Spring Data JPA：解析JpaSpecificationExecutor & Specification

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]