Annotate interfaces with @FunctionaInterface

Closes gh-6857

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/CacheStatisticsProvider.java

@@ -27,6 +27,7 @@ import org.springframework.cache.CacheManager;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface CacheStatisticsProvider<C extends Cache> {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java

@@ -29,6 +29,7 @@ import org.springframework.boot.actuate.metrics.Metric;
  * @author Dave Syer
  * @see SystemPublicMetrics SystemPublicMetrics for an example implementation
  */
+@FunctionalInterface
 public interface PublicMetrics {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/EndpointHandlerMappingCustomizer.java

@@ -22,6 +22,7 @@ package org.springframework.boot.actuate.endpoint.mvc;
  * @author Dave Syer
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface EndpointHandlerMappingCustomizer {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/ManagementServletContext.java

@@ -22,6 +22,7 @@ package org.springframework.boot.actuate.endpoint.mvc;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface ManagementServletContext {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthAggregator.java

@@ -34,6 +34,7 @@ import java.util.Map;
  * @author Christian Dupuis
  * @since 1.1.0
  */
+@FunctionalInterface
 public interface HealthAggregator {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicator.java

@@ -22,6 +22,7 @@ package org.springframework.boot.actuate.health;
  * @author Dave Syer
  * @see ApplicationHealthIndicator
  */
+@FunctionalInterface
 public interface HealthIndicator {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/info/InfoContributor.java

@@ -22,6 +22,7 @@ package org.springframework.boot.actuate.info;
  * @author Stephane Nicoll
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface InfoContributor {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/GaugeService.java

@@ -27,6 +27,7 @@ package org.springframework.boot.actuate.metrics;
  *
  * @author Dave Syer
  */
+@FunctionalInterface
 public interface GaugeService {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/Exporter.java

@@ -27,6 +27,7 @@ package org.springframework.boot.actuate.metrics.export;
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface Exporter {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbNamingStrategy.java

@@ -22,6 +22,7 @@ package org.springframework.boot.actuate.metrics.opentsdb;
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface OpenTsdbNamingStrategy {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/reader/PrefixMetricReader.java

@@ -23,6 +23,7 @@ import org.springframework.boot.actuate.metrics.Metric;
  *
  * @author Dave Syer
  */
+@FunctionalInterface
 public interface PrefixMetricReader {
 
 	/**

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/writer/GaugeWriter.java

@@ -24,6 +24,7 @@ import org.springframework.boot.actuate.metrics.Metric;
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface GaugeWriter {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/CacheManagerCustomizer.java

@@ -26,6 +26,7 @@ import org.springframework.cache.CacheManager;
  * @author Stephane Nicoll
  * @since 1.3.3
  */
+@FunctionalInterface
 public interface CacheManagerCustomizer<T extends CacheManager> {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/JCacheManagerCustomizer.java

@@ -25,6 +25,7 @@ import javax.cache.CacheManager;
  * @author Stephane Nicoll
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface JCacheManagerCustomizer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayMigrationStrategy.java

@@ -25,6 +25,7 @@ import org.flywaydb.core.Flyway;
  * @author Andreas Ahlenstorf
  * @author Phillip Webb
  */
+@FunctionalInterface
 public interface FlywayMigrationStrategy {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/Jackson2ObjectMapperBuilderCustomizer.java

@@ -28,6 +28,7 @@ import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
  * @author Grzegorz Poznachowski
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface Jackson2ObjectMapperBuilderCustomizer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/metadata/DataSourcePoolMetadataProvider.java

@@ -24,6 +24,7 @@ import javax.sql.DataSource;
  * @author Stephane Nicoll
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface DataSourcePoolMetadataProvider {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jersey/ResourceConfigCustomizer.java

@@ -25,6 +25,7 @@ import org.glassfish.jersey.server.ResourceConfig;
  * @author Eddú Meléndez
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface ResourceConfigCustomizer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jms/artemis/ArtemisConfigurationCustomizer.java

@@ -29,6 +29,7 @@ import org.apache.activemq.artemis.jms.server.embedded.EmbeddedJMS;
  * @since 1.3.0
  * @see ArtemisAutoConfiguration
  */
+@FunctionalInterface
 public interface ArtemisConfigurationCustomizer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/AuthoritiesExtractor.java

