Declare interfaces as @FunctionalInterface where feasible

This commit declares each of the following public interfaces as a
@FunctionalInterface.

- org.springframework.context.ApplicationContextInitializer
- org.springframework.test.web.servlet.DispatcherServletCustomizer
- org.springframework.validation.MessageCodeFormatter
- org.springframework.util.IdGenerator
- org.springframework.beans.factory.config.YamlProcessor.MatchCallback
- org.springframework.beans.factory.config.YamlProcessor.DocumentMatcher

Closes gh-25580

spring-beans/src/main/java/org/springframework/beans/factory/config/YamlProcessor.java

@@ -351,6 +351,7 @@ public abstract class YamlProcessor {
 	/**
 	 * Callback interface used to process the YAML parsing results.
 	 */
+	@FunctionalInterface
 	public interface MatchCallback {
 
 		/**
@@ -367,6 +368,7 @@ public abstract class YamlProcessor {
 	/**
 	 * Strategy interface used to test if properties match.
 	 */
+	@FunctionalInterface
 	public interface DocumentMatcher {
 
 		/**

spring-beans/src/test/java/org/springframework/beans/factory/config/YamlPropertiesFactoryBeanTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,7 +24,6 @@ import org.yaml.snakeyaml.Yaml;
 import org.yaml.snakeyaml.constructor.DuplicateKeyException;
 import org.yaml.snakeyaml.scanner.ScannerException;
 
-import org.springframework.beans.factory.config.YamlProcessor.DocumentMatcher;
 import org.springframework.beans.factory.config.YamlProcessor.MatchStatus;
 import org.springframework.beans.factory.config.YamlProcessor.ResolutionMethod;
 import org.springframework.core.io.ByteArrayResource;
@@ -39,10 +38,10 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
  * @author Dave Syer
  * @author Juergen Hoeller
  */
-public class YamlPropertiesFactoryBeanTests {
+class YamlPropertiesFactoryBeanTests {
 
 	@Test
-	public void loadResource() {
+	void loadResource() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("foo: bar\nspam:\n  foo: baz".getBytes()));
 		Properties properties = factory.getObject();
@@ -51,7 +50,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void badResource() {
+	void badResource() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("foo: bar\ncd\nspam:\n  foo: baz".getBytes()));
 		assertThatExceptionOfType(ScannerException.class)
@@ -60,7 +59,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourcesWithOverride() {
+	void loadResourcesWithOverride() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(
 				new ByteArrayResource("foo: bar\nspam:\n  foo: baz".getBytes()),
@@ -72,7 +71,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourcesWithInternalOverride() {
+	void loadResourcesWithInternalOverride() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource(
 				"foo: bar\nspam:\n  foo: baz\nfoo: bucket".getBytes()));
@@ -80,7 +79,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourcesWithNestedInternalOverride() {
+	void loadResourcesWithNestedInternalOverride() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource(
 				"foo:\n  bar: spam\n  foo: baz\nbreak: it\nfoo: bucket".getBytes()));
@@ -88,7 +87,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourceWithMultipleDocuments() {
+	void loadResourceWithMultipleDocuments() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource(
 				"foo: bar\nspam: baz\n---\nfoo: bag".getBytes()));
@@ -98,7 +97,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourceWithSelectedDocuments() {
+	void loadResourceWithSelectedDocuments() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource(
 				"foo: bar\nspam: baz\n---\nfoo: bag\nspam: bad".getBytes()));
@@ -110,7 +109,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourceWithDefaultMatch() {
+	void loadResourceWithDefaultMatch() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setMatchDefault(true);
 		factory.setResources(new ByteArrayResource(
@@ -129,20 +128,17 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourceWithoutDefaultMatch() {
+	void loadResourceWithoutDefaultMatch() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setMatchDefault(false);
 		factory.setResources(new ByteArrayResource(
 				"one: two\n---\nfoo: bar\nspam: baz\n---\nfoo: bag\nspam: bad".getBytes()));
-		factory.setDocumentMatchers(new DocumentMatcher() {
+		factory.setDocumentMatchers(properties -> {
-			@Override
-			public MatchStatus matches(Properties properties) {
 				if (!properties.containsKey("foo")) {
 					return MatchStatus.ABSTAIN;
 				}
 				return ("bag".equals(properties.getProperty("foo")) ?
 						MatchStatus.FOUND : MatchStatus.NOT_FOUND);
-			}
 		});
 		Properties properties = factory.getObject();
 		assertThat(properties.getProperty("foo")).isEqualTo("bag");
@@ -151,7 +147,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadResourceWithDefaultMatchSkippingMissedMatch() {
+	void loadResourceWithDefaultMatchSkippingMissedMatch() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setMatchDefault(true);
 		factory.setResources(new ByteArrayResource(
@@ -170,7 +166,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadNonExistentResource() {
+	void loadNonExistentResource() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResolutionMethod(ResolutionMethod.OVERRIDE_AND_IGNORE);
 		factory.setResources(new ClassPathResource("no-such-file.yml"));
@@ -179,7 +175,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadNull() {
+	void loadNull() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("foo: bar\nspam:".getBytes()));
 		Properties properties = factory.getObject();
@@ -188,7 +184,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadEmptyArrayValue() {
+	void loadEmptyArrayValue() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("a: alpha\ntest: []".getBytes()));
 		Properties properties = factory.getObject();
@@ -197,7 +193,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadArrayOfString() {
+	void loadArrayOfString() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("foo:\n- bar\n- baz".getBytes()));
 		Properties properties = factory.getObject();
@@ -207,7 +203,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadArrayOfInteger() {
+	void loadArrayOfInteger() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource("foo:\n- 1\n- 2".getBytes()));
 		Properties properties = factory.getObject();
@@ -217,7 +213,7 @@ public class YamlPropertiesFactoryBeanTests {
 	}
 
