该问题出现原因多样，大多数是因为配置不当的问题，首先要确定自己问题是不是与本问题出现原因一样。
背景
项目在IDEA突然就报错了
Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2019-06-18 16:52:29.850 ERROR 9248 --- [ main] o.s.boot.SpringApplication : Application run failed
org.springframework.context.ApplicationContextException: Unable to start web server; nested exception is org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:157) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) ~[spring-context-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at com.ynkg.pongal.PongalApplication.main(PongalApplication.java:22) [classes/:na]
Caused by: org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.getWebServerFactory(ServletWebServerApplicationContext.java:206) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:180) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:154) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
... 8 common frames omitted一大串错误的核心信息是
org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.
网上查找内容大多说的是不是配置没配好之类的，然而都没法解决我的情况，所以只好自己研究了。
过程
一阵断定定位，最终到了
//ServletWebServerApplicationContext
protected ServletWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
String[] beanNames = getBeanFactory()
.getBeanNamesForType(ServletWebServerFactory.class);
if (beanNames.length == 0) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to missing "
+ "ServletWebServerFactory bean.");
}
if (beanNames.length > 1) {
throw new ApplicationContextException(
"Unable to start ServletWebServerApplicationContext due to multiple "
+ "ServletWebServerFactory beans : "
+ StringUtils.arrayToCommaDelimitedString(beanNames));
}
return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
}貌似没啥特殊的，大概就是说加载不到ServletWebServerFactory，IDEA定位下，可以发现，这个工厂接口有3个实现类JettyServletWebServerFactory,TomcatServletWebServerFactory,UndertowServletWebServerFactory。
回到问题，无法实例化Bean出现的错误，那么是不是可以考虑手动标记呢？
处理
给启动类添加上Bean声明，也就是当启动时，web服务容器会加载这么一个bean
@Bean
ServletWebServerFactory servletWebServerFactory(){
return new TomcatServletWebServerFactory();
}运行项目，发现依然报错了，万幸的是，这次错误信息有变化了，很长一段。
Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2019-06-18 17:10:46.352 ERROR 11224 --- [ main] o.s.boot.SpringApplication : Application run failed
org.springframework.context.ApplicationContextException: Unable to start web server; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'servletWebServerFactory' defined in com.ynkg.pongal.PongalApplication: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.web.servlet.server.ServletWebServerFactory]: Factory method 'servletWebServerFactory' threw exception; nested exception is java.lang.NoClassDefFoundError: org/apache/catalina/core/AprLifecycleListener
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:157) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) ~[spring-context-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at com.ynkg.pongal.PongalApplication.main(PongalApplication.java:22) [classes/:na]
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'servletWebServerFactory' defined in com.ynkg.pongal.PongalApplication: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.web.servlet.server.ServletWebServerFactory]: Factory method 'servletWebServerFactory' threw exception; nested exception is java.lang.NoClassDefFoundError: org/apache/catalina/core/AprLifecycleListener
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:627) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:456) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1321) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1160) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:515) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:204) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.getWebServerFactory(ServletWebServerApplicationContext.java:216) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:180) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:154) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
... 8 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.web.servlet.server.ServletWebServerFactory]: Factory method 'servletWebServerFactory' threw exception; nested exception is java.lang.NoClassDefFoundError: org/apache/catalina/core/AprLifecycleListener
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:622) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
... 20 common frames omitted
Caused by: java.lang.NoClassDefFoundError: org/apache/catalina/core/AprLifecycleListener
at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getDefaultLifecycleListeners(TomcatServletWebServerFactory.java:158) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.<init>(TomcatServletWebServerFactory.java:114) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
at com.ynkg.pongal.PongalApplication.servletWebServerFactory(PongalApplication.java:33) [classes/:na]
at com.ynkg.pongal.PongalApplication$$EnhancerBySpringCGLIB$$951efaf5.CGLIB$servletWebServerFactory$1(<generated>) ~[classes/:na]
at com.ynkg.pongal.PongalApplication$$EnhancerBySpringCGLIB$$951efaf5$$FastClassBySpringCGLIB$$d7e2ff01.invoke(<generated>) ~[classes/:na]
at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) ~[spring-core-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:363) ~[spring-context-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at com.ynkg.pongal.PongalApplication$$EnhancerBySpringCGLIB$$951efaf5.servletWebServerFactory(<generated>) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_191]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_191]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_191]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_191]
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
... 21 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.apache.catalina.core.AprLifecycleListener
at java.net.URLClassLoader.findClass(URLClassLoader.java:382) ~[na:1.8.0_191]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_191]
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_191]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_191]
... 34 common frames omitted核心错误信息是Caused by: java.lang.ClassNotFoundException: org.apache.catalina.core.AprLifecycleListener.
继续定位该类AprLifecycleListener，发现是tomcat-embed-core-9.0.19.jar包下的东西，那为何会找不到呢？
<!--pom.xml-->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provide</scope>
</dependency>这里的scope配置的是provide，也就是说，在编译打包时，lib里不会存在这个artifact，以防止与容器冲突。
然而在idea的时候，却因此读取不到这个class导致了问题（具体原因目前已触及我知识盲区）。
总之，把scope的值改为compile（默认值）即可，到发布正式环境时记得改回去就可以了（不需要配置TomcatServletWebServerFactory bean）。