Copy default headers, cookies in WebClient builder

This commit makes copies of the default headers and cookies when a
WebClient is built, so that subsequent changes to these do not affect
previously built clients.

Closes: spring-projectsgh-25992

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClientBuilder.java

@@ -263,9 +263,14 @@ public WebClient build() {
 				.reduce(ExchangeFilterFunction::andThen)
 				.map(filter -> filter.apply(exchange))
 				.orElse(exchange) : exchange);
+
+		HttpHeaders defaultHeaders = copyDefaultHeaders();
+
+		MultiValueMap<String, String> defaultCookies = copyDefaultCookies();
+
 		return new DefaultWebClient(filteredExchange, initUriBuilderFactory(),
-				this.defaultHeaders != null ? HttpHeaders.readOnlyHttpHeaders(this.defaultHeaders) : null,
-				this.defaultCookies != null ? CollectionUtils.unmodifiableMultiValueMap(this.defaultCookies) : null,
+				defaultHeaders,
+				defaultCookies,
 				this.defaultRequest, new DefaultWebClientBuilder(this));
 	}
 
@@ -302,4 +307,28 @@ private UriBuilderFactory initUriBuilderFactory() {
 		return factory;
 	}
 
+	@Nullable
+	private HttpHeaders copyDefaultHeaders() {
+		if (this.defaultHeaders != null) {
+			HttpHeaders copy = new HttpHeaders();
+			this.defaultHeaders.forEach((key, values) -> copy.put(key, new ArrayList<>(values)));
+			return HttpHeaders.readOnlyHttpHeaders(copy);
+		}
+		else {
+			return null;
+		}
+	}
+
+	@Nullable
+	private MultiValueMap<String, String> copyDefaultCookies() {
+		if (this.defaultCookies != null) {
+			MultiValueMap<String, String> copy = new LinkedMultiValueMap<>(this.defaultCookies.size());
+			this.defaultCookies.forEach((key, values) -> copy.put(key, new ArrayList<>(values)));
+			return CollectionUtils.unmodifiableMultiValueMap(copy);
+		}
+		else {
+			return null;
+		}
+	}
+
 }

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -130,7 +130,8 @@ public void requestHeaderAndCookie() {
 	@Test
 	public void defaultHeaderAndCookie() {
 		WebClient client = this.builder
-				.defaultHeader("Accept", "application/json").defaultCookie("id", "123")
+				.defaultHeader("Accept", "application/json")
+				.defaultCookie("id", "123")
 				.build();
 
 		client.get().uri("/path").exchange().block(Duration.ofSeconds(10));
@@ -157,6 +158,35 @@ public void defaultHeaderAndCookieOverrides() {
 		assertThat(request.cookies().getFirst("id")).isEqualTo("456");
 	}
 
+	@Test
+	public void defaultHeaderAndCookieCopies() {
+		WebClient client1 = this.builder
+				.defaultHeader("Accept", "application/json")
+				.defaultCookie("id", "123")
+				.build();
+		WebClient client2 = this.builder
+				.defaultHeader("Accept", "application/xml")
+				.defaultCookies(cookies -> cookies.set("id", "456"))
+				.build();
+
+		client1.get().uri("/path")
+				.exchange().block(Duration.ofSeconds(10));
+
+		ClientRequest request = verifyAndGetRequest();
+		assertThat(request.headers().getFirst("Accept")).isEqualTo("application/json");
+		assertThat(request.cookies().getFirst("id")).isEqualTo("123");
+
+
+		client2.get().uri("/path")
+				.exchange().block(Duration.ofSeconds(10));
+
+		request = verifyAndGetRequest();
+		assertThat(request.headers().getFirst("Accept")).isEqualTo("application/xml");
+		assertThat(request.cookies().getFirst("id")).isEqualTo("456");
+
+
+	}
+
 	@Test
 	public void defaultRequest() {
 		ThreadLocal<String> context = new NamedThreadLocal<>("foo");