 	@Test
-	public void loadArrayOfObject() {
+	void loadArrayOfObject() {
 		YamlPropertiesFactoryBean factory = new YamlPropertiesFactoryBean();
 		factory.setResources(new ByteArrayResource(
 				"foo:\n- bar:\n    spam: crap\n- baz\n- one: two\n  three: four".getBytes()
@@ -232,7 +228,7 @@ public class YamlPropertiesFactoryBeanTests {
 
 	@Test
 	@SuppressWarnings("unchecked")
-	public void yaml() {
+	void yaml() {
 		Yaml yaml = new Yaml();
 		Map<String, ?> map = yaml.loadAs("foo: bar\nspam:\n  foo: baz", Map.class);
 		assertThat(map.get("foo")).isEqualTo("bar");

spring-context/src/main/java/org/springframework/context/ApplicationContextInitializer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,7 +28,7 @@ package org.springframework.context;
  *
  * <p>{@code ApplicationContextInitializer} processors are encouraged to detect
  * whether Spring's {@link org.springframework.core.Ordered Ordered} interface has been
- * implemented or if the @{@link org.springframework.core.annotation.Order Order}
+ * implemented or if the {@link org.springframework.core.annotation.Order @Order}
  * annotation is present and to sort instances accordingly if so prior to invocation.
  *
  * @author Chris Beams
@@ -39,6 +39,7 @@ package org.springframework.context;
  * @see org.springframework.web.servlet.FrameworkServlet#setContextInitializerClasses
  * @see org.springframework.web.servlet.FrameworkServlet#applyInitializers
  */
+@FunctionalInterface
 public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
 
 	/**

spring-context/src/main/java/org/springframework/validation/MessageCodeFormatter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@ import org.springframework.lang.Nullable;
  * @see DefaultMessageCodesResolver
  * @see DefaultMessageCodesResolver.Format
  */
+@FunctionalInterface
 public interface MessageCodeFormatter {
 
 	/**

spring-context/src/test/java/org/springframework/validation/DefaultMessageCodesResolverTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,22 +28,20 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Phillip Webb
  */
-public class DefaultMessageCodesResolverTests {
+class DefaultMessageCodesResolverTests {
+
+	private final DefaultMessageCodesResolver resolver = new DefaultMessageCodesResolver();
 
-	private DefaultMessageCodesResolver resolver = new DefaultMessageCodesResolver();
 
