DATAJDBC-529 - Use specialized ResultSetExtractor for exists queries.

Exists queries do not look at the value returned in the ResultSet but just check if there is a row.

Original pull request: #212.

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/query/AbstractJdbcQuery.java

@@ -80,10 +80,10 @@ public abstract class AbstractJdbcQuery implements RepositoryQuery {
 	 * @param queryMethod must not be {@literal null}.
 	 * @param extractor must not be {@literal null}.
 	 * @param rowMapper must not be {@literal null}.
-	 * @return
+	 * @return a JdbcQueryExecution appropriate for {@literal queryMethod}. Guaranteed to be not {@literal null}.
 	 */
 	protected JdbcQueryExecution<?> getQueryExecution(JdbcQueryMethod queryMethod,
-			@Nullable ResultSetExtractor<Object> extractor, RowMapper<Object> rowMapper) {
+			@Nullable ResultSetExtractor<?> extractor, RowMapper<?> rowMapper) {
 
 		if (queryMethod.isModifyingQuery()) {
 			return createModifyingQueryExecutor();

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/query/PartTreeJdbcQuery.java

@@ -24,10 +24,13 @@ import org.springframework.data.relational.repository.query.RelationalParameterA
 import org.springframework.data.relational.repository.query.RelationalParametersParameterAccessor;
 import org.springframework.data.repository.query.Parameters;
 import org.springframework.data.repository.query.parser.PartTree;
+import org.springframework.jdbc.core.ResultSetExtractor;
 import org.springframework.jdbc.core.RowMapper;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
 import org.springframework.util.Assert;
 
+import java.sql.ResultSet;
+
 /**
  * An {@link AbstractJdbcQuery} implementation based on a {@link PartTree}.
  *
@@ -72,7 +75,9 @@ public class PartTreeJdbcQuery extends AbstractJdbcQuery {
 		this.tree = new PartTree(queryMethod.getName(), queryMethod.getEntityInformation().getJavaType());
 		JdbcQueryCreator.validate(this.tree, this.parameters, this.converter.getMappingContext());
 
-		this.execution = getQueryExecution(queryMethod, null, rowMapper);
+		ResultSetExtractor<Boolean> extractor = tree.isExistsProjection() ? (ResultSet::next) : null;
+
+		this.execution = getQueryExecution(queryMethod, extractor, rowMapper);
 	}
 
 	private Sort getDynamicSort(RelationalParameterAccessor accessor) {

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryIntegrationTests.java

@@ -26,11 +26,11 @@ import java.time.Instant;
 import java.util.ArrayList;
 import java.util.List;
 
+import org.assertj.core.api.SoftAssertions;
 import org.junit.Before;
 import org.junit.ClassRule;
 import org.junit.Rule;
 import org.junit.Test;
-
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.config.PropertiesFactoryBean;
 import org.springframework.context.ApplicationListener;
@@ -355,6 +355,22 @@ public class JdbcRepositoryIntegrationTests {
 		assertThat(loaded.pointInTime).isNull();
 	}
 
+	@Test // DATAJDBC-529
+	public void existsWorksAsExpected() {
+
+		DummyEntity dummy = repository.save(createDummyEntity());
+
+		SoftAssertions.assertSoftly(softly -> {
+
+			softly.assertThat(repository.existsByName(dummy.getName())) //
+					.describedAs("Positive") //
+					.isTrue();
+			softly.assertThat(repository.existsByName("not an existing name")) //
+					.describedAs("Positive") //
+					.isFalse();
+		});
+	}
+
 	private static DummyEntity createDummyEntity() {
 
 		DummyEntity entity = new DummyEntity();
@@ -380,6 +396,8 @@ public class JdbcRepositoryIntegrationTests {
 
 		@Query("SELECT id_Prop from dummy_entity where id_Prop = :id")
 		DummyEntity withMissingColumn(@Param("id") Long id);
+
+		boolean existsByName(String name);
 	}
 
 	@Data