DATAJPA-257 - Make count query execution in SimpleJpaRepository more robust.

The execution of a count query can potentially return multiple values instead of just a single one. This causes persistence providers to throw an exception as we trigger ….getSingleResult() in SimpleJpaRepository. We're now calling …getResultList() and sum up all values returned.

src/main/java/org/springframework/data/jpa/repository/query/JpaQueryExecution.java

@@ -97,6 +97,7 @@ public abstract class JpaQueryExecution {
 
 			// Execute query to compute total
 			TypedQuery<Long> projection = repositoryQuery.createCountQuery(values);
+
 			List<Long> totals = projection.getResultList();
 			Long total = totals.size() == 1 ? totals.get(0) : totals.size();
 
@@ -160,4 +161,4 @@ public abstract class JpaQueryExecution {
 			return result;
 		}
 	}
-}
+}

src/main/java/org/springframework/data/jpa/repository/query/QueryUtils.java

@@ -27,6 +27,7 @@ import java.util.regex.Pattern;
 import javax.persistence.EntityManager;
 import javax.persistence.Parameter;
 import javax.persistence.Query;
+import javax.persistence.TypedQuery;
 import javax.persistence.criteria.CriteriaBuilder;
 import javax.persistence.criteria.Expression;
 import javax.persistence.criteria.From;
@@ -270,6 +271,26 @@ public abstract class QueryUtils {
 		return orders;
 	}
 
+	/**
+	 * Executes a count query and transparently sums up all values returned.
+	 * 
+	 * @param query must not be {@literal null}.
+	 * @return
+	 */
+	public static Long executeCountQuery(TypedQuery<Long> query) {
+
+		Assert.notNull(query);
+
+		List<Long> totals = query.getResultList();
+		Long total = 0L;
+
+		for (Long element : totals) {
+			total += element == null ? 0 : element;
+		}
+
+		return total;
+	}
+
 	/**
 	 * Creates a criteria API {@link javax.persistence.criteria.Order} from the given {@link Order}.
 	 * 

src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -40,6 +40,7 @@ import org.springframework.data.domain.Sort;
 import org.springframework.data.jpa.domain.Specification;
 import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.data.jpa.repository.JpaSpecificationExecutor;
+import org.springframework.data.jpa.repository.query.QueryUtils;
 import org.springframework.transaction.annotation.Transactional;
 import org.springframework.util.Assert;
 
@@ -402,7 +403,7 @@ public class SimpleJpaRepository<T, ID extends Serializable> implements JpaRepos
 		query.setFirstResult(pageable.getOffset());
 		query.setMaxResults(pageable.getPageSize());
 
-		Long total = getCountQuery(spec).getSingleResult();
+		Long total = QueryUtils.executeCountQuery(getCountQuery(spec));
 		List<T> content = total > pageable.getOffset() ? query.getResultList() : Collections.<T> emptyList();
 
 		return new PageImpl<T>(content, pageable, total);