Allow multiple values to be specified when configuring a default header

See gh-17286

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

@@ -91,7 +91,7 @@ public class RestTemplateBuilder {
 
 	private final BasicAuthentication basicAuthentication;
 
-	private final Map<String, String> defaultHeaders;
+	private final Map<String, List<String>> defaultHeaders;
 
 	private final Set<RestTemplateCustomizer> customizers;
 
@@ -122,7 +122,7 @@ public class RestTemplateBuilder {
 			String rootUri, Set<HttpMessageConverter<?>> messageConverters,
 			Set<ClientHttpRequestInterceptor> interceptors, Supplier<ClientHttpRequestFactory> requestFactorySupplier,
 			UriTemplateHandler uriTemplateHandler, ResponseErrorHandler errorHandler,
-			BasicAuthentication basicAuthentication, Map<String, String> defaultHeaders,
+			BasicAuthentication basicAuthentication, Map<String, List<String>> defaultHeaders,
 			Set<RestTemplateCustomizer> customizers, Set<RestTemplateRequestCustomizer<?>> requestCustomizers) {
 		this.requestFactoryCustomizer = requestFactoryCustomizer;
 		this.detectRequestFactory = detectRequestFactory;
@@ -395,15 +395,17 @@ public class RestTemplateBuilder {
 	 * Add a default header that will be set if not already present on the outgoing
 	 * {@link HttpClientRequest}.
 	 * @param name the name of the header
-	 * @param value the header value
+	 * @param values the header values
 	 * @return a new builder instance
 	 * @since 2.2.0
 	 */
-	public RestTemplateBuilder defaultHeader(String name, String value) {
+	public RestTemplateBuilder defaultHeader(String name, String... values) {
+		Assert.notNull(name, "Name must not be null");
+		Assert.notNull(values, "Values must not be null");
 		return new RestTemplateBuilder(this.requestFactoryCustomizer, this.detectRequestFactory, this.rootUri,
 				this.messageConverters, this.interceptors, this.requestFactory, this.uriTemplateHandler,
-				this.errorHandler, this.basicAuthentication, append(this.defaultHeaders, name, value), this.customizers,
-				this.requestCustomizers);
+				this.errorHandler, this.basicAuthentication, append(this.defaultHeaders, name, values),
+				this.customizers, this.requestCustomizers);
 	}
 
 	/**
@@ -683,6 +685,10 @@ public class RestTemplateBuilder {
 		return Collections.unmodifiableSet(new LinkedHashSet<>(collection));
 	}
 
+	private static <T> List<T> listOf(T[] items) {
+		return Collections.unmodifiableList(new ArrayList<>(Arrays.asList(items)));
+	}
+
 	private static <T> Set<T> append(Collection<? extends T> collection, Collection<? extends T> additions) {
 		Set<T> result = new LinkedHashSet<>((collection != null) ? collection : Collections.emptySet());
 		if (additions != null) {
@@ -691,9 +697,11 @@ public class RestTemplateBuilder {
 		return Collections.unmodifiableSet(result);
 	}
 
-	private static <K, V> Map<K, V> append(Map<K, V> map, K key, V value) {
-		Map<K, V> result = new LinkedHashMap<>((map != null) ? map : Collections.emptyMap());
-		result.put(key, value);
+	private static <K, V> Map<K, List<V>> append(Map<K, List<V>> map, K key, V[] values) {
+		Map<K, List<V>> result = new LinkedHashMap<>((map != null) ? map : Collections.emptyMap());
+		if (values != null) {
+			result.put(key, listOf(values));
+		}
 		return Collections.unmodifiableMap(result);
 	}
 

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

@@ -18,6 +18,7 @@ package org.springframework.boot.web.client;
 
 import java.io.IOException;
 import java.net.URI;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -39,12 +40,12 @@ class RestTemplateBuilderClientHttpRequestFactoryWrapper extends AbstractClientH
 
 	private final BasicAuthentication basicAuthentication;
 
