Spring 定时器 No qualifying bean of type [org.springframework.scheduling.TaskScheduler] is defined

最近项目里面，用了spring的定时任务，一直以来，项目运行的不错。定时器也能正常使用。可是，今天启动项目测试的时候，盯着启动Log看了一阵子，突然间发现，启动的Log中居然有一个异常，虽然一闪而过，但是那熟悉的异常格式还是让我浑身一颤。这个项目一直运行的很好啊，没发现有什么问题，怎么会出现异常呢？

于是我仔细的翻啊翻啊，查啊找啊，嘿，还真的找到了两个异常：

[2016-10-12 22:49:10,184] [DEBUG] (ScheduledAnnotationBeanPostProcessor.java:finishRegistration:207) 
Could not find default TaskScheduler bean
org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.springframework.scheduling.TaskScheduler] is defined
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:372)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:332)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.finishRegistration(ScheduledAnnotationBeanPostProcessor.java:192)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.onApplicationEvent(ScheduledAnnotationBeanPostProcessor.java:171)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.onApplicationEvent(ScheduledAnnotationBeanPostProcessor.java:86)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:163)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:136)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:381)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:335)
	at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:855)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:541)
	at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:444)
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:326)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4812)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5255)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1408)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1398)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
[2016-10-12 22:49:10,188] [DEBUG] (ScheduledAnnotationBeanPostProcessor.java:finishRegistration:219) 
Could not find default ScheduledExecutorService bean
org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.springframework.scheduling.TaskScheduler] is defined
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:372)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:332)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.finishRegistration(ScheduledAnnotationBeanPostProcessor.java:192)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.onApplicationEvent(ScheduledAnnotationBeanPostProcessor.java:171)
	at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.onApplicationEvent(ScheduledAnnotationBeanPostProcessor.java:86)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:163)
	at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:136)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:381)
	at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:335)
	at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:855)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:541)
	at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:444)
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:326)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4812)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5255)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1408)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1398)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

晕啊，很明显这是定时任务的异常啊。可是定时任务明明能够正常运行啊，怎么回事？

尝试着删掉了所有的定时任务配置，果然异常消失了。但是只要启用任何一个定时任务，异常都会再次出现。这可真可怕。作为一个强迫症患者，不能放任这样的异常存在，于是仔细的查找了半天，却依旧没有结果。最后只好放大招，读spring的源码啦，这一读，可好，差点没把我郁闷死。

为什么郁闷呢？ 不知道读这篇文章的朋友们有没有细心的，有没有注意这两个异常有一个非常特别的地方。什么地方呢？ 这两个异常的级别，不是ERROR 也不是 WARNING

 居然是DEBUG

原来，Spring的定时任务调度器会尝试获取一个注册过的 task scheduler来做任务调度，它会尝试通过BeanFactory.getBean的方法来获取一个注册过的scheduler bean，获取的步骤如下：

1.尝试从配置中找到一个TaskScheduler Bean

2.寻找ScheduledExecutorService Bean

3.使用默认的scheduler

前两步，如果找不到的话，就会以debug的方式抛出异常，分别是：

logger.debug("Could not find default TaskScheduler bean", ex);
logger.debug("Could not find default ScheduledExecutorService bean", ex);

所以，日志中打印出来的两个异常，根本不是什么错误信息，也不会影响定时器的使用，只不过是spring的自己打印的一些信息罢了，不过没搞明白，为什么非要用异常的方式打出来，估计是为了看这清晰点吧。也或者，这里面有一些重要的信息需要提示开发者。具体是什么原因，只能有机会进一步再去了解了。