@@ -28,6 +28,7 @@ import org.springframework.security.core.GrantedAuthority;
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface AuthoritiesExtractor {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/JwtAccessTokenConverterConfigurer.java

@@ -25,6 +25,7 @@ import org.springframework.security.oauth2.provider.token.store.JwtAccessTokenCo
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface JwtAccessTokenConverterConfigurer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/PrincipalExtractor.java

@@ -25,6 +25,7 @@ import java.util.Map;
  * @author Phillip Webb
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface PrincipalExtractor {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/UserInfoRestTemplateCustomizer.java

@@ -29,6 +29,7 @@ import org.springframework.security.oauth2.client.OAuth2RestTemplate;
  * @author Dave Syer
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface UserInfoRestTemplateCustomizer {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/template/TemplateAvailabilityProvider.java

@@ -26,6 +26,7 @@ import org.springframework.core.io.ResourceLoader;
  * @author Andy Wilkinson
  * @since 1.1.0
  */
+@FunctionalInterface
 public interface TemplateAvailabilityProvider {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorController.java

@@ -24,6 +24,7 @@ import org.springframework.stereotype.Controller;
  *
  * @author Phillip Webb
  */
+@FunctionalInterface
 public interface ErrorController {
 
 	/**

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorViewResolver.java

@@ -29,6 +29,7 @@ import org.springframework.web.servlet.ModelAndView;
  * @author Phillip Webb
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface ErrorViewResolver {
 
 	/**

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/CommandFactory.java

@@ -25,6 +25,7 @@ import java.util.ServiceLoader;
  *
  * @author Dave Syer
  */
+@FunctionalInterface
 public interface CommandFactory {
 
 	/**

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/install/DependencyResolver.java

@@ -26,6 +26,7 @@ import java.util.List;
  * @author Andy Wilkinson
  * @since 1.2.0
  */
+@FunctionalInterface
 interface DependencyResolver {
 
 	/**

spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/SpringBootAstTransformation.java

@@ -24,6 +24,7 @@ import org.codehaus.groovy.transform.ASTTransformation;
  *
  * @author Dave Syer
  */
+@FunctionalInterface
 public interface SpringBootAstTransformation extends ASTTransformation {
 
 }

spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/ProgressReporter.java

@@ -21,6 +21,7 @@ package org.springframework.boot.cli.compiler.grape;
  *
  * @author Andy Wilkinson
  */
+@FunctionalInterface
 interface ProgressReporter {
 
 	/**

spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/RepositorySystemSessionAutoConfiguration.java

@@ -25,6 +25,7 @@ import org.eclipse.aether.RepositorySystem;
  *
  * @author Andy Wilkinson
  */
+@FunctionalInterface
 public interface RepositorySystemSessionAutoConfiguration {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathRestartStrategy.java

@@ -27,6 +27,7 @@ import org.springframework.boot.devtools.filewatch.ChangedFile;
  * @since 1.3.0
  * @see PatternClassPathRestartStrategy
  */
+@FunctionalInterface
 public interface ClassPathRestartStrategy {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileChangeListener.java

@@ -25,6 +25,7 @@ import java.util.Set;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface FileChangeListener {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherFactory.java

@@ -22,6 +22,7 @@ package org.springframework.boot.devtools.filewatch;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface FileSystemWatcherFactory {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/AccessManager.java

@@ -24,6 +24,7 @@ import org.springframework.http.server.ServerHttpRequest;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface AccessManager {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/Handler.java

@@ -27,6 +27,7 @@ import org.springframework.http.server.ServerHttpResponse;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface Handler {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/HandlerMapper.java

@@ -25,6 +25,7 @@ import org.springframework.http.server.ServerHttpRequest;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface HandlerMapper {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/FailureHandler.java

@@ -22,6 +22,7 @@ package org.springframework.boot.devtools.restart;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface FailureHandler {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartInitializer.java

@@ -25,6 +25,7 @@ import java.net.URL;
  * @since 1.3.0
  * @see DefaultRestartInitializer
  */
+@FunctionalInterface
 public interface RestartInitializer {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartListener.java

@@ -22,6 +22,7 @@ package org.springframework.boot.devtools.restart;
  * @author Andy Wilkinson
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface RestartListener {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFileRepository.java

