diff --git a/spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java b/spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java
index 55454b00f20..9b37dc433a2 100644
--- a/spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java
+++ b/spring-web/src/main/java/org/springframework/http/client/JdkClientHttpRequest.java
@@ -216,7 +216,7 @@ class JdkClientHttpRequest extends AbstractStreamingClientHttpRequest {
 	 * {@code jdk.httpclient.allowRestrictedHeaders} system property.
 	 * @see jdk.internal.net.http.common.Utils#getDisallowedHeaders()
 	 */
-	private static Set<String> disallowedHeaders() {
+	static Set<String> disallowedHeaders() {
 		TreeSet<String> headers = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
 		headers.addAll(Set.of("connection", "content-length", "expect", "host", "upgrade"));
 
diff --git a/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestFactoryTests.java b/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestFactoryTests.java
index f44246b3d8a..e41ec8177b9 100644
--- a/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestFactoryTests.java
+++ b/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestFactoryTests.java
@@ -21,9 +21,6 @@ import java.net.URI;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 
-import org.jspecify.annotations.Nullable;
-import org.junit.jupiter.api.AfterAll;
-import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.condition.EnabledForJreRange;
 import org.junit.jupiter.api.condition.JRE;
@@ -32,7 +29,6 @@ import org.junit.jupiter.params.provider.ValueSource;
 
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
-import org.springframework.http.HttpStatusCode;
 import org.springframework.util.StreamUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -45,26 +41,6 @@ import static org.assertj.core.api.Assertions.assertThat;
  */
 class JdkClientHttpRequestFactoryTests extends AbstractHttpRequestFactoryTests {
 
-	private static @Nullable String originalPropertyValue;
-
-
-	@BeforeAll
-	static void setProperty() {
-		originalPropertyValue = System.getProperty("jdk.httpclient.allowRestrictedHeaders");
-		System.setProperty("jdk.httpclient.allowRestrictedHeaders", "expect");
-	}
-
-	@AfterAll
-	static void restoreProperty() {
-		if (originalPropertyValue != null) {
-			System.setProperty("jdk.httpclient.allowRestrictedHeaders", originalPropertyValue);
-		}
-		else {
-			System.clearProperty("jdk.httpclient.allowRestrictedHeaders");
-		}
-	}
-
-
 	@Override
 	protected ClientHttpRequestFactory createRequestFactory() {
 		return new JdkClientHttpRequestFactory();
@@ -77,17 +53,6 @@ class JdkClientHttpRequestFactoryTests extends AbstractHttpRequestFactoryTests {
 		assertHttpMethod("patch", HttpMethod.PATCH);
 	}
 
-	@Test
-	void customizeDisallowedHeaders() throws IOException {
-		URI uri = URI.create(this.baseUrl + "/status/299");
-		ClientHttpRequest request = this.factory.createRequest(uri, HttpMethod.PUT);
-		request.getHeaders().set("Expect", "299");
-
-		try (ClientHttpResponse response = request.execute()) {
-			assertThat(response.getStatusCode()).as("Invalid status code").isEqualTo(HttpStatusCode.valueOf(299));
-		}
-	}
-
 	@Test // gh-31451
 	void contentLength0() throws IOException {
 		URI uri = URI.create(this.baseUrl + "/methods/get");
diff --git a/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestTests.java b/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestTests.java
index 5b2f0bdc42b..e0e58f9c120 100644
--- a/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestTests.java
+++ b/spring-web/src/test/java/org/springframework/http/client/JdkClientHttpRequestTests.java
@@ -24,16 +24,19 @@ import java.net.http.HttpRequest;
 import java.net.http.HttpResponse;
 import java.net.http.HttpTimeoutException;
 import java.time.Duration;
+import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.AutoClose;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
@@ -71,6 +74,53 @@ class JdkClientHttpRequestTests {
 				.isExactlyInstanceOf(IOException.class);
 	}
 
+	@Test
+	void disallowedHeadersByDefault() {
+		assertThat(disallowedHeaders(null))
+				.containsExactlyInAnyOrder("connection", "content-length", "expect", "host", "upgrade");
+	}
+
+	@Test
+	void disallowedHeadersWithSingleHeaderAllowed() {
+		assertThat(disallowedHeaders("expect"))
+				.containsExactlyInAnyOrder("connection", "content-length", "host", "upgrade");
+	}
+
+	@Test
+	void disallowedHeadersWithMultipleHeadersAllowed() {
+		assertThat(disallowedHeaders("expect,host"))
+				.containsExactlyInAnyOrder("connection", "content-length", "upgrade");
+	}
+
+	@Test
+	void disallowedHeadersAreCaseInsensitive() {
+		// AssertJ's contains() uses equals(), so go through Set.contains() instead.
+		assertThat(disallowedHeaders(null).contains("EXPECT")).isTrue();
+		assertThat(disallowedHeaders("Expect").contains("expect")).isFalse();
+	}
+
+	private static Set<String> disallowedHeaders(@Nullable String allowRestrictedHeaders) {
+		String key = "jdk.httpclient.allowRestrictedHeaders";
+		String original = System.getProperty(key);
+		try {
+			if (allowRestrictedHeaders != null) {
+				System.setProperty(key, allowRestrictedHeaders);
+			}
+			else {
+				System.clearProperty(key);
+			}
+			return JdkClientHttpRequest.disallowedHeaders();
+		}
+		finally {
+			if (original != null) {
+				System.setProperty(key, original);
+			}
+			else {
+				System.clearProperty(key);
+			}
+		}
+	}
+
 	private JdkClientHttpRequest createRequest(Duration timeout) {
 		return new JdkClientHttpRequest(client, URI.create("https://abc.com"), HttpMethod.GET, executor, timeout, false);
 	}