Force initialization of configuration class in mainline thread

Closes gh-36844

(cherry picked from commit af2b96192d)

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

@@ -197,6 +197,16 @@ class BackgroundBootstrapTests {
 		}
 	}
 
+	@Test
+	@Timeout(10)
+	@EnabledForTestGroups(LONG_RUNNING)
+	void bootstrapWithCustomExecutorAndLazyConfig() {
+		ConfigurableApplicationContext ctx = new AnnotationConfigApplicationContext(CustomExecutorLazyBeanConfig.class);
+		assertThat(ctx.getBeanFactory().containsSingleton("testBean1")).isTrue();
+		assertThat(ctx.getBeanFactory().containsSingleton("testBean2")).isTrue();
+		ctx.close();
+	}
+
 
 	@Configuration(proxyBeanMethods = false)
 	static class UnmanagedThreadBeanConfig {
@@ -542,4 +552,36 @@ class BackgroundBootstrapTests {
 		}
 	}
 
+
+	@Configuration(proxyBeanMethods = false)
+	static class CustomExecutorLazyBeanConfig {
+
+		@Bean
+		public ThreadPoolTaskExecutor bootstrapExecutor() {
+			ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+			executor.setThreadNamePrefix("Custom-");
+			executor.setCorePoolSize(2);
+			executor.initialize();
+			return executor;
+		}
+
+		@Configuration(proxyBeanMethods = false)
+		@Lazy
+		static class LazyBeanConfig {
+
+			@Bean(bootstrap = BACKGROUND)
+			public TestBean testBean1() throws InterruptedException {
+				Thread.sleep(6000);
+				return new TestBean();
+			}
+
+			@Bean(bootstrap = BACKGROUND)
+			@Lazy
+			public TestBean testBean2() throws InterruptedException {
+				Thread.sleep(6000);
+				return new TestBean();
+			}
+		}
+	}
+
 }