該findAll方法實質上是JpaSpecificationExecutor提供的findAll方法
Specification
從本文前面的描述得知，在調用JpaSpecificationExecutor接口提供的幾個方法時需要構造Specification類型的參數。
在前面關於SimpleJpaRepository的文章[地址]中提到了構造Specification類型參數的方式：匿名內部類或者ExampleSpecification的實例，本小節來剖析一下Specification的細節。
類圖
解讀：
從類圖可知，ByIdsSpecification、ExampleSpecification實現了Specification接口
進一步發掘，可以發現ByIdsSpecification、ExampleSpecification都是SimpleJpaRepository的內部類，如下圖所示：
源碼
Specification定義在包路徑org.springframework.data.jpa.domain下，其定義如下：
/** * Specification in the sense of Domain Driven Design. * * @author Oliver Gierke * @author Thomas Darimont * @author Krzysztof Rzymkowski * @author Sebastian Staudt * @author Mark Paluch * @author Jens Schauder */ public interface Specification<T> extends Serializable { long serialVersionUID = 1L; /** * Negates the given {@link Specification}. * * @param <T> the type of the {@link Root} the resulting {@literal Specification} operates on. * @param spec can be {@literal null}. * @return guaranteed to be not {@literal null}. * @since 2.0 */ static <T> Specification<T> not(@Nullable Specification<T> spec) { return spec == null // ? (root, query, builder) -> null // : (root, query, builder) -> builder.not(spec.toPredicate(root, query, builder)); } /** * Simple static factory method to add some syntactic sugar around a {@link Specification}. * * @param <T> the type of the {@link Root} the resulting {@literal Specification} operates on. * @param spec can be {@literal null}. * @return guaranteed to be not {@literal null}. * @since 2.0 */ static <T> Specification<T> where(@Nullable Specification<T> spec) { return spec == null ? (root, query, builder) -> null : spec; } /** * ANDs the given {@link Specification} to the current one. * * @param other can be {@literal null}. * @return The conjunction of the specifications * @since 2.0 */ default Specification<T> and(@Nullable Specification<T> other) { return SpecificationComposition.composed(this, other, CriteriaBuilder::and); } /** * ORs the given specification to the current one. * * @param other can be {@literal null}. * @return The disjunction of the specifications * @since 2.0 */ default Specification<T> or(@Nullable Specification<T> other) { return SpecificationComposition.composed(this, other, CriteriaBuilder::or); } /** * Creates a WHERE clause for a query of the referenced entity in form of a {@link Predicate} for the given * {@link Root} and {@link CriteriaQuery}. * * @param root must not be {@literal null}. * @param query must not be {@literal null}. * @param criteriaBuilder must not be {@literal null}. * @return a {@link Predicate}, may be {@literal null}. */ @Nullable Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder); }
解讀：
其中只有toPredicate方法抽象方法，所以通過匿名內部類的形式構造Specification的實例時只需實現toPredicate方法即可。
Root、Path
Specification中toPredicate方法的的第一個參數為Root<T> root，前面示例的Service層在實現toPredicate方法時通過調用如下語句獲得Path類型的變量
Path<Integer> path = root.get("id");
Root、Path均定義在包路徑javax.persistence.criteria下。
下圖展示了Root、Path之間的關係：
解讀：
從上圖可知，Root接口間接繼承了Path接口，前述調用語句中的get方法由Path接口定義
相關代碼如下：
<Y> Path<Y> get(String attributeName);
參加[官方Doc]
