Merge pull request #14298 from ayudovin:make-possible-disable-backgroundpreinitializer

* pr/14298:
  Polish "Make it possible to disable the BackgroundPreinitializer"
  Make it possible to disable the BackgroundPreinitializer

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java

@@ -39,15 +39,29 @@ import org.springframework.http.converter.support.AllEncompassingFormHttpMessage
 /**
  * {@link ApplicationListener} to trigger early initialization in a background thread of
  * time consuming tasks.
+ * <p>
+ * Set the {@value IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME} system property to
+ * {@code true} to disable this mechanism and let such initialization happen in the
+ * foreground.
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Artsiom Yudovin
  * @since 1.3.0
  */
 @Order(LoggingApplicationListener.DEFAULT_ORDER + 1)
 public class BackgroundPreinitializer
 		implements ApplicationListener<SpringApplicationEvent> {
 
+	/**
+	 * System property that instructs Spring Boot how to run pre initialization. When the
+	 * property is set to {@code true}, no pre intialization happens and each item is
+	 * initialized in the foreground as it needs to. When the property is {@code false}
+	 * (default), pre initialization runs in a separate thread in the background.
+	 * @since 2.1.0
+	 */
+	public static final String IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME = "spring.backgroundpreinitializer.ignore";
+
 	private static final AtomicBoolean preinitializationStarted = new AtomicBoolean(
 			false);
 
@@ -55,7 +69,8 @@ public class BackgroundPreinitializer
 
 	@Override
 	public void onApplicationEvent(SpringApplicationEvent event) {
-		if (event instanceof ApplicationStartingEvent
+		if (!Boolean.getBoolean(IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME)
+				&& event instanceof ApplicationStartingEvent
 				&& preinitializationStarted.compareAndSet(false, true)) {
 			performPreinitialization();
 		}