getBean 的默认实现的入口是在 AbstractBeanFactory#doGetBean 方法
结合源码来看，创建 bean 的线程安全是通过可并发容器 + 加锁 synchronized 保证的
比如列举几个可说的点：

• 根据 beanName 获取是否存在早期已缓存的单例 bean，存在 get、判空、put、remove 操作，所以加了锁。如源码1

• 合并 BeanDefinition 成 RootDefinition 时，AbstractBeanFactory#getMergedBeanDefinition 方法也加了锁。如源码 2

类似之处还有很多，可结合源码进行查看

源码1：
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return s

源码2：
protected RootBeanDefinition getMergedBeanDefinition(
            String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
            throws BeanDefinitionStoreException {

        synchronized (this.mergedBeanDefinitions) {
            RootBeanDefinition mbd = null;
            RootBeanDefinition previous = null;