該問題出現(xiàn)原因多樣，大多數(shù)是因為配置不當?shù)膯栴}，首先要確定自己問題是不是與本問題出現(xiàn)原因一樣。

背景

項目在IDEA突然就報錯了

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?

一大串錯誤的核心信息是

org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.

網(wǎng)上查找內(nèi)容大多說的是不是配置沒配好之類的，然而都沒法解決我的情況，所以只好自己研究了。

過程

一陣斷定定位，最終到了

//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);
	}

貌似沒啥特殊的，

大概就是說加載不到ServletWebServerFactory，IDEA定位下，可以發(fā)現(xiàn)，這個工廠接口有3個實現(xiàn)類

• JettyServletWebServerFactory
• TomcatServletWebServerFactory
• UndertowServletWebServerFactory

回到問題，無法實例化Bean出現(xiàn)的錯誤，那么是不是可以考慮手動標記呢？

處理

給啟動類添加上Bean聲明，也就是當啟動時，web服務容器會加載這么一個bean

	@Bean
	ServletWebServerFactory servletWebServerFactory(){
		return new TomcatServletWebServerFactory();
	}

運行項目，發(fā)現(xiàn)依然報錯了，萬幸的是，這次錯誤信息有變化了，很長一段。

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

核心錯誤信息是

Caused by: java.lang.ClassNotFoundException: org.apache.catalina.core.AprLifecycleListener.

繼續(xù)定位該類AprLifecycleListener，發(fā)現(xiàn)是tomcat-embed-core-9.0.19.jar包下的東西，那為何會找不到呢？

<!--pom.xml-->
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-tomcat</artifactId>
	<scope>provide</scope>
</dependency>

這里的scope配置的是provide，也就是說，在編譯打包時，lib里不會存在這個artifact，以防止與容器沖突。

然而在idea的時候，卻因此讀取不到這個class導致了問題（具體原因目前已觸及我知識盲區(qū)）。

總之，把scope的值改為compile（默認值）即可，到發(fā)布正式環(huán)境時記得改回去就可以了（不需要配置TomcatServletWebServerFactory bean）。

總結(jié)

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。