 	@Test
-	public void shouldResolveMessageCode() throws Exception {
+	void shouldResolveMessageCode() throws Exception {
 		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName");
-		assertThat(codes).containsExactly(
+		assertThat(codes).containsExactly("errorCode.objectName", "errorCode");
-				"errorCode.objectName",
-				"errorCode");
 	}
 
 	@Test
-	public void shouldResolveFieldMessageCode() throws Exception {
+	void shouldResolveFieldMessageCode() throws Exception {
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"errorCode.objectName.field",
 				"errorCode.field",
@@ -52,9 +50,8 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldResolveIndexedFieldMessageCode() throws Exception {
+	void shouldResolveIndexedFieldMessageCode() throws Exception {
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "a.b[3].c[5].d",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "a.b[3].c[5].d", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"errorCode.objectName.a.b[3].c[5].d",
 				"errorCode.objectName.a.b[3].c.d",
@@ -68,19 +65,16 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldResolveMessageCodeWithPrefix() throws Exception {
+	void shouldResolveMessageCodeWithPrefix() throws Exception {
 		resolver.setPrefix("prefix.");
 		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName");
-		assertThat(codes).containsExactly(
+		assertThat(codes).containsExactly("prefix.errorCode.objectName", "prefix.errorCode");
-				"prefix.errorCode.objectName",
-				"prefix.errorCode");
 	}
 
 	@Test
-	public void shouldResolveFieldMessageCodeWithPrefix() throws Exception {
+	void shouldResolveFieldMessageCodeWithPrefix() throws Exception {
 		resolver.setPrefix("prefix.");
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"prefix.errorCode.objectName.field",
 				"prefix.errorCode.field",
@@ -89,10 +83,9 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldSupportNullPrefix() throws Exception {
+	void shouldSupportNullPrefix() throws Exception {
 		resolver.setPrefix(null);
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"errorCode.objectName.field",
 				"errorCode.field",
@@ -101,9 +94,8 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldSupportMalformedIndexField() throws Exception {
+	void shouldSupportMalformedIndexField() throws Exception {
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field[",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field[", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"errorCode.objectName.field[",
 				"errorCode.field[",
@@ -112,9 +104,8 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldSupportNullFieldType() throws Exception {
+	void shouldSupportNullFieldType() throws Exception {
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field", null);
-				null);
 		assertThat(codes).containsExactly(
 				"errorCode.objectName.field",
 				"errorCode.field",
@@ -122,19 +113,16 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldSupportPostfixFormat() throws Exception {
+	void shouldSupportPostfixFormat() throws Exception {
 		resolver.setMessageCodeFormatter(Format.POSTFIX_ERROR_CODE);
 		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName");
-		assertThat(codes).containsExactly(
+		assertThat(codes).containsExactly("objectName.errorCode", "errorCode");
-				"objectName.errorCode",
-				"errorCode");
 	}
 
 	@Test
-	public void shouldSupportFieldPostfixFormat() throws Exception {
+	void shouldSupportFieldPostfixFormat() throws Exception {
 		resolver.setMessageCodeFormatter(Format.POSTFIX_ERROR_CODE);
-		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field",
+		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName", "field", TestBean.class);
-				TestBean.class);
 		assertThat(codes).containsExactly(
 				"objectName.field.errorCode",
 				"field.errorCode",
@@ -143,18 +131,11 @@ public class DefaultMessageCodesResolverTests {
 	}
 
 	@Test
-	public void shouldSupportCustomFormat() throws Exception {
+	void shouldSupportCustomFormat() throws Exception {
-		resolver.setMessageCodeFormatter(new MessageCodeFormatter() {
+		resolver.setMessageCodeFormatter((errorCode, objectName, field) ->
-			@Override
+				DefaultMessageCodesResolver.Format.toDelimitedString("CUSTOM-" + errorCode, objectName, field));
-			public String format(String errorCode, String objectName, String field) {
-				return DefaultMessageCodesResolver.Format.toDelimitedString(
-						"CUSTOM-" + errorCode, objectName, field);
-			}
-		});
 		String[] codes = resolver.resolveMessageCodes("errorCode", "objectName");
-		assertThat(codes).containsExactly(
+		assertThat(codes).containsExactly("CUSTOM-errorCode.objectName", "CUSTOM-errorCode");
-				"CUSTOM-errorCode.objectName",
-				"CUSTOM-errorCode");
 	}
 
