Fixes build failures with JDK16.

`JdbcMappingContext` sets the SimpleTypeHolder to `JdbcSimpleTypeHolder.HOLDER` by default.
Fixing test failures where properties of type `UUID` were considered an entity by the `MappingContext`.
This in turn triggered failures when it tried to make fields of `UUID` accessible.

Removed the `UUID` field from a test entity in SD Relational, since `UUID` is not a simple type for Relational, nor a proper entity.

Replaced `@PostConstruct` with `InitializingBean` in test data source conifgurations in order avoid the requirement of importing javax.annotation-api.

Closes #975

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/mapping/JdbcMappingContext.java

@@ -46,6 +46,7 @@ public class JdbcMappingContext extends RelationalMappingContext {
 	 */
 	public JdbcMappingContext() {
 		super();
+		setSimpleTypeHolder(JdbcSimpleTypes.HOLDER);
 	}
 
 	/**
@@ -55,6 +56,7 @@ public class JdbcMappingContext extends RelationalMappingContext {
 	 */
 	public JdbcMappingContext(NamingStrategy namingStrategy) {
 		super(namingStrategy);
+		setSimpleTypeHolder(JdbcSimpleTypes.HOLDER);
 	}
 
 	/*

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

@@ -19,6 +19,7 @@ import static org.assertj.core.api.Assertions.*;
 import static org.mockito.Mockito.*;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Consumer;
 
@@ -27,6 +28,8 @@ import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.convert.converter.Converter;
+import org.springframework.data.convert.WritingConverter;
 import org.springframework.data.jdbc.core.JdbcAggregateTemplate;
 import org.springframework.data.jdbc.core.convert.DataAccessStrategy;
 import org.springframework.data.jdbc.core.convert.JdbcConverter;
@@ -46,7 +49,7 @@ import org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate;
 public class AbstractJdbcConfigurationIntegrationTests {
 
 	@Test // DATAJDBC-395
-	public void configuresInfrastructureComponents() {
+	void configuresInfrastructureComponents() {
 
 		assertApplicationContext(context -> {
 
@@ -63,6 +66,18 @@ public class AbstractJdbcConfigurationIntegrationTests {
 		}, AbstractJdbcConfigurationUnderTest.class, Infrastructure.class);
 	}
 
+	@Test // #975
+	void registersSimpleTypesFromCustomConversions() {
+
+		assertApplicationContext(context -> {
+			JdbcMappingContext mappingContext = context.getBean(JdbcMappingContext.class);
+			assertThat( //
+					mappingContext.getPersistentEntity(AbstractJdbcConfigurationUnderTest.Blah.class) //
+			).describedAs("Blah should not be an entity, since there is a WritingConversion configured for it") //
+					.isNull();
+		}, AbstractJdbcConfigurationUnderTest.class, Infrastructure.class);
+	}
+
 	protected static void assertApplicationContext(Consumer<ConfigurableApplicationContext> verification,
 			Class<?>... configurationClasses) {
 
@@ -93,6 +108,25 @@ public class AbstractJdbcConfigurationIntegrationTests {
 		public Dialect jdbcDialect(NamedParameterJdbcOperations operations) {
 			return HsqlDbDialect.INSTANCE;
 		}
+
+		@Override
+		public JdbcCustomConversions jdbcCustomConversions() {
+			return new JdbcCustomConversions(Collections.singletonList(Blah2BlubbConverter.INSTANCE));
+		}
+
+		@WritingConverter
+		enum Blah2BlubbConverter implements Converter<Blah, Blubb> {
+			INSTANCE;
+
+			@Override
+			public Blubb convert(Blah blah) {
+				return new Blubb();
+			}
+		}
+
+		private static class Blah {}
+
+		private static class Blubb {}
 	}
 
 }

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/testing/MariaDBDataSourceConfiguration.java

@@ -18,10 +18,10 @@ package org.springframework.data.jdbc.testing;
 import java.sql.Connection;
 import java.sql.SQLException;
 
-import javax.annotation.PostConstruct;
 import javax.sql.DataSource;
 
 import org.mariadb.jdbc.MariaDbDataSource;
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Profile;
 import org.springframework.core.io.ByteArrayResource;
@@ -33,10 +33,11 @@ import org.testcontainers.containers.MariaDBContainer;
  *
  * @author Christoph Preißner
  * @author Mark Paluch
+ * @author Jens Schauder
  */
 @Configuration
 @Profile("mariadb")
-class MariaDBDataSourceConfiguration extends DataSourceConfiguration {
+class MariaDBDataSourceConfiguration extends DataSourceConfiguration implements InitializingBean {
 
 	private static MariaDBContainer<?> MARIADB_CONTAINER;
 
@@ -49,9 +50,7 @@ class MariaDBDataSourceConfiguration extends DataSourceConfiguration {
 
 		if (MARIADB_CONTAINER == null) {
 
-			MariaDBContainer<?> container = new MariaDBContainer<>("mariadb:10.5")
-					.withUsername("root")
-					.withPassword("")
+			MariaDBContainer<?> container = new MariaDBContainer<>("mariadb:10.5").withUsername("root").withPassword("")
 					.withConfigurationOverride("");
 			container.start();
 
