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源码

在前面关于SimpleJpaRepository的文章[地址]中可以得知,SimpleJpaRepository间接实现了JpaSpecificationExecutor接口,本文就详细探究一下该接口。

JpaSpecificationExecutor的定义如下:

/**
 * 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);
}

解读:

上述接口提供了一个findOne方法以及三个接受不同参数的findAll方法,这几个方法接受Specification类型的参数

 

示例

在实际开发中,通常按如下示例中展示的方式使用JpaSpecificationExecutor接口

Repository层:

@Repository
public interface UserRepository extends JpaRepository<User, Integer>, JpaSpecificationExecutor<User> {

}

Service层:

  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);
  }

解读:

此处Service调用了userRepository的findAll方法,参数为Specification的实例以及Pageable的实例,该findAll方法实质上是JpaSpecificationExecutor提供的findAll方法

 

Specification

从本文前面的描述得知,在调用JpaSpecificationExecutor接口提供的几个方法时需要构造Specification类型的参数。

在前面关于SimpleJpaRepository的文章[地址]中提到了构造Specification类型参数的方式:匿名内部类或者ExampleSpecification的实例,本小节来剖析一下Specification的细节。

类图

解读:

从类图可知,ByIdsSpecification、ExampleSpecification实现了Specification接口

 

进一步发掘,可以发现ByIdsSpecification、ExampleSpecification都是SimpleJpaRepository的内部类,如下图所示:

 

源码

Specification定义在包路径org.springframework.data.jpa.domain下,其定义如下:

/**
 * 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);
}

解读:

其中只有toPredicate方法抽象方法,所以通过匿名内部类的形式构造Specification的实例时只需实现toPredicate方法即可。

 

Root、Path

Specification中toPredicate方法的的第一个参数为Root<T> root,前面示例的Service层在实现toPredicate方法时通过调用如下语句获得Path类型的变量

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

下图展示了Root、Path之间的关系:

 

解读:

从上图可知,Root接口间接继承了Path接口,前述调用语句中的get方法由Path接口定义

相关代码如下:

 

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

参加[官方Doc]

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