@@ -25,6 +25,7 @@ package org.springframework.boot.devtools.restart.classloader;
  * @since 1.3.0
  * @see ClassLoaderFile
  */
+@FunctionalInterface
 public interface ClassLoaderFileRepository {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/SourceFolderUrlFilter.java

@@ -26,6 +26,7 @@ import java.net.URL;
  * @since 1.3.0
  * @see DefaultSourceFolderUrlFilter
  */
+@FunctionalInterface
 public interface SourceFolderUrlFilter {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/client/TunnelConnection.java

@@ -25,6 +25,7 @@ import java.nio.channels.WritableByteChannel;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface TunnelConnection {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/PortProvider.java

@@ -23,6 +23,7 @@ package org.springframework.boot.devtools.tunnel.server;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface PortProvider {
 
 	/**

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/TargetServerConnection.java

@@ -25,6 +25,7 @@ import java.nio.channels.ByteChannel;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface TargetServerConnection {
 
 	/**

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/restdocs/RestDocsMockMvcConfigurationCustomizer.java

@@ -29,6 +29,7 @@ import org.springframework.restdocs.mockmvc.MockMvcRestDocumentationConfigurer;
  * @author Andy Wilkinson
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface RestDocsMockMvcConfigurationCustomizer {
 
 	/**

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/servlet/MockMvcBuilderCustomizer.java

@@ -28,6 +28,7 @@ import org.springframework.test.web.servlet.setup.ConfigurableMockMvcBuilder;
  * @since 1.4.0
  * @see MockMvcAutoConfiguration
  */
+@FunctionalInterface
 public interface MockMvcBuilderCustomizer {
 
 	/**

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/FieldValuesParser.java

@@ -30,6 +30,7 @@ import org.springframework.boot.configurationprocessor.fieldvalues.javac.JavaCom
  * @since 1.1.2
  * @see JavaCompilerFieldValuesParser
  */
+@FunctionalInterface
 public interface FieldValuesParser {
 
 	/**

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/TreeVisitor.java

@@ -22,6 +22,7 @@ package org.springframework.boot.configurationprocessor.fieldvalues.javac;
  * @author Phillip Webb
  * @since 1.2.0
  */
+@FunctionalInterface
 interface TreeVisitor {
 
 	void visitVariable(VariableTree variable) throws Exception;

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/PluginFeatures.java

@@ -24,6 +24,7 @@ import org.gradle.api.Project;
  *
  * @author Phillip Webb
  */
+@FunctionalInterface
 public interface PluginFeatures {
 
 	/**

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LaunchScript.java

@@ -22,6 +22,7 @@ package org.springframework.boot.loader.tools;
  * @author Phillip Webb
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface LaunchScript {
 
 	/**

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Libraries.java

@@ -23,6 +23,7 @@ import java.io.IOException;
  *
  * @author Phillip Webb
  */
+@FunctionalInterface
 public interface Libraries {
 
 	/**

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LibraryCallback.java

@@ -24,6 +24,7 @@ import java.io.IOException;
  *
  * @author Phillip Webb
  */
+@FunctionalInterface
 public interface LibraryCallback {
 
 	/**

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarEntryFilter.java

@@ -21,6 +21,7 @@ package org.springframework.boot.loader.jar;
  *
  * @author Phillip Webb
  */
+@FunctionalInterface
 interface JarEntryFilter {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/ApplicationRunner.java

@@ -29,6 +29,7 @@ import org.springframework.core.annotation.Order;
  * @since 1.3.0
  * @see CommandLineRunner
  */
+@FunctionalInterface
 public interface ApplicationRunner {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/Banner.java

@@ -28,6 +28,7 @@ import org.springframework.core.env.Environment;
  * @author Jeremy Rickard
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface Banner {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/CommandLineRunner.java

@@ -31,6 +31,7 @@ import org.springframework.core.annotation.Order;
  * @author Dave Syer
  * @see ApplicationRunner
  */
+@FunctionalInterface
 public interface CommandLineRunner {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/ExitCodeExceptionMapper.java

@@ -23,6 +23,7 @@ package org.springframework.boot;
  * @author Phillip Webb
  * @since 1.3.2
  */
+@FunctionalInterface
 public interface ExitCodeExceptionMapper {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/ExitCodeGenerator.java

