Annotate interfaces with @FunctionaInterface

Closes gh-6857

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 {
 
 	/**