Fix NPE with Hikari when DatabaseDriver has null driverClassName

Fixes gh-44994

Author: wilkinsona

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

@@ -190,7 +190,10 @@ public T build() {
 				&& this.values.containsKey(DataSourceProperty.URL)) {
 			String url = this.values.get(DataSourceProperty.URL);
 			DatabaseDriver driver = DatabaseDriver.fromJdbcUrl(url);
-			properties.set(dataSource, DataSourceProperty.DRIVER_CLASS_NAME, driver.getDriverClassName());
+			String driverClassName = driver.getDriverClassName();
+			if (driverClassName != null) {
+				properties.set(dataSource, DataSourceProperty.DRIVER_CLASS_NAME, driver.getDriverClassName());
+			}
 		}
 		return dataSource;
 	}

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

@@ -474,6 +474,15 @@ void buildWhenC3P0TypeSpecifiedReturnsExpectedDataSource() {
 		assertThat(c3p0DataSource.getDriverClass()).isEqualTo("com.example.Driver");
 	}
 
+	@Test
+	void buildWhenJdbcUrlIsFromUnknownDriverLeavesDriverClassNameUnset() {
+		this.dataSource = DataSourceBuilder.create()
+			.url("jdbc:example://localhost:1234/example")
+			.type(HikariDataSource.class)
+			.build();
+		assertThat(((HikariDataSource) this.dataSource).getDriverClassName()).isNull();
+	}
+
 	private DataSource wrap(DataSource target) {
 		return new DataSourceWrapper(target);
 	}