Add missing "abstract" keywords

Closes gh-11303

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/AbstractFreeMarkerConfiguration.java

@@ -25,7 +25,7 @@ import org.springframework.ui.freemarker.FreeMarkerConfigurationFactory;
  *
  * @author Brian Clozel
  */
-class AbstractFreeMarkerConfiguration {
+abstract class AbstractFreeMarkerConfiguration {
 
 	private final FreeMarkerProperties properties;
 

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/AbstractSessionCondition.java

@@ -35,7 +35,7 @@ import org.springframework.core.type.AnnotationMetadata;
  * @author Madhura Bhave
  * @author Andy Wilkinson
  */
-class AbstractSessionCondition extends SpringBootCondition {
+abstract class AbstractSessionCondition extends SpringBootCondition {
 
 	private final WebApplicationType webApplicationType;
 

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/AbstractSpringBootTestEmbeddedReactiveWebEnvironmentTests.java

@@ -88,7 +88,7 @@ public abstract class AbstractSpringBootTestEmbeddedReactiveWebEnvironmentTests
 		assertThat(this.value).isEqualTo(123);
 	}
 
-	protected static class AbstractConfig {
+	protected static abstract class AbstractConfig {
 
 		@Value("${server.port:8080}")
 		private int port = 8080;

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/context/AbstractSpringBootTestWebServerWebEnvironmentTests.java

@@ -92,7 +92,7 @@ public abstract class AbstractSpringBootTestWebServerWebEnvironmentTests {
 				WebApplicationContextUtils.getWebApplicationContext(this.servletContext));
 	}
 
-	protected static class AbstractConfig {
+	protected static abstract class AbstractConfig {
 
 		@Value("${server.port:8080}")
 		private int port = 8080;

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/AbstractExecutableArchiveLauncherTests.java

@@ -43,7 +43,7 @@ import org.springframework.util.FileCopyUtils;
  *
  * @author Andy Wilkinson
  */
-public class AbstractExecutableArchiveLauncherTests {
+public abstract class AbstractExecutableArchiveLauncherTests {
 
 	@Rule
 	public TemporaryFolder temp = new TemporaryFolder();

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/server/AbstractConfigurableWebServerFactory.java

@@ -37,7 +37,7 @@ import org.springframework.util.Assert;
  * @author Brian Clozel
  * @since 2.0.0
  */
-public class AbstractConfigurableWebServerFactory
+public abstract class AbstractConfigurableWebServerFactory
 		implements ConfigurableWebServerFactory {
 
 	private int port = 8080;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -1277,7 +1277,7 @@ public class SpringApplicationTests {
 
 	}
 
-	static class AbstractTestRunner implements ApplicationContextAware, Ordered {
+	static abstract class AbstractTestRunner implements ApplicationContextAware, Ordered {
 
 		private final String[] expectedBefore;