Fix NPE in SimpleDataSourceProperties when driver is null

- Prevent NullPointerException in SimpleDataSourceProperties by adding null check before calling getDriver().getClass().
- Fixes potential crash when driver is not explicitly configured.
- Add null checks in convertToString method to handle null Class values
- Add test case to verify NPE-free operation when deriving DataSource

Signed-off-by: chanbinme <gksmfcksqls@gmail.com>

Author: chanbinme

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jdbc/DataSourceBuilder.java

@@ -508,6 +508,9 @@ private V convertFromString(String value) {
 		}
 
 		private String convertToString(V value) {
+			if (value == null) {
+				return null;
+			}
 			if (String.class.equals(this.type)) {
 				return (String) value;
 			}
@@ -712,7 +715,8 @@ private static class SimpleDataSourceProperties extends MappedDataSourceProperti
 		@SuppressWarnings("unchecked")
 		SimpleDataSourceProperties() {
 			add(DataSourceProperty.URL, SimpleDriverDataSource::getUrl, SimpleDriverDataSource::setUrl);
-			add(DataSourceProperty.DRIVER_CLASS_NAME, Class.class, (dataSource) -> dataSource.getDriver().getClass(),
+			add(DataSourceProperty.DRIVER_CLASS_NAME, Class.class,
+					(dataSource) -> dataSource.getDriver() == null ? null : dataSource.getDriver().getClass(),
 					SimpleDriverDataSource::setDriverClass);
 			add(DataSourceProperty.USERNAME, SimpleDriverDataSource::getUsername, SimpleDriverDataSource::setUsername);
 			add(DataSourceProperty.PASSWORD, SimpleDriverDataSource::getPassword, SimpleDriverDataSource::setPassword);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/jdbc/DataSourceBuilderTests.java

@@ -51,8 +51,11 @@
 import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatNoException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 
 /**
  * Tests for {@link DataSourceBuilder}.
@@ -522,6 +525,16 @@ void buildWhenViburTypeSpecifiedReturnsExpectedDataSource() {
 		assertThat(viburDataSource.getDriverClassName()).isEqualTo("com.example.Driver");
 	}
 
+	@Test
+	void buildWhenDerivedFromSimpleDriverDataSourceWithDriverNotSetSucceeds() {
+		SimpleDriverDataSource dataSource = new SimpleDriverDataSource();
+		dataSource.setUsername("test");
+		dataSource.setPassword("secret");
+		dataSource.setUrl("jdbc:postgresql://localhost:5432/postgres");
+		assertDoesNotThrow(
+				() -> DataSourceBuilder.derivedFrom(dataSource).type(SimpleDriverDataSource.class).build());
+	}
+
 	@Test
 	void buildWhenJdbcUrlIsFromUnknownDriverLeavesDriverClassNameUnset() {
 		this.dataSource = DataSourceBuilder.create()