Introduce background bootstrapping for individual singleton beans

Closes gh-13410
Closes gh-19487
See gh-23501

spring-context/src/test/java/org/springframework/context/annotation/BackgroundBootstrapTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.annotation;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+
+import org.springframework.beans.testfixture.beans.TestBean;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.core.testfixture.EnabledForTestGroups;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+
+import static org.springframework.context.annotation.Bean.Bootstrap.BACKGROUND;
+import static org.springframework.core.testfixture.TestGroup.LONG_RUNNING;
+
+/**
+ * @author Juergen Hoeller
+ * @since 6.2
+ */
+class BackgroundBootstrapTests {
+
+	@Test
+	@Timeout(5)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithCustomExecutor() {
+		ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(CustomExecutorBeanConfig.class);
+		ctx.getBean("testBean1", TestBean.class);
+		ctx.getBean("testBean2", TestBean.class);
+		ctx.getBean("testBean3", TestBean.class);
+		ctx.close();
+	}
+
+
+	@Configuration
+	static class CustomExecutorBeanConfig {
+
+		@Bean
+		public ThreadPoolTaskExecutor bootstrapExecutor() {
+			ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+			executor.setThreadNamePrefix("Custom-");
+			executor.setCorePoolSize(2);
+			executor.initialize();
+			return executor;
+		}
+
+		@Bean(bootstrap = BACKGROUND) @DependsOn("testBean3")
+		public TestBean testBean1(TestBean testBean3) throws InterruptedException{
+			Thread.sleep(3000);
+			return new TestBean();
+		}
+
+		@Bean(bootstrap = BACKGROUND) @Lazy
+		public TestBean testBean2() throws InterruptedException {
+			Thread.sleep(3000);
+			return new TestBean();
+		}
+
+		@Bean @Lazy
+		public TestBean testBean3() {
+			return new TestBean();
+		}
+
+		@Bean
+		public String dependent(@Lazy TestBean testBean1, @Lazy TestBean testBean2, @Lazy TestBean testBean3) {
+			return "";
+		}
+	}
+
+}