diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/CacheStatisticsProvider.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/CacheStatisticsProvider.java
index d27e44dc8e..879b19e21a 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/cache/CacheStatisticsProvider.java
+++ b/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> {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java
index cdcde530c5..8530465342 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/PublicMetrics.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/EndpointHandlerMappingCustomizer.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/EndpointHandlerMappingCustomizer.java
index de0c8ff373..6702d9dab9 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/EndpointHandlerMappingCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/ManagementServletContext.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/ManagementServletContext.java
index ca4c592fe3..6bfca43cd2 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/mvc/ManagementServletContext.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthAggregator.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthAggregator.java
index 6ca10c29c2..68011f15cc 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthAggregator.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicator.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicator.java
index c477433755..aec17a5db7 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthIndicator.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/info/InfoContributor.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/info/InfoContributor.java
index f4c7628e75..a5306572e9 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/info/InfoContributor.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/GaugeService.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/GaugeService.java
index 699d9b2f63..18a25c766e 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/GaugeService.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/Exporter.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/Exporter.java
index 6664b8e3c6..4a0d32fb88 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/Exporter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbNamingStrategy.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbNamingStrategy.java
index 156fe75a54..36884731d7 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/opentsdb/OpenTsdbNamingStrategy.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/reader/PrefixMetricReader.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/reader/PrefixMetricReader.java
index 19a06232b2..de580c9648 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/reader/PrefixMetricReader.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/writer/GaugeWriter.java b/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/writer/GaugeWriter.java
index b9e7017f62..61aab7a187 100644
--- a/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/writer/GaugeWriter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/CacheManagerCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/CacheManagerCustomizer.java
index 6ea91c457d..0b9693da8f 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/CacheManagerCustomizer.java
+++ b/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> {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/JCacheManagerCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/JCacheManagerCustomizer.java
index 02d241fb3d..850a396755 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/cache/JCacheManagerCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayMigrationStrategy.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayMigrationStrategy.java
index 7d8dc0e892..aed57e671a 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway/FlywayMigrationStrategy.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/Jackson2ObjectMapperBuilderCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/Jackson2ObjectMapperBuilderCustomizer.java
index 8cf88109ff..511c519be1 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/Jackson2ObjectMapperBuilderCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/metadata/DataSourcePoolMetadataProvider.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/metadata/DataSourcePoolMetadataProvider.java
index 4ec0ee5545..d40a010227 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/metadata/DataSourcePoolMetadataProvider.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jersey/ResourceConfigCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jersey/ResourceConfigCustomizer.java
index 0bb08cc030..9ee7c9ba69 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jersey/ResourceConfigCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jms/artemis/ArtemisConfigurationCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jms/artemis/ArtemisConfigurationCustomizer.java
index 1716a46d87..4fecb641ac 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jms/artemis/ArtemisConfigurationCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/AuthoritiesExtractor.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/AuthoritiesExtractor.java
index 44f8866e07..93e71589d9 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/AuthoritiesExtractor.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/JwtAccessTokenConverterConfigurer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/JwtAccessTokenConverterConfigurer.java
index 2e43ce43d4..8e7ad5c1aa 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/JwtAccessTokenConverterConfigurer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/PrincipalExtractor.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/PrincipalExtractor.java
index 224b20ce58..5c139a1c13 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/PrincipalExtractor.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/UserInfoRestTemplateCustomizer.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/UserInfoRestTemplateCustomizer.java
index 26b5cc4f66..c8afa1d328 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/oauth2/resource/UserInfoRestTemplateCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/template/TemplateAvailabilityProvider.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/template/TemplateAvailabilityProvider.java
index 9f01d70c25..73bbd90023 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/template/TemplateAvailabilityProvider.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorController.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorController.java
index 7d605e8622..661c0ec7a1 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorController.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorViewResolver.java b/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorViewResolver.java