@@ -70,13 +69,12 @@ class MariaDBDataSourceConfiguration extends DataSourceConfiguration {
 		}
 	}
 
-	@PostConstruct
-	public void initDatabase() throws SQLException {
+	@Override
+	public void afterPropertiesSet() throws Exception {
 
 		try (Connection connection = createDataSource().getConnection()) {
 			ScriptUtils.executeSqlScript(connection,
 					new ByteArrayResource("DROP DATABASE test;CREATE DATABASE test;".getBytes()));
 		}
 	}
-
 }

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/testing/MySqlDataSourceConfiguration.java

@@ -16,18 +16,18 @@
 package org.springframework.data.jdbc.testing;
 
 import java.sql.Connection;
-import java.sql.SQLException;
 
-import javax.annotation.PostConstruct;
 import javax.sql.DataSource;
 
-import com.mysql.cj.jdbc.MysqlDataSource;
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Profile;
 import org.springframework.core.io.ByteArrayResource;
 import org.springframework.jdbc.datasource.init.ScriptUtils;
 import org.testcontainers.containers.MySQLContainer;
 
+import com.mysql.cj.jdbc.MysqlDataSource;
+
 /**
  * {@link DataSource} setup for MySQL. Starts a docker container with a MySql database and sets up a database name
  * "test" in it.
@@ -39,7 +39,7 @@ import org.testcontainers.containers.MySQLContainer;
  */
 @Configuration
 @Profile("mysql")
-class MySqlDataSourceConfiguration extends DataSourceConfiguration {
+class MySqlDataSourceConfiguration extends DataSourceConfiguration implements InitializingBean {
 
 	private static MySQLContainer<?> MYSQL_CONTAINER;
 
@@ -52,9 +52,7 @@ class MySqlDataSourceConfiguration extends DataSourceConfiguration {
 
 		if (MYSQL_CONTAINER == null) {
 
-			MySQLContainer<?> container = new MySQLContainer<>("mysql:8.0.24")
-					.withUsername("test")
-					.withPassword("test")
+			MySQLContainer<?> container = new MySQLContainer<>("mysql:8.0.24").withUsername("test").withPassword("test")
 					.withConfigurationOverride("");
 
 			container.start();
@@ -71,8 +69,8 @@ class MySqlDataSourceConfiguration extends DataSourceConfiguration {
 		return dataSource;
 	}
 
-	@PostConstruct
-	public void initDatabase() throws SQLException {
+	@Override
+	public void afterPropertiesSet() throws Exception {
 
 		try (Connection connection = createDataSource().getConnection()) {
 			ScriptUtils.executeSqlScript(connection,

spring-data-relational/src/test/java/org/springframework/data/relational/core/mapping/BasicRelationalPersistentPropertyUnitTests.java

@@ -25,7 +25,6 @@ import java.time.LocalDateTime;
 import java.time.ZonedDateTime;
 import java.util.Date;
 import java.util.List;
-import java.util.UUID;
 import java.util.function.BiConsumer;
 
 import org.assertj.core.api.SoftAssertions;
@@ -47,7 +46,6 @@ public class BasicRelationalPersistentPropertyUnitTests {
 	RelationalMappingContext context = new RelationalMappingContext();
 	RelationalPersistentEntity<?> entity = context.getRequiredPersistentEntity(DummyEntity.class);
 
-
 	@Test // DATAJDBC-106
 	public void detectsAnnotatedColumnName() {
 
@@ -61,9 +59,12 @@ public class BasicRelationalPersistentPropertyUnitTests {
 
 		RelationalPersistentProperty listProperty = entity.getRequiredPersistentProperty("someList");
 
-		PersistentPropertyPath<RelationalPersistentProperty> path = context.findPersistentPropertyPaths(DummyEntity.class, p -> p.getName().equals("someList")).getFirst().orElseThrow(() -> new AssertionFailedError("Couldn't find path for 'someList'"));
+		PersistentPropertyPath<RelationalPersistentProperty> path = context
+				.findPersistentPropertyPaths(DummyEntity.class, p -> p.getName().equals("someList")).getFirst()
+				.orElseThrow(() -> new AssertionFailedError("Couldn't find path for 'someList'"));
 
-		assertThat(listProperty.getReverseColumnName(new PersistentPropertyPathExtension(context, path))).isEqualTo(quoted("dummy_column_name"));
+		assertThat(listProperty.getReverseColumnName(new PersistentPropertyPathExtension(context, path)))
+				.isEqualTo(quoted("dummy_column_name"));
 		assertThat(listProperty.getKeyColumn()).isEqualTo(quoted("dummy_key_column_name"));
 	}
 
@@ -127,7 +128,6 @@ public class BasicRelationalPersistentPropertyUnitTests {
 		softly.assertAll();
 	}
 
-
 	@Data
 	@SuppressWarnings("unused")
 	private static class DummyEntity {
@@ -136,7 +136,6 @@ public class BasicRelationalPersistentPropertyUnitTests {
 		private final SomeEnum someEnum;
 		private final LocalDateTime localDateTime;
 		private final ZonedDateTime zonedDateTime;
-		private final UUID uuid;
 
 		// DATAJDBC-259
 		private final List<String> listOfString;