Spring boot 启动报错：Unable to start web server

今天启动一个小项目，发现启动直接报错，找找原因看看。。。

2020-05-08 10:07:21.287  INFO 1988 --- [  restartedMain] com.example.demo.DemoApplication         : Starting DemoApplication on DESKTOP-3TAK5M9 with PID 1988 (E:\Learning\SpringBoot\demo-master\target\classes started by renco in E:\Learning\SpringBoot\demo-master)
2020-05-08 10:07:21.291  INFO 1988 --- [  restartedMain] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2020-05-08 10:07:21.346  INFO 1988 --- [  restartedMain] .e.DevToolsPropertyDefaultsPostProcessor : Devtools property defaults active! Set 'spring.devtools.add-properties' to 'false' to disable
2020-05-08 10:07:21.346  INFO 1988 --- [  restartedMain] .e.DevToolsPropertyDefaultsPostProcessor : For additional web related logging consider setting the 'logging.level.web' property to 'DEBUG'
2020-05-08 10:07:22.256  INFO 1988 --- [  restartedMain] .s.d.r.c.RepositoryConfigurationDelegate : Multiple Spring Data modules found, entering strict repository configuration mode!
2020-05-08 10:07:22.258  INFO 1988 --- [  restartedMain] .s.d.r.c.RepositoryConfigurationDelegate : Bootstrapping Spring Data repositories in DEFAULT mode.
2020-05-08 10:07:22.292  INFO 1988 --- [  restartedMain] .s.d.r.c.RepositoryConfigurationDelegate : Finished Spring Data repository scanning in 17ms. Found 0 repository interfaces.
2020-05-08 10:07:22.701  INFO 1988 --- [  restartedMain] trationDelegate$BeanPostProcessorChecker : Bean 'org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration' of type [org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration] is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)
2020-05-08 10:07:22.736  WARN 1988 --- [  restartedMain] ConfigServletWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: 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.
2020-05-08 10:07:22.745  INFO 1988 --- [  restartedMain] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2020-05-08 10:07:22.814 ERROR 1988 --- [  restartedMain] 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:156) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:544) ~[spring-context-5.2.1.RELEASE.jar:5.2.1.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) [spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at com.example.demo.DemoApplication.main(DemoApplication.java:17) [classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
	at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.2.1.RELEASE.jar:2.2.1.RELEASE]
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:203) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:179) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:153) ~[spring-boot-2.2.1.RELEASE.jar:2.2.1.RELEASE]
	... 13 common frames omitted

Disconnected from the target VM, address: '127.0.0.1:8689', transport: 'socket'

Process finished with exit code 0
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看帖子有说启动项添加 @EnableAutoConfiguration 注解，然而我的还是不好使。。。
继续看。。。
有同学删除这个

<dependency>
	<groupId>org.apache.tomcat.embed</groupId>
	<artifactId>tomcat-embed-jasper</artifactId>
	<scope>provided</scope>
</dependency>
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可是吧，我的压根就没有添加这个Maven，直接早就注释掉了。

突然想起来项目是用Spring Boot Application 直接启动的，没有使用 Tomcat，切换到 Tomcat 启动，直接可以启动。BUT，还是没找到原因。。。继续看看。。。
考虑项目使用Tomcat启动没有问题，证明代码没有问题，往启动项目方向考虑。。。查看 pom.xml 发现有一个：

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<!--发布时移除tomcat-->
	<!--<exclusions>
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</exclusion>
	</exclusions>-->
</dependency>
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这里有有一个发布时排除内置tomcat，将其注释掉可以正常启动。

<exclusions>
	<exclusion>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-tomcat</artifactId>
	</exclusion>
</exclusions>
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但是这样又有一个问题。当我使用tomcat启动项目的时候必须要将其放开，不然也是无法启动。。。
找找原因。。。
把上边tomcat配置放出来，单独再写一个，现在使用Application 和 Tomcat启动项目都没有问题。。

<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
			<!--发布时移除tomcat-->
			<!--使用DemoApplication启动项目是需要将此项注释掉-->
			<!--使用Tomcat启动项目时需要将此项打开-->
			<!--如果下边Tomcat放开的话这个就不需要添加了-->
			<exclusions>
				<exclusion>
					<groupId>org.springframework.boot</groupId>
					<artifactId>spring-boot-starter-tomcat</artifactId>
				</exclusion>
			</exclusions>
		</dependency>

		<!--tomcat-->
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
			<scope>provided</scope>
		</dependency>
		<dependency>
			<groupId>org.apache.tomcat.embed</groupId>
			<artifactId>tomcat-embed-jasper</artifactId>
			<scope>provided</scope>
		</dependency>
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但是把，我还是没明白，不过应该就是tomcat启动的时候冲突了。。继续看看原因
应该是这个原因了：
最开始启动的时候没有加载tomcat，只有发布时排除tomcat，所以Application 启动报错，无法加载Bean 然后使用 Tomcat 可以正常启动。
现在添加 Tomcat 依赖后，可以直接使用 Application 启动，不过需要添加

<exclusions>
	<exclusion>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-tomcat</artifactId>
	</exclusion>
</exclusions>
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在项目部署时将 Tomcat 排除，不然还是会有冲突报错。
BUT！！！我清楚的记得这个项目可以正常启动的。。。。不管是使用 Application 还是使用 Tomcat 包括项目打包发布都是没有任何问题的。。