spring异常之BeanCreationNotAllowedException

BeanCreationNotAllowedException：遇到异常方不方？
程序猿：慌毛线，我们跟着trace看一下

org.springframework.beans.factory.BeanCreationNotAllowedException: Error creating bean with name 'stringRedisTemplate': Singleton bean
 creation not allowed while singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy metho
d implementation!)
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:216) 
~[spring-beans-4.3.6.RELEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.6.
RELEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:220) ~[spring-beans-4.3.6.RE
LEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1018)
 ~[spring-beans-4.3.6.RELEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:345) ~[spring-
beans-4.3.6.RELEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:340) ~[spring-
beans-4.3.6.RELEASE.jar!/:4.3.6.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1093) ~[spring-conte
xt-4.3.6.RELEASE.jar!/:4.3.6.RELEASE]
...

首先看下异常信息，优秀的框架日志都很好很清晰，基本可以一目了然，起码能给你指引
日志已经很清楚的说明了：单例bean不允许创建，在单例们的工厂destruction销毁的时候
碰到该异常基本是在应用停止时，因为停止时会销毁工厂，在销毁的期间如果有业务线程进来处理业务，这时从工厂获取bean的时候可能获取不到，因为缓存均都已经清空，缓存取不到当然要创建bean了，在创建的时候发现工厂当前已经被标识为销毁状态，于是就抛出了该异常，那么下面我们来跟着代码一步步验证我们的结论

1. 销毁bean：AbstractApplicationContext的关闭钩子

AbstractApplicationContext实现了ConfigurableApplicationContext接口的钩子方法，而且方法中调用了doClose方法

public void registerShutdownHook() {
	if (this.shutdownHook == null) {
		// No shutdown hook registered yet.
		this.shutdownHook = new Thread() {
			@Override
			public void run() {
				synchronized (startupShutdownMonitor) {
					doClose();
				}
			}
		};
		Runtime.getRuntime().addShutdownHook(this.shutdownHook);
	}
}

AbstractApplicationContext中的doClose方法中先销毁了beans，然后销毁了bean工厂
销毁bean是调用工厂的销毁方法destroySingletons
DefaultListableBeanFactory中的销毁方法调用父类DefaultSingletonBeanRegistry进行对各种bean销毁，而且会对singletonsCurrentlyInDestruction标识置为true

public void destroySingletons() {
	...
	synchronized (this.singletonObjects) {
		this.singletonsCurrentlyInDestruction = true;
	}
	...
}

2. 获取Bean：此时正在执行的业务流程会有可能调用到getBean

然后我们看下此时getBean中是否判断了此标识并抛出了相应的异常
AbstractBeanFactory根据名称获取getBean调用doGetBean
调用父类getSingleton根据名称从缓存中获取bean（如果此时工厂正在销毁，此时取到的是可能会为null的），如果为null则会重新创建，首先获取bean定义判断是否是单例类型，如果是则按照单例的方式创建：AbstractBeanFactory根据beanName与ObjectFactory工厂获取共享实例getSingleton
调用DefaultSingletonBeanRegistry实现的getSingleton
可以看到这里首先判断了singletonsCurrentlyInDestruction标识，如果工厂正常销毁则抛出异常BeanCreationNotAllowedException

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	...
			if (this.singletonsCurrentlyInDestruction) {
				throw new BeanCreationNotAllowedException(beanName,
						"Singleton bean creation not allowed while singletons of this factory are in destruction " +
						"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
			}
			...
}