Apply TTL invocation caching on reactor types

Update `CachingOperationInvoker` so that TTL caching is applied directly
to reactive types. Prior to this commit, a `Mono` would be cached, but
the values that it emitted would not.

See gh-18339

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/invoker/cache/CachingOperationInvoker.java

@@ -16,9 +16,12 @@
 
 package org.springframework.boot.actuate.endpoint.invoker.cache;
 
+import java.time.Duration;
 import java.util.Map;
 import java.util.Objects;
 
+import reactor.core.publisher.Mono;
+
 import org.springframework.boot.actuate.endpoint.InvocationContext;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
 import org.springframework.util.Assert;
@@ -67,13 +70,20 @@ public class CachingOperationInvoker implements OperationInvoker {
 		long accessTime = System.currentTimeMillis();
 		CachedResponse cached = this.cachedResponse;
 		if (cached == null || cached.isStale(accessTime, this.timeToLive)) {
-			Object response = this.invoker.invoke(context);
+			Object response = handleMonoResponse(this.invoker.invoke(context));
 			this.cachedResponse = new CachedResponse(response, accessTime);
 			return response;
 		}
 		return cached.getResponse();
 	}
 
+	private Object handleMonoResponse(Object response) {
+		if (response instanceof Mono) {
+			return ((Mono) response).cache(Duration.ofMillis(this.timeToLive));
+		}
+		return response;
+	}
+
 	private boolean hasInput(InvocationContext context) {
 		if (context.getSecurityContext().getPrincipal() != null) {
 			return true;

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/invoker/cache/CachingOperationInvokerTests.java

@@ -17,15 +17,20 @@
 package org.springframework.boot.actuate.endpoint.invoker.cache;
 
 import java.security.Principal;
+import java.time.Duration;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
+import org.junit.Rule;
 import org.junit.Test;
+import reactor.core.publisher.Mono;
 
 import org.springframework.boot.actuate.endpoint.InvocationContext;
 import org.springframework.boot.actuate.endpoint.SecurityContext;
+import org.springframework.boot.actuate.endpoint.invoke.MissingParametersException;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
+import org.springframework.boot.test.rule.OutputCapture;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
@@ -42,6 +47,9 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
  */
 public class CachingOperationInvokerTests {
 
+	@Rule
+	public OutputCapture outputCapture = new OutputCapture();
+
 	@Test
 	public void createInstanceWithTtlSetToZero() {
 		assertThatIllegalArgumentException()
@@ -62,6 +70,21 @@ public class CachingOperationInvokerTests {
 		assertCacheIsUsed(parameters);
 	}
 
+	@Test
+	public void cacheInTtlWithMonoResponse() {
+		MonoOperationInvoker target = new MonoOperationInvoker();
+		InvocationContext context = new InvocationContext(mock(SecurityContext.class), Collections.emptyMap());
+		CachingOperationInvoker invoker = new CachingOperationInvoker(target, 500L);
+		Object monoResponse = invoker.invoke(context);
+		assertThat(monoResponse).isInstanceOf(Mono.class);
+		Object response = ((Mono) monoResponse).block(Duration.ofSeconds(30));
+		Object cachedMonoResponse = invoker.invoke(context);
+		assertThat(cachedMonoResponse).isInstanceOf(Mono.class);
+		Object cachedResponse = ((Mono) cachedMonoResponse).block(Duration.ofSeconds(30));
+		assertThat(response).isSameAs(cachedResponse);
+		assertThat(this.outputCapture.toString()).containsOnlyOnce("invoked");
+	}
+
 	private void assertCacheIsUsed(Map<String, Object> parameters) {
 		OperationInvoker target = mock(OperationInvoker.class);
 		Object expected = new Object();
@@ -119,4 +142,18 @@ public class CachingOperationInvokerTests {
 		verify(target, times(2)).invoke(context);
 	}
 
+	private static class MonoOperationInvoker implements OperationInvoker {
+
+		@Override
+		public Object invoke(InvocationContext context) throws MissingParametersException {
+			return Mono.fromCallable(this::printInvocation);
+		}
+
+		private Mono<String> printInvocation() {
+			System.out.println("MonoOperationInvoker invoked");
+			return Mono.just("test");
+		}
+
+	}
+
 }