Polishing.

Add SpecificationFluentQuery to include specification-related overloads. Also, add slice(…) terminal method to obtain a slice only without running a count query.

See #3727

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/JpaSpecificationExecutor.java

@@ -19,6 +19,8 @@ import jakarta.persistence.criteria.CriteriaBuilder;
 import jakarta.persistence.criteria.CriteriaQuery;
 import jakarta.persistence.criteria.Root;
 
+import java.util.Arrays;
+import java.util.Collection;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.List;
@@ -31,6 +33,7 @@ import org.jspecify.annotations.Nullable;
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.Slice;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.jpa.domain.DeleteSpecification;
 import org.springframework.data.jpa.domain.PredicateSpecification;
@@ -229,7 +232,7 @@ public interface JpaSpecificationExecutor<T> {
 	 * @since 4.0
 	 */
 	default <S extends T, R> R findBy(PredicateSpecification<T> spec,
-			Function<FluentQuery.FetchableFluentQuery<S>, R> queryFunction) {
+			Function<? super SpecificationFluentQuery<S>, R> queryFunction) {
 		return findBy(Specification.where(spec), queryFunction);
 	}
 
@@ -275,6 +278,21 @@ public interface JpaSpecificationExecutor<T> {
 		@Override
 		SpecificationFluentQuery<T> project(Collection<String> properties);
 
+		/**
+		 * Get a page of matching elements for {@link Pageable} and provide a custom {@link Specification count
+		 * specification}.
+		 *
+		 * @param pageable the pageable to request a paged result, can be {@link Pageable#unpaged()}, must not be
+		 *          {@literal null}. The given {@link Pageable} will override any previously specified {@link Sort sort} if
+		 *          the {@link Sort} object is not {@link Sort#isUnsorted()}. Any potentially specified {@link #limit(int)}
+		 *          will be overridden by {@link Pageable#getPageSize()}.
+		 * @param countSpec specification used to count results.
+		 * @return
+		 */
+		default Page<T> page(Pageable pageable, PredicateSpecification<?> countSpec) {
+			return page(pageable, Specification.where(countSpec));
+		}
+
 		/**
 		 * Get a page of matching elements for {@link Pageable} and provide a custom {@link Specification count
 		 * specification}.

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/FetchableFluentQueryBySpecification.java

@@ -226,6 +226,26 @@ class FetchableFluentQueryBySpecification<S, R> extends FluentQuerySupport<S, R>
 
 	private Slice<R> readSlice(Pageable pageable) {
 
+		TypedQuery<S> pagedQuery = createSortedAndProjectedQuery();
+
+		if (pageable.isPaged()) {
+			pagedQuery.setFirstResult(PageableUtils.getOffsetAsInteger(pageable));
+			pagedQuery.setMaxResults(pageable.getPageSize() + 1);
+		}
+
+		List<S> resultList = pagedQuery.getResultList();
+		boolean hasNext = resultList.size() > pageable.getPageSize();
+		if (hasNext) {
+			resultList = resultList.subList(0, pageable.getPageSize());
+		}
+
+		List<R> slice = convert(resultList);
+
+		return new SliceImpl<>(slice, pageable, hasNext);
+	}
+
+	private Slice<R> readSlice(Pageable pageable, @Nullable Specification<S> countSpec) {
+
 		TypedQuery<S> pagedQuery = createSortedAndProjectedQuery(pageable.getSort());
 
 		if (pageable.isPaged()) {

spring-data-jpa/src/test/java/org/springframework/data/jpa/repository/UserRepositoryTests.java

@@ -2800,6 +2800,44 @@ class UserRepositoryTests {
 		assertThat(page0.getTotalElements()).isEqualTo(3L);
 	}
 
+	@Test // GH-2274
+	void findByFluentSpecificationSlice() {
+
+		flushTestUsers();
+
+		Slice<User> slice = repository.findBy(userHasFirstnameLike("v"),
+				q -> q.sortBy(Sort.by("firstname")).slice(PageRequest.of(0, 2)));
+
+		assertThat(slice).isNotInstanceOf(Page.class);
+		assertThat(slice.getContent()).containsExactly(thirdUser, firstUser);
+		assertThat(slice.hasNext()).isTrue();
+
+		slice = repository.findBy(userHasFirstnameLike("v"),
+				q -> q.sortBy(Sort.by("firstname")).slice(PageRequest.of(0, 3)));
+
+		assertThat(slice).isNotInstanceOf(Page.class);
+		assertThat(slice).hasSize(3);
+		assertThat(slice.hasNext()).isFalse();
+	}
+
+	@Test // GH-3727
+	void findByFluentSpecificationPageCustomCountSpec() {
+
+		flushTestUsers();
+
+		Page<User> page0 = repository.findBy(userHasFirstnameLike("v"),
+				q -> q.sortBy(Sort.by("firstname")).page(PageRequest.of(0, 2), (root, query, criteriaBuilder) -> null));
+
+		assertThat(page0.getContent()).containsExactly(thirdUser, firstUser);
+		assertThat(page0.getTotalElements()).isEqualTo(4L);
+
+		page0 = repository.findBy(userHasFirstnameLike("v"),
+				q -> q.sortBy(Sort.by("firstname")).page(PageRequest.of(0, 2)));
+
+		assertThat(page0.getContent()).containsExactly(thirdUser, firstUser);
+		assertThat(page0.getTotalElements()).isEqualTo(3L);
+	}
+
 	@Test // GH-2274, GH-3716
 	void findByFluentSpecificationWithInterfaceBasedProjection() {