原始碼

在前面關於SimpleJpaRepository的文章[地址]中可以得知，SimpleJpaRepository間接實現了JpaSpecificationExecutor介面，本文就詳細探究一下該介面。

JpaSpecificationExecutor的定義如下：

  1. /**
  2.  * Interface to allow execution of {@link Specification}s based on the JPA criteria API.
  3.  *
  4.  * @author Oliver Gierke
  5.  * @author Christoph Strobl
  6.  */
  7. public interface JpaSpecificationExecutor<T> {
  8.  
  9.     /**
  10.      * Returns a single entity matching the given {@link Specification} or {@link Optional#empty()} if none found.
  11.      *
  12.      * @param spec can be {@literal null}.
  13.      * @return never {@literal null}.
  14.      * @throws org.springframework.dao.IncorrectResultSizeDataAccessException if more than one entity found.
  15.      */
  16.     Optional<T> findOne(@Nullable Specification<T> spec);
  17.  
  18.     /**
  19.      * Returns all entities matching the given {@link Specification}.
  20.      *
  21.      * @param spec can be {@literal null}.
  22.      * @return never {@literal null}.
  23.      */
  24.     List<T> findAll(@Nullable Specification<T> spec);
  25.  
  26.     /**
  27.      * Returns a {@link Page} of entities matching the given {@link Specification}.
  28.      *
  29.      * @param spec can be {@literal null}.
  30.      * @param pageable must not be {@literal null}.
  31.      * @return never {@literal null}.
  32.      */
  33.     Page<T> findAll(@Nullable Specification<T> spec, Pageable pageable);
  34.  
  35.     /**
  36.      * Returns all entities matching the given {@link Specification} and {@link Sort}.
  37.      *
  38.      * @param spec can be {@literal null}.
  39.      * @param sort must not be {@literal null}.
  40.      * @return never {@literal null}.
  41.      */
  42.     List<T> findAll(@Nullable Specification<T> spec, Sort sort);
  43.  
  44.     /**
  45.      * Returns the number of instances that the given {@link Specification} will return.
  46.      *
  47.      * @param spec the {@link Specification} to count instances for. Can be {@literal null}.
  48.      * @return the number of instances.
  49.      */
  50.     long count(@Nullable Specification<T> spec);
  51. }

解讀：

上述介面提供了一個findOne方法以及三個接受不同引數的findAll方法，這幾個方法接受Specification型別的引數

示例

在實際開發中，通常按如下示例中展示的方式使用JpaSpecificationExecutor介面

Repository層：

  1. @Repository
  2. public interface UserRepository extends JpaRepository<User, Integer>, JpaSpecificationExecutor<User> {
  3.  
  4. }

Service層：

  1.   public Page<User> getUsers(Integer id, Integer pageNum, Integer pageSize) {
  2.     Sort sort = Sort.by(Sort.Direction.DESC, "id");
  3.     Pageable pageable = PageRequest.of(pageNum, pageSize, sort);
  4.  
  5.     Specification<User> specification = (Specification<User>) (root, query, cb) -> {
  6.       Path<Integer> path = root.get("id");
  7.       return cb.lt(path, id);
  8.     };
  9.     return userRepository.findAll(specification, pageable);
  10.   }

解讀：

此處Service呼叫了userRepository的findAll方法，引數為Specification的例項以及Pageable的例項，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，在呼叫JpaSpecificationExecutor介面提供的幾個方法時需要構造Specification型別的引數。

在前面關於SimpleJpaRepository的文章[地址]中提到了構造Specification型別引數的方式：匿名內部類或者ExampleSpecification的例項，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介面

進一步發掘，可以發現ByIdsSpecification、ExampleSpecification都是SimpleJpaRepository的內部類，如下圖所示：

原始碼

Specification定義在包路徑org.springframework.data.jpa.domain下，其定義如下：

  1. /**
  2.  * Specification in the sense of Domain Driven Design.
  3.  *
  4.  * @author Oliver Gierke
  5.  * @author Thomas Darimont
  6.  * @author Krzysztof Rzymkowski
  7.  * @author Sebastian Staudt
  8.  * @author Mark Paluch
  9.  * @author Jens Schauder
  10.  */
  11. public interface Specification<T> extends Serializable {
  12.  
  13.     long serialVersionUID = 1L;
  14.  
  15.     /**
  16.      * Negates the given {@link Specification}.
  17.      *
  18.      * @param <T> the type of the {@link Root} the resulting {@literal Specification} operates on.
  19.      * @param spec can be {@literal null}.
  20.      * @return guaranteed to be not {@literal null}.
  21.      * @since 2.0
  22.      */
  23.     static <T> Specification<T> not(@Nullable Specification<T> spec) {
  24.  
  25.         return spec == null //
  26.                 ? (root, query, builder) -> null //
  27.                 : (root, query, builder) -> builder.not(spec.toPredicate(root, query, builder));
  28.     }
  29.  
  30.     /**
  31.      * Simple static factory method to add some syntactic sugar around a {@link Specification}.
  32.      *
  33.      * @param <T> the type of the {@link Root} the resulting {@literal Specification} operates on.
  34.      * @param spec can be {@literal null}.
  35.      * @return guaranteed to be not {@literal null}.
  36.      * @since 2.0
  37.      */
  38.     static <T> Specification<T> where(@Nullable Specification<T> spec) {
  39.         return spec == null ? (root, query, builder) -> null : spec;
  40.     }
  41.  
  42.     /**
  43.      * ANDs the given {@link Specification} to the current one.
  44.      *
  45.      * @param other can be {@literal null}.
  46.      * @return The conjunction of the specifications
  47.      * @since 2.0
  48.      */
  49.     default Specification<T> and(@Nullable Specification<T> other) {
  50.         return SpecificationComposition.composed(this, other, CriteriaBuilder::and);
  51.     }
  52.  
  53.     /**
  54.      * ORs the given specification to the current one.
  55.      *
  56.      * @param other can be {@literal null}.
  57.      * @return The disjunction of the specifications
  58.      * @since 2.0
  59.      */
  60.     default Specification<T> or(@Nullable Specification<T> other) {
  61.         return SpecificationComposition.composed(this, other, CriteriaBuilder::or);
  62.     }
  63.  
  64.     /**
  65.      * Creates a WHERE clause for a query of the referenced entity in form of a {@link Predicate} for the given
  66.      * {@link Root} and {@link CriteriaQuery}.
  67.      *
  68.      * @param root must not be {@literal null}.
  69.      * @param query must not be {@literal null}.
  70.      * @param criteriaBuilder must not be {@literal null}.
  71.      * @return a {@link Predicate}, may be {@literal null}.
  72.      */
  73.     @Nullable
  74.     Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuilder criteriaBuilder);
  75. }

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的例項時只需實現toPredicate方法即可。

Root、Path

Specification中toPredicate方法的的第一個引數為Root<T> root，前面示例的Service層在實現toPredicate方法時通過呼叫如下語句獲得Path型別的變數

  1. Path<Integer> path = root.get("id");

下圖展示了Root、Path之間的關係：

解讀：

從上圖可知，Root介面間接繼承了Path介面，前述呼叫語句中的get方法由Path介面定義

相關程式碼如下：

  1. <Y> Path<Y> get(String attributeName);

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