Add support for GET requests for /actuator/startup

See gh-24717

spring-boot-project/spring-boot-actuator-autoconfigure/src/docs/asciidoc/endpoints/startup.adoc

@@ -16,7 +16,9 @@ The resulting response is similar to the following:
 
 include::{snippets}/startup/http-response.adoc[]
 
-
+NOTE: The above call resets the application startup steps buffer - subsequent calls to the endpoint will
+not include the returned steps. To retrieve a snapshot of the steps recorded so far without removing them
+from the startup buffer, make a `GET` request to `/actuator/startup`.
 
 [[startup-retrieving-response-structure]]
 === Response Structure

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/startup/StartupEndpoint.java

@@ -18,6 +18,7 @@ package org.springframework.boot.actuate.startup;
 
 import org.springframework.boot.SpringBootVersion;
 import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
+import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
 import org.springframework.boot.actuate.endpoint.annotation.WriteOperation;
 import org.springframework.boot.context.metrics.buffering.BufferingApplicationStartup;
 import org.springframework.boot.context.metrics.buffering.StartupTimeline;
@@ -28,6 +29,7 @@ import org.springframework.boot.context.metrics.buffering.StartupTimeline;
  * application startup}.
  *
  * @author Brian Clozel
+ * @author Chris Bono
  * @since 2.4.0
  */
 @Endpoint(id = "startup")
@@ -50,6 +52,12 @@ public class StartupEndpoint {
 		return new StartupResponse(startupTimeline);
 	}
 
+	@ReadOperation
+	public StartupResponse startupSnapshot() {
+		StartupTimeline startupTimeline = this.applicationStartup.getBufferedTimeline();
+		return new StartupResponse(startupTimeline);
+	}
+
 	/**
 	 * A description of an application startup, primarily intended for serialization to
 	 * JSON.

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/startup/StartupEndpointTests.java

@@ -30,6 +30,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link StartupEndpoint}.
  *
  * @author Brian Clozel
+ * @author Chris Bono
  */
 class StartupEndpointTests {
 
@@ -60,6 +61,19 @@ class StartupEndpointTests {
 		});
 	}
 
+	@Test
+	void bufferIsNotDrained() {
+		BufferingApplicationStartup applicationStartup = new BufferingApplicationStartup(256);
+		ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+				.withInitializer((context) -> context.setApplicationStartup(applicationStartup))
+				.withUserConfiguration(EndpointConfiguration.class);
+		contextRunner.run((context) -> {
+			StartupEndpoint.StartupResponse startup = context.getBean(StartupEndpoint.class).startupSnapshot();
+			assertThat(startup.getTimeline().getEvents()).isNotEmpty();
+			assertThat(applicationStartup.getBufferedTimeline().getEvents()).isNotEmpty();
+		});
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class EndpointConfiguration {