Add RFC-8246 support to CacheControl

This commit adds the "immutable" Cache-Control directives in `CacheControl`.

Closes gh-29955

spring-web/src/main/java/org/springframework/http/CacheControl.java

@@ -77,6 +77,8 @@ public class CacheControl {
 	@Nullable
 	private Duration sMaxAge;
 
+	private boolean immutable = false;
+
 
 	/**
 	 * Create an empty CacheControl instance.
@@ -320,6 +322,18 @@ public class CacheControl {
 		return this;
 	}
 
+	/**
+	 * Add an "immutable" directive.
+	 * <p>This directive indicates that the origin server will not update the
+	 * representation of that resource during the freshness lifetime of the response.
+	 * @return {@code this}, to facilitate method chaining
+	 * @see <a href="https://tools.ietf.org/html/rfc8246">rfc8246</a>
+	 */
+	public CacheControl immutable() {
+		this.immutable = true;
+		return this;
+	}
+
 	/**
 	 * Return the "Cache-Control" header value, if any.
 	 * @return the header value, or {@code null} if no directive was added
@@ -369,6 +383,9 @@ public class CacheControl {
 		if (this.staleWhileRevalidate != null) {
 			appendDirective(headerValue, "stale-while-revalidate=" + this.staleWhileRevalidate.getSeconds());
 		}
+		if (this.immutable) {
+			appendDirective(headerValue, "immutable");
+		}
 		return headerValue.toString();
 	}
 

spring-web/src/test/java/org/springframework/http/CacheControlTests.java

@@ -101,4 +101,10 @@ public class CacheControlTests {
 		assertThat(cc.getHeaderValue()).isEqualTo("max-age=3600, stale-while-revalidate=7200");
 	}
 
+	@Test
+	public void immutable() throws Exception {
+		CacheControl cc = CacheControl.maxAge(Duration.ofHours(1)).immutable();
+		assertThat(cc.getHeaderValue()).isEqualTo("max-age=3600, immutable");
+	}
+
 }