@@ -25,6 +25,7 @@ package org.springframework.boot;
  * @see SpringApplication#exit(org.springframework.context.ApplicationContext,
  * ExitCodeGenerator...)
  */
+@FunctionalInterface
 public interface ExitCodeGenerator {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/bind/PropertyNamePatternsMatcher.java

@@ -22,6 +22,7 @@ package org.springframework.boot.bind;
  * @author Phillip Webb
  * @since 1.2.0
  */
+@FunctionalInterface
 interface PropertyNamePatternsMatcher {
 
 	PropertyNamePatternsMatcher ALL = new PropertyNamePatternsMatcher() {

spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerCustomizer.java

@@ -32,6 +32,7 @@ import org.springframework.beans.factory.config.BeanPostProcessor;
  * @author Dave Syer
  * @see EmbeddedServletContainerCustomizerBeanPostProcessor
  */
+@FunctionalInterface
 public interface EmbeddedServletContainerCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerFactory.java

@@ -33,6 +33,7 @@ import org.springframework.boot.web.servlet.ServletContextInitializer;
  * @see JettyEmbeddedServletContainerFactory
  * @see TomcatEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface EmbeddedServletContainerFactory {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyServerCustomizer.java

@@ -24,6 +24,7 @@ import org.eclipse.jetty.server.Server;
  * @author Dave Syer
  * @see JettyEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface JettyServerCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatConnectorCustomizer.java

@@ -24,6 +24,7 @@ import org.apache.catalina.connector.Connector;
  * @author Dave Syer
  * @see TomcatEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface TomcatConnectorCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatContextCustomizer.java

@@ -24,6 +24,7 @@ import org.apache.catalina.Context;
  * @author Dave Syer
  * @see TomcatEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface TomcatContextCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowBuilderCustomizer.java

@@ -25,6 +25,7 @@ import io.undertow.Undertow.Builder;
  * @since 1.2.0
  * @see UndertowEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface UndertowBuilderCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowDeploymentInfoCustomizer.java

@@ -25,6 +25,7 @@ import io.undertow.servlet.api.DeploymentInfo;
  * @since 1.2.0
  * @see UndertowEmbeddedServletContainerFactory
  */
+@FunctionalInterface
 public interface UndertowDeploymentInfoCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalysisReporter.java

@@ -22,6 +22,7 @@ package org.springframework.boot.diagnostics;
  * @author Andy Wilkinson
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface FailureAnalysisReporter {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalyzer.java

@@ -23,6 +23,7 @@ package org.springframework.boot.diagnostics;
  * @author Andy Wilkinson
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface FailureAnalyzer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/env/EnvironmentPostProcessor.java

@@ -37,6 +37,7 @@ import org.springframework.core.env.Environment;
  * @author Stephane Nicoll
  * @since 1.3.0
  */
+@FunctionalInterface
 public interface EnvironmentPostProcessor {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/jta/XAConnectionFactoryWrapper.java

@@ -27,6 +27,7 @@ import javax.transaction.TransactionManager;
  * @author Phillip Webb
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface XAConnectionFactoryWrapper {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/jta/XADataSourceWrapper.java

@@ -27,6 +27,7 @@ import javax.transaction.TransactionManager;
  * @author Phillip Webb
  * @since 1.2.0
  */
+@FunctionalInterface
 public interface XADataSourceWrapper {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateCustomizer.java

@@ -25,6 +25,7 @@ import org.springframework.web.client.RestTemplate;
  * @since 1.4.0
  * @see RestTemplateBuilder
  */
+@FunctionalInterface
 public interface RestTemplateCustomizer {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistrar.java

@@ -22,6 +22,7 @@ package org.springframework.boot.web.servlet;
  * @author Phillip Webb
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface ErrorPageRegistrar {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistry.java

@@ -22,6 +22,7 @@ package org.springframework.boot.web.servlet;
  * @author Phillip Webb
  * @since 1.4.0
  */
+@FunctionalInterface
 public interface ErrorPageRegistry {
 
 	/**

spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializer.java

@@ -38,6 +38,7 @@ import org.springframework.web.WebApplicationInitializer;
  * @since 1.4.0
  * @see WebApplicationInitializer
  */
+@FunctionalInterface
 public interface ServletContextInitializer {
 
 	/**