现象

java9 启动 spring 项目时报错：

org.springframework.context.ApplicationContextException: Unable to start web server; nested exception is org.springframework.boot.web.server.WebServerException: Unable to start embedded Tomcat
    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 cn.devmgr.springcloud.register.eureka.Application.main(Application.java:14) [classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na]
    at org.springframework.boot.maven.AbstractRunMojo$LaunchRunner.run(AbstractRunMojo.java:558) [spring-boot-maven-plugin-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at java.base/java.lang.Thread.run(Thread.java:844) [na:na]
Caused by: org.springframework.boot.web.server.WebServerException: Unable to start embedded Tomcat
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:125) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.<init>(TomcatWebServer.java:86) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getTomcatWebServer(TomcatServletWebServerFactory.java:427) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getWebServer(TomcatServletWebServerFactory.java:180) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:181) ~[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]
    ... 14 common frames omitted
Caused by: java.lang.IllegalStateException: StandardEngine[Tomcat].StandardHost[localhost].TomcatEmbeddedContext[] failed to start
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.rethrowDeferredStartupExceptions(TomcatWebServer.java:171) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:109) ~[spring-boot-2.1.5.RELEASE.jar:2.1.5.RELEASE]
    ... 19 common frames omitted

[WARNING] 
java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at org.springframework.boot.maven.AbstractRunMojo$LaunchRunner.run(AbstractRunMojo.java:558)
    at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: org.springframework.context.ApplicationContextException: Unable to start web server; nested exception is org.springframework.boot.web.server.WebServerException: Unable to start embedded Tomcat
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:157)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:316)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248)
    at cn.devmgr.springcloud.register.eureka.Application.main(Application.java:14)
    ... 6 more
Caused by: org.springframework.boot.web.server.WebServerException: Unable to start embedded Tomcat
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:125)
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.<init>(TomcatWebServer.java:86)
    at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getTomcatWebServer(TomcatServletWebServerFactory.java:427)
    at org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory.getWebServer(TomcatServletWebServerFactory.java:180)
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.createWebServer(ServletWebServerApplicationContext.java:181)
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.onRefresh(ServletWebServerApplicationContext.java:154)
    ... 14 more
Caused by: java.lang.IllegalStateException: StandardEngine[Tomcat].StandardHost[localhost].TomcatEmbeddedContext[] failed to start
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.rethrowDeferredStartupExceptions(TomcatWebServer.java:171)
    at org.springframework.boot.web.embedded.tomcat.TomcatWebServer.initialize(TomcatWebServer.java:109)
    ... 19 more

原因和解决办法

这是java9 默认没有加载 JAXB-API 造成的，java9开始分模块，默认加载的模块不包含 jaxb-api。有几个办法可以解决：

命令后增加参数 --add-modules java.xml.bind 引入此模块
换java8

手工增加 jaxb的依赖

<dependency>
     <groupId>javax.xml.bind</groupId>
     <artifactId>jaxb-api</artifactId>
     <version>2.3.1</version>
 </dependency>
 <dependency>
     <groupId>com.sun.xml.bind</groupId>
     <artifactId>jaxb-impl</artifactId>
     <version>2.3.1</version>
 </dependency>
 <dependency>
     <groupId>org.glassfish.jaxb</groupId>
     <artifactId>jaxb-runtime</artifactId>
     <version>2.3.1</version>
 </dependency>
 <dependency>
     <groupId>javax.activation</groupId>
     <artifactId>activation</artifactId>
     <version>1.1.1</version>
 </dependency>