Add Cache-Control must-understand directive

Signed-off-by: heka1024 <heka1024@gmail.com>

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

@@ -58,6 +58,8 @@ public class CacheControl {
 
 	private boolean noStore = false;
 
+	private boolean mustUnderstand = false;
+
 	private boolean mustRevalidate = false;
 
 	private boolean noTransform = false;
@@ -165,6 +167,20 @@ public class CacheControl {
 		return cc;
 	}
 
+	/**
+	 * Add a "must-understand" directive.
+	 * <p>This directive limits caching of the response to caches that
+	 * understand and conform to the requirements for the response status code.
+	 * The {@link #noStore()} directive should also be set as a fallback for
+	 * caches that do not implement the "must-understand" directive.
+	 * @return {@code this}, to facilitate method chaining
+	 * @since 7.1
+	 * @see <a href="https://www.rfc-editor.org/rfc/rfc9111#section-5.2.2.3">rfc9111 section 5.2.2.3</a>
+	 */
+	public CacheControl mustUnderstand() {
+		this.mustUnderstand = true;
+		return this;
+	}
 
 	/**
 	 * Add a "must-revalidate" directive.
@@ -358,6 +374,9 @@ public class CacheControl {
 		if (this.noStore) {
 			appendDirective(headerValue, "no-store");
 		}
+		if (this.mustUnderstand) {
+			appendDirective(headerValue, "must-understand");
+		}
 		if (this.mustRevalidate) {
 			appendDirective(headerValue, "must-revalidate");
 		}

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

@@ -78,6 +78,12 @@ class CacheControlTests {
 		assertThat(cc.getHeaderValue()).isEqualTo("no-store");
 	}
 
+	@Test
+	void mustUnderstand() {
+		CacheControl cc = CacheControl.noStore().mustUnderstand();
+		assertThat(cc.getHeaderValue()).isEqualTo("no-store, must-understand");
+	}
+
 	@Test
 	void staleIfError() {
 		CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS).staleIfError(2, TimeUnit.HOURS);