 }

spring-core/src/main/java/org/springframework/util/IdGenerator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,11 +19,12 @@ package org.springframework.util;
 import java.util.UUID;
 
 /**
- * Contract for generating universally unique identifiers {@link UUID (UUIDs)}.
+ * Contract for generating universally unique identifiers ({@link UUID UUIDs}).
  *
  * @author Rossen Stoyanchev
  * @since 4.0
  */
+@FunctionalInterface
 public interface IdGenerator {
 
 	/**

spring-messaging/src/test/java/org/springframework/messaging/support/MessageBuilderTests.java

@@ -26,7 +26,6 @@ import org.junit.jupiter.api.Test;
 
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
-import org.springframework.util.IdGenerator;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
@@ -36,16 +35,16 @@ import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
  * @author Mark Fisher
  * @author Rossen Stoyanchev
  */
-public class MessageBuilderTests {
+class MessageBuilderTests {
 
 	@Test
-	public void testSimpleMessageCreation() {
+	void simpleMessageCreation() {
 		Message<String> message = MessageBuilder.withPayload("foo").build();
 		assertThat(message.getPayload()).isEqualTo("foo");
 	}
 
 	@Test
-	public void testHeaderValues() {
+	void headerValues() {
 		Message<String> message = MessageBuilder.withPayload("test")
 				.setHeader("foo", "bar")
 				.setHeader("count", 123)
@@ -55,7 +54,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testCopiedHeaderValues() {
+	void copiedHeaderValues() {
 		Message<String> message1 = MessageBuilder.withPayload("test1")
 				.setHeader("foo", "1")
 				.setHeader("bar", "2")
@@ -74,21 +73,21 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testIdHeaderValueReadOnly() {
+	void idHeaderValueReadOnly() {
 		UUID id = UUID.randomUUID();
 		assertThatIllegalArgumentException().isThrownBy(() ->
 				MessageBuilder.withPayload("test").setHeader(MessageHeaders.ID, id));
 	}
 
 	@Test
-	public void testTimestampValueReadOnly() {
+	void timestampValueReadOnly() {
 		Long timestamp = 12345L;
 		assertThatIllegalArgumentException().isThrownBy(() ->
 				MessageBuilder.withPayload("test").setHeader(MessageHeaders.TIMESTAMP, timestamp).build());
 	}
 
 	@Test
-	public void copyHeadersIfAbsent() {
+	void copyHeadersIfAbsent() {
 		Message<String> message1 = MessageBuilder.withPayload("test1")
 				.setHeader("foo", "bar").build();
 		Message<String> message2 = MessageBuilder.withPayload("test2")
@@ -100,7 +99,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void createFromMessage() {
+	void createFromMessage() {
 		Message<String> message1 = MessageBuilder.withPayload("test")
 				.setHeader("foo", "bar").build();
 		Message<String> message2 = MessageBuilder.fromMessage(message1).build();
@@ -109,7 +108,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test // gh-23417
-	public void createErrorMessageFromErrorMessage() {
+	void createErrorMessageFromErrorMessage() {
 		Message<String> source = MessageBuilder.withPayload("test").setHeader("foo", "bar").build();
 		RuntimeException ex = new RuntimeException();
 		ErrorMessage errorMessage1 = new ErrorMessage(ex, Collections.singletonMap("baz", "42"), source);
@@ -122,7 +121,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void createIdRegenerated() {
+	void createIdRegenerated() {
 		Message<String> message1 = MessageBuilder.withPayload("test")
 				.setHeader("foo", "bar").build();
 		Message<String> message2 = MessageBuilder.fromMessage(message1).setHeader("another", 1).build();
@@ -131,7 +130,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testRemove() {
+	void remove() {
 		Message<Integer> message1 = MessageBuilder.withPayload(1)
 				.setHeader("foo", "bar").build();
 		Message<Integer> message2 = MessageBuilder.fromMessage(message1)
@@ -141,7 +140,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testSettingToNullRemoves() {
+	void settingToNullRemoves() {
 		Message<Integer> message1 = MessageBuilder.withPayload(1)
 				.setHeader("foo", "bar").build();
 		Message<Integer> message2 = MessageBuilder.fromMessage(message1)
@@ -151,28 +150,28 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testNotModifiedSameMessage() throws Exception {
+	void notModifiedSameMessage() throws Exception {
 		Message<?> original = MessageBuilder.withPayload("foo").build();
 		Message<?> result = MessageBuilder.fromMessage(original).build();
 		assertThat(result).isEqualTo(original);
 	}
 
