Added exists method with specification to JpaSpecificationExecutor.

It is now possible to make existence checks based on a `Specification`. This is an addition to checking existence with using an `Example`.

Closes #2388
Original pull request #2449

Author: DiegoKrupitza

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

@@ -29,6 +29,7 @@
  *
  * @author Oliver Gierke
  * @author Christoph Strobl
+ * @author Diego Krupitza
  */
 public interface JpaSpecificationExecutor<T> {
 
@@ -74,4 +75,13 @@ public interface JpaSpecificationExecutor<T> {
 	 * @return the number of instances.
 	 */
 	long count(@Nullable Specification<T> spec);
+
+	/**
+	 * Checks whether the data store contains elements that match the given {@link Specification}.
+	 * 
+	 * @param spec the {@link Specification} to use for the existence check. Must not be {@literal null}.
+	 * @return <code>true</code> if the data store contains elements that match the given {@link Specification} otherwise
+	 *         <code>false</code>.
+	 */
+	boolean exists(Specification<T> spec);
 }

Diff for: spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -40,11 +40,7 @@
 import jakarta.persistence.criteria.Root;
 
 import org.springframework.dao.EmptyResultDataAccessException;
-import org.springframework.data.domain.Example;
-import org.springframework.data.domain.Page;
-import org.springframework.data.domain.PageImpl;
-import org.springframework.data.domain.Pageable;
-import org.springframework.data.domain.Sort;
+import org.springframework.data.domain.*;
 import org.springframework.data.jpa.convert.QueryByExamplePredicateBuilder;
 import org.springframework.data.jpa.domain.Specification;
 import org.springframework.data.jpa.provider.PersistenceProvider;
@@ -81,6 +77,7 @@
  * @author Greg Turnquist
  * @author Yanming Zhou
  * @author Ernst-Jan van der Laan
+ * @author Diego Krupitza
  */
 @Repository
 @Transactional(readOnly = true)
@@ -464,6 +461,20 @@ public <S extends T> boolean exists(Example<S> example) {
 		return query.setMaxResults(1).getResultList().size() == 1;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.jpa.repository.JpaSpecificationExecutor#exists(org.springframework.data.jpa.domain.Specification)
+	 */
+	@Override
+	public boolean exists(Specification<T> spec) {
+
+		CriteriaQuery<Integer> cq = this.em.getCriteriaBuilder().createQuery(Integer.class);
+		cq.select(this.em.getCriteriaBuilder().literal(1));
+		applySpecificationToCriteria(spec, getDomainClass(), cq);
+		TypedQuery<Integer> query = applyRepositoryMethodMetadata(this.em.createQuery(cq));
+		return query.setMaxResults(1).getResultList().size() == 1;
+	}
+
 	@Override
 	public <S extends T> List<S> findAll(Example<S> example) {
 		return getQuery(new ExampleSpecification<>(example, escapeCharacter), example.getProbeType(), Sort.unsorted())

Diff for: spring-data-jpa/src/test/java/org/springframework/data/jpa/domain/sample/UserSpecifications.java

@@ -26,6 +26,7 @@
  * Collection of {@link Specification}s for a {@link User}.
  *
  * @author Oliver Gierke
+ * @author Diego Krupitza
  */
 public class UserSpecifications {
 
@@ -69,6 +70,17 @@ public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBu
 		};
 	}
 
+	/**
+	 * A {@link Specification} to do an age check.
+	 *
+	 * @param age upper (exclusive) bound of the age
+	 * @return
+	 */
+	public static Specification<User> userHasAgeLess(final Integer age) {
+
+		return (root, query, cb) -> cb.lessThan(root.get("age").as(Integer.class), age);
+	}
+
 	/**
 	 * A {@link Specification} to do a like-match on a {@link User}'s lastname but also adding a sort order on the
 	 * firstname.

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

@@ -96,6 +96,7 @@
  * @author Sander Krabbenborg
  * @author Jesse Wouters
  * @author Greg Turnquist
+ * @author Diego Krupitza
  */
 @ExtendWith(SpringExtension.class)
 @ContextConfiguration("classpath:application-context.xml")
@@ -2640,6 +2641,17 @@ void readsDerivedInterfaceProjections() {
 		assertThat(repository.findAllInterfaceProjectedBy()).hasSize(4);
 	}
 
+	@Test // GH-2388
+	void existsWithSpec() {
+		flushTestUsers();
+
+		Specification<User> minorSpec = userHasAgeLess(18);
+		Specification<User> hundredYearsOld = userHasAgeLess(100);
+
+		assertThat(repository.exists(minorSpec)).isFalse();
+		assertThat(repository.exists(hundredYearsOld)).isTrue();
+	}
+
 	private Page<User> executeSpecWithSort(Sort sort) {
 
 		flushTestUsers();