index d2fdaf75fc..2ff8df1613 100644
--- a/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorViewResolver.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/CommandFactory.java b/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/CommandFactory.java
index 0b6ff1a2e5..82d7724709 100644
--- a/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/CommandFactory.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/install/DependencyResolver.java b/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/install/DependencyResolver.java
index de664e512e..4ffd99bcc5 100644
--- a/spring-boot-cli/src/main/java/org/springframework/boot/cli/command/install/DependencyResolver.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/SpringBootAstTransformation.java b/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/SpringBootAstTransformation.java
index 2e8f4f6575..0114cd839c 100644
--- a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/SpringBootAstTransformation.java
+++ b/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 {
 
 }
diff --git a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/ProgressReporter.java b/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/ProgressReporter.java
index e0283979c5..ca612771ef 100644
--- a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/ProgressReporter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/RepositorySystemSessionAutoConfiguration.java b/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/RepositorySystemSessionAutoConfiguration.java
index 44dc2c25c6..69fd7034fb 100644
--- a/spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/grape/RepositorySystemSessionAutoConfiguration.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathRestartStrategy.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathRestartStrategy.java
index 456dd4d5a9..cec5f792ae 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathRestartStrategy.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileChangeListener.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileChangeListener.java
index f6f3264575..0b5a779650 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileChangeListener.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherFactory.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherFactory.java
index 15971a425d..13288e55f1 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherFactory.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/AccessManager.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/AccessManager.java
index c196516500..0dc0a22890 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/AccessManager.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/Handler.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/Handler.java
index 1bff5b3f9c..3123396d42 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/Handler.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/HandlerMapper.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/HandlerMapper.java
index 499274070e..cf78248a9e 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/HandlerMapper.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/FailureHandler.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/FailureHandler.java
index 851b337b2b..1f78d46f4b 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/FailureHandler.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartInitializer.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartInitializer.java
index d029bc5d73..a57edad234 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartInitializer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartListener.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartListener.java
index f490e71797..5807bb0066 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartListener.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFileRepository.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFileRepository.java
index 4f8c8a8559..34e9ac7c24 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFileRepository.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/SourceFolderUrlFilter.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/SourceFolderUrlFilter.java
index 815d611ed6..e7c23daa06 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/SourceFolderUrlFilter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/client/TunnelConnection.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/client/TunnelConnection.java
index 8ac5662b01..939c77a733 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/client/TunnelConnection.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/PortProvider.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/PortProvider.java
index 1f5298c312..ef002e0edb 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/PortProvider.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/TargetServerConnection.java b/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/TargetServerConnection.java
index 69dcfaa5e4..6495477f2b 100644
--- a/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/TargetServerConnection.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/restdocs/RestDocsMockMvcConfigurationCustomizer.java b/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/restdocs/RestDocsMockMvcConfigurationCustomizer.java
index 0662da72ca..ace7ef0790 100644
--- a/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/restdocs/RestDocsMockMvcConfigurationCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/servlet/MockMvcBuilderCustomizer.java b/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/servlet/MockMvcBuilderCustomizer.java
index ecf62f8b74..a3431e8b07 100644
--- a/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/servlet/MockMvcBuilderCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/FieldValuesParser.java b/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/FieldValuesParser.java
index c91d0c5fa4..45154a498d 100644
--- a/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/FieldValuesParser.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/TreeVisitor.java b/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/TreeVisitor.java
index b73cbc28d5..9c785feeaa 100644
--- a/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/TreeVisitor.java
+++ b/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;
diff --git a/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/PluginFeatures.java b/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/PluginFeatures.java
index 6566ac1c0a..b16946a35a 100644