 	@Test
-	public void testContainsHeaderNotModifiedSameMessage() throws Exception {
+	void containsHeaderNotModifiedSameMessage() throws Exception {
 		Message<?> original = MessageBuilder.withPayload("foo").setHeader("bar", 42).build();
 		Message<?> result = MessageBuilder.fromMessage(original).build();
 		assertThat(result).isEqualTo(original);
 	}
 
 	@Test
-	public void testSameHeaderValueAddedNotModifiedSameMessage() throws Exception {
+	void sameHeaderValueAddedNotModifiedSameMessage() throws Exception {
 		Message<?> original = MessageBuilder.withPayload("foo").setHeader("bar", 42).build();
 		Message<?> result = MessageBuilder.fromMessage(original).setHeader("bar", 42).build();
 		assertThat(result).isEqualTo(original);
 	}
 
 	@Test
-	public void testCopySameHeaderValuesNotModifiedSameMessage() throws Exception {
+	void copySameHeaderValuesNotModifiedSameMessage() throws Exception {
 		Date current = new Date();
 		Map<String, Object> originalHeaders = new HashMap<>();
 		originalHeaders.put("b", "xyz");
@@ -187,7 +186,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testBuildMessageWithMutableHeaders() {
+	void buildMessageWithMutableHeaders() {
 		MessageHeaderAccessor accessor = new MessageHeaderAccessor();
 		accessor.setLeaveMutable(true);
 		MessageHeaders headers = accessor.getMessageHeaders();
@@ -199,7 +198,7 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testBuildMessageWithDefaultMutability() {
+	void buildMessageWithDefaultMutability() {
 		MessageHeaderAccessor accessor = new MessageHeaderAccessor();
 		MessageHeaders headers = accessor.getMessageHeaders();
 		Message<?> message = MessageBuilder.createMessage("foo", headers);
@@ -212,21 +211,16 @@ public class MessageBuilderTests {
 	}
 
 	@Test
-	public void testBuildMessageWithoutIdAndTimestamp() {
+	void buildMessageWithoutIdAndTimestamp() {
 		MessageHeaderAccessor headerAccessor = new MessageHeaderAccessor();
-		headerAccessor.setIdGenerator(new IdGenerator() {
+		headerAccessor.setIdGenerator(() -> MessageHeaders.ID_VALUE_NONE);
-			@Override
-			public UUID generateId() {
-				return MessageHeaders.ID_VALUE_NONE;
-			}
-		});
 		Message<?> message = MessageBuilder.createMessage("foo", headerAccessor.getMessageHeaders());
 		assertThat(message.getHeaders().getId()).isNull();
 		assertThat(message.getHeaders().getTimestamp()).isNull();
 	}
 
 	@Test
-	public void testBuildMultipleMessages() {
+	void buildMultipleMessages() {
 		MessageHeaderAccessor headerAccessor = new MessageHeaderAccessor();
 		MessageBuilder<?> messageBuilder = MessageBuilder.withPayload("payload").setHeaders(headerAccessor);
 
@@ -243,4 +237,5 @@ public class MessageBuilderTests {
 		assertThat(message2.getHeaders().get("foo")).isEqualTo("bar2");
 		assertThat(message3.getHeaders().get("foo")).isEqualTo("bar3");
 	}
+
 }

spring-test/src/main/java/org/springframework/test/web/servlet/DispatcherServletCustomizer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ import org.springframework.web.servlet.DispatcherServlet;
  * @author Stephane Nicoll
  * @since 4.3.4
  */
+@FunctionalInterface
 public interface DispatcherServletCustomizer {
 
 	/**