【Spring源码】单例创建期间进行同步可能会导致死锁？

	@Nullable
	private CompletableFuture<?> preInstantiateSingleton(String beanName, RootBeanDefinition mbd) {
		if (mbd.isBackgroundInit()) {
			Executor executor = getBootstrapExecutor(); // 获取Executor
			if (executor != null) {
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						getBean(dep);
					}
				}
				CompletableFuture<?> future = CompletableFuture.runAsync(
						() -> instantiateSingletonInBackgroundThread(beanName), executor);
				addSingletonFactory(beanName, () -> {
					try {
						future.join();
					}
					catch (CompletionException ex) {
						ReflectionUtils.rethrowRuntimeException(ex.getCause());
					}
					return future;  // not to be exposed, just to lead to ClassCastException in case of mismatch
				});
				return (!mbd.isLazyInit() ? future : null);
			}
			else if (logger.isInfoEnabled()) {
				logger.info("Bean '" + beanName + "' marked for background initialization " +
						"without bootstrap executor configured - falling back to mainline initialization");
			}
		}
		if (!mbd.isLazyInit()) {
			instantiateSingleton(beanName);
		}
		return null;
	}

	private void instantiateSingletonInBackgroundThread(String beanName) {
		this.preInstantiationThread.set(PreInstantiation.BACKGROUND);
		try {
			instantiateSingleton(beanName);
		}
		catch (RuntimeException | Error ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Failed to instantiate singleton bean '" + beanName + "' in background thread", ex);
			}
			throw ex;
		}
		finally {
			this.preInstantiationThread.set(null);
		}
	}

	private void instantiateSingleton(String beanName) {
		if (isFactoryBean(beanName)) {
			Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
			if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {
				getBean(beanName);
			}
		}
		else {
			getBean(beanName);
		}
	}