-	private final Map<String, String> defaultHeaders;
+	private final Map<String, List<String>> defaultHeaders;
 
 	private final Set<RestTemplateRequestCustomizer<?>> requestCustomizers;
 
 	RestTemplateBuilderClientHttpRequestFactoryWrapper(ClientHttpRequestFactory requestFactory,
-			BasicAuthentication basicAuthentication, Map<String, String> defaultHeaders,
+			BasicAuthentication basicAuthentication, Map<String, List<String>> defaultHeaders,
 			Set<RestTemplateRequestCustomizer<?>> requestCustomizers) {
 		super(requestFactory);
 		this.basicAuthentication = basicAuthentication;
@@ -61,7 +62,7 @@ class RestTemplateBuilderClientHttpRequestFactoryWrapper extends AbstractClientH
 		if (this.basicAuthentication != null) {
 			this.basicAuthentication.applyTo(headers);
 		}
-		this.defaultHeaders.forEach(headers::addIfAbsent);
+		this.defaultHeaders.forEach(headers::putIfAbsent);
 		LambdaSafe.callbacks(RestTemplateRequestCustomizer.class, this.requestCustomizers, request)
 				.invoke((customizer) -> customizer.customize(request));
 		return request;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/client/RestTemplateBuilderClientHttpRequestFactoryWrapperTests.java

@@ -18,9 +18,11 @@ package org.springframework.boot.web.client;
 
 import java.io.IOException;
 import java.net.URI;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -80,16 +82,16 @@ public class RestTemplateBuilderClientHttpRequestFactoryWrapperTests {
 	@Test
 	void createRequestWhenHasDefaultHeadersAddsMissing() throws IOException {
 		this.headers.add("one", "existing");
-		Map<String, String> defaultHeaders = new LinkedHashMap<>();
-		defaultHeaders.put("one", "1");
-		defaultHeaders.put("two", "2");
-		defaultHeaders.put("three", "3");
+		Map<String, List<String>> defaultHeaders = new LinkedHashMap<>();
+		defaultHeaders.put("one", Collections.singletonList("1"));
+		defaultHeaders.put("two", Arrays.asList("2", "3"));
+		defaultHeaders.put("three", Collections.singletonList("4"));
 		this.requestFactory = new RestTemplateBuilderClientHttpRequestFactoryWrapper(this.requestFactory, null,
 				defaultHeaders, Collections.emptySet());
 		ClientHttpRequest request = createRequest();
 		assertThat(request.getHeaders().get("one")).containsExactly("existing");
-		assertThat(request.getHeaders().get("two")).containsExactly("2");
-		assertThat(request.getHeaders().get("three")).containsExactly("3");
+		assertThat(request.getHeaders().get("two")).containsExactly("2", "3");
+		assertThat(request.getHeaders().get("three")).containsExactly("4");
 	}
 
 	@Test

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/client/RestTemplateBuilderTests.java

@@ -20,6 +20,7 @@ import java.io.IOException;
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Set;
 import java.util.function.Supplier;
@@ -33,6 +34,7 @@ import org.mockito.MockitoAnnotations;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
 import org.springframework.http.client.BufferingClientHttpRequestFactory;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpRequestFactory;
@@ -321,6 +323,16 @@ class RestTemplateBuilderTests {
 		assertThat(request.getHeaders()).contains(entry("spring", Collections.singletonList("boot")));
 	}
 
+	@Test
+	void defaultHeaderAddsHeaderValues() throws IOException {
+		String name = HttpHeaders.ACCEPT;
+		String[] values = { MediaType.APPLICATION_JSON_VALUE, MediaType.APPLICATION_XML_VALUE };
+		RestTemplate template = this.builder.defaultHeader(name, values).build();
+		ClientHttpRequestFactory requestFactory = template.getRequestFactory();
+		ClientHttpRequest request = requestFactory.createRequest(URI.create("http://localhost"), HttpMethod.GET);
+		assertThat(request.getHeaders()).contains(entry(name, Arrays.asList(values)));
+	}
+
 	@Test
 	void requestCustomizersAddsCustomizers() throws IOException {
 		RestTemplate template = this.builder