IDEA中Spring boot应用如何从main方法运行

IDEA中在main方法中启动Spring Boot应用报错:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'statViewServlet' defined in class path resource [org/feiyu/utilproject/springboot/DruidConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.web.servlet.ServletRegistrationBean]: Factory method 'statViewServlet' threw exception; nested exception is java.lang.NoClassDefFoundError: javax/servlet/http/HttpServlet
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:599) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1173) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1067) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693) [spring-boot-1.5.10.RELEASE.jar:1.5.10.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:360) [spring-boot-1.5.10.RELEASE.jar:1.5.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:303) [spring-boot-1.5.10.RELEASE.jar:1.5.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118) [spring-boot-1.5.10.RELEASE.jar:1.5.10.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107) [spring-boot-1.5.10.RELEASE.jar:1.5.10.RELEASE]
    at org.feiyu.utilproject.springboot.SpringbootApplication.main(SpringbootApplication.java:10) [classes/:na]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.boot.web.servlet.ServletRegistrationBean]: Factory method 'statViewServlet' threw exception; nested exception is java.lang.NoClassDefFoundError: javax/servlet/http/HttpServlet
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    ... 17 common frames omitted
Caused by: java.lang.NoClassDefFoundError: javax/servlet/http/HttpServlet
    at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_144]
    at java.lang.ClassLoader.defineClass(ClassLoader.java:763) ~[na:1.8.0_144]
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[na:1.8.0_144]
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:467) ~[na:1.8.0_144]
    at java.net.URLClassLoader.access$100(URLClassLoader.java:73) ~[na:1.8.0_144]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:368) ~[na:1.8.0_144]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:362) ~[na:1.8.0_144]
    at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_144]
    at java.net.URLClassLoader.findClass(URLClassLoader.java:361) ~[na:1.8.0_144]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_144]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335) ~[na:1.8.0_144]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_144]
    at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_144]
    at java.lang.ClassLoader.defineClass(ClassLoader.java:763) ~[na:1.8.0_144]
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[na:1.8.0_144]
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:467) ~[na:1.8.0_144]
    at java.net.URLClassLoader.access$100(URLClassLoader.java:73) ~[na:1.8.0_144]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:368) ~[na:1.8.0_144]
    at java.net.URLClassLoader$1.run(URLClassLoader.java:362) ~[na:1.8.0_144]
    at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_144]
    at java.net.URLClassLoader.findClass(URLClassLoader.java:361) ~[na:1.8.0_144]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_144]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:335) ~[na:1.8.0_144]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_144]
    at org.feiyu.utilproject.springboot.DruidConfiguration.statViewServlet(DruidConfiguration.java:19) ~[classes/:na]
    at org.feiyu.utilproject.springboot.DruidConfiguration$$EnhancerBySpringCGLIB$$1a10e622.CGLIB$statViewServlet$1() ~[classes/:na]
    at org.feiyu.utilproject.springboot.DruidConfiguration$$EnhancerBySpringCGLIB$$1a10e622$$FastClassBySpringCGLIB$$8347397f.invoke() ~[classes/:na]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228) ~[spring-core-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:358) ~[spring-context-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    at org.feiyu.utilproject.springboot.DruidConfiguration$$EnhancerBySpringCGLIB$$1a10e622.statViewServlet() ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_144]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_144]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_144]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_144]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ~[spring-beans-4.3.14.RELEASE.jar:4.3.14.RELEASE]
    ... 18 common frames omitted

依赖的定义

Spring Boot的应用是可以通过main方法中运行SpringApplication.run()来启动的。如果项目使用了gradle或者maven，则可以通过gradle bootRun或者mvn spring-boot:run等插件来运行。

但是运行起来是有不同的，因为通过gradle或者maven运行，他们运行起来之后能够正确的处理一些runtime和provided的依赖，但是通过main方法运行的cp则是IDE中固定的。

Maven默认的提供的依赖级别有compile、test、runtime、provided，后两者分别对于仅仅打包（运行）需要和打包（运行）不需要。

Scope的分类##

compile

默认就是compile，什么都不配置也就是意味着compile。compile表示被依赖项目需要参与当前项目的编译，当然后续的测试，运行周期也参与其中，是一个比较强的依赖。打包的时候通常需要包含进去。

test

scope为test表示依赖项目仅仅参与测试相关的工作，包括测试代码的编译，执行。比较典型的如junit。

runntime

runntime表示被依赖项目无需参与项目的编译，不过后期的测试和运行周期需要其参与。与compile相比，跳过编译而已，说实话在终端的项目（非开源，企业内部系统）中，和compile区别不是很大。比较常见的如JSR×××的实现，对应的API jar是compile的，具体实现是runtime的，compile只需要知道接口就足够了。oracle jdbc驱动架包就是一个很好的例子，一般scope为runntime。另外runntime的依赖通常和optional搭配使用，optional为true。我可以用A实现，也可以用B实现。

provided

provided意味着打包的时候可以不用包进去，别的设施(Web Container)会提供。事实上该依赖理论上可以参与编译，测试，运行等周期。相当于compile，但是在打包阶段做了exclude的动作。

system

从参与度来说，也provided相同，不过被依赖项不会从maven仓库抓，而是从本地文件系统拿，一定需要配合systemPath属性使用。

问题

因为项目需要最终打成war包，放在Tomcat中运行。所以对于Tomcat的依赖，项目中这么定义的:

providedRuntime("org.springframework.boot:spring-boot-starter-tomcat")

通过main方法在IDE（IDEA）中运行时，会找不到报找不到Servlet API的错误。

原因

这样的话，通过bootRun运行，tomcat能够正确依赖，打包时，tomcat依赖不会被加入到war包中。但是问题是，如果项目导入IDEA中，会被解析为provided，也就是说，运行的时候，tomcat依赖会找不到。但是实际对于providedRuntime来说，它就是runtime，只是打包的时候不加入而已。

解决

解决方法是将项目的Project Structure 中dependencies中tomcat相关jar文件的scope修改为compile或在maven引用spring-boot-starter-tomcat包时将scope属性注释掉。