--- a/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/PluginFeatures.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LaunchScript.java b/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LaunchScript.java
index 9f0c5f25e1..24855b5299 100644
--- a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LaunchScript.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Libraries.java b/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Libraries.java
index 41af72d18c..ce5ccc0cfd 100644
--- a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Libraries.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LibraryCallback.java b/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LibraryCallback.java
index df030a91d5..d21fe00812 100644
--- a/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/LibraryCallback.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarEntryFilter.java b/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarEntryFilter.java
index 3346b89168..bbf7f95ea8 100644
--- a/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarEntryFilter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/ApplicationRunner.java b/spring-boot/src/main/java/org/springframework/boot/ApplicationRunner.java
index 69fc8d5626..03031ce2de 100644
--- a/spring-boot/src/main/java/org/springframework/boot/ApplicationRunner.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/Banner.java b/spring-boot/src/main/java/org/springframework/boot/Banner.java
index 079a2cf940..7959500cd9 100644
--- a/spring-boot/src/main/java/org/springframework/boot/Banner.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/CommandLineRunner.java b/spring-boot/src/main/java/org/springframework/boot/CommandLineRunner.java
index 115ff3bf7c..55b8d46bb4 100644
--- a/spring-boot/src/main/java/org/springframework/boot/CommandLineRunner.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/ExitCodeExceptionMapper.java b/spring-boot/src/main/java/org/springframework/boot/ExitCodeExceptionMapper.java
index 280b34a72c..f548c348e2 100644
--- a/spring-boot/src/main/java/org/springframework/boot/ExitCodeExceptionMapper.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/ExitCodeGenerator.java b/spring-boot/src/main/java/org/springframework/boot/ExitCodeGenerator.java
index 20b7ec7bed..f21efd1bfe 100644
--- a/spring-boot/src/main/java/org/springframework/boot/ExitCodeGenerator.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/bind/PropertyNamePatternsMatcher.java b/spring-boot/src/main/java/org/springframework/boot/bind/PropertyNamePatternsMatcher.java
index 7d186f7024..e51ce3f7d8 100644
--- a/spring-boot/src/main/java/org/springframework/boot/bind/PropertyNamePatternsMatcher.java
+++ b/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() {
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerCustomizer.java
index 91db26fcb6..d897b39444 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerFactory.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerFactory.java
index b459eadebb..80486a804c 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerFactory.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyServerCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyServerCustomizer.java
index 90614a0f7d..af39bea2f7 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyServerCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatConnectorCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatConnectorCustomizer.java
index d621290158..50f7156b8e 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatConnectorCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatContextCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatContextCustomizer.java
index 36c94cb018..4ee3a7ce13 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatContextCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowBuilderCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowBuilderCustomizer.java
index 234b2890a8..3f9819c161 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowBuilderCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowDeploymentInfoCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowDeploymentInfoCustomizer.java
index 11a195a5f5..5bf4822eb5 100644
--- a/spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowDeploymentInfoCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalysisReporter.java b/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalysisReporter.java
index 31dca6fdcb..72f402a295 100644
--- a/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalysisReporter.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalyzer.java b/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalyzer.java
index a3f3192047..5c7e5d233f 100644
--- a/spring-boot/src/main/java/org/springframework/boot/diagnostics/FailureAnalyzer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/env/EnvironmentPostProcessor.java b/spring-boot/src/main/java/org/springframework/boot/env/EnvironmentPostProcessor.java
index 3c29532da7..634b9d90fb 100644
--- a/spring-boot/src/main/java/org/springframework/boot/env/EnvironmentPostProcessor.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/jta/XAConnectionFactoryWrapper.java b/spring-boot/src/main/java/org/springframework/boot/jta/XAConnectionFactoryWrapper.java
index 2817a5bf6a..c8dc41542f 100644
--- a/spring-boot/src/main/java/org/springframework/boot/jta/XAConnectionFactoryWrapper.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/jta/XADataSourceWrapper.java b/spring-boot/src/main/java/org/springframework/boot/jta/XADataSourceWrapper.java
index d8186688a1..26019f18ad 100644
--- a/spring-boot/src/main/java/org/springframework/boot/jta/XADataSourceWrapper.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateCustomizer.java b/spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateCustomizer.java
index fb902d41aa..df9b715aed 100644
--- a/spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateCustomizer.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistrar.java b/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistrar.java
index 65fefa75e6..1860391759 100644
--- a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistrar.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistry.java b/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistry.java
index a4eaccde52..e177508707 100644
--- a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistry.java
+++ b/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 {
 
 	/**
diff --git a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializer.java b/spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializer.java
index ea4c2546d2..fb926e6f1e 100644
--- a/spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializer.java
+++ b/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 {
 
 	/**