Caused by: java.lang.NoClassDefFoundError:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'identityService': Lookup method resolution failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [com.example.new_oa.identity.service.impl.IdentityServiceImpl] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]

Caused by: java.lang.NoClassDefFoundError: javax/servlet/http/HttpSession
    at java.lang.Class.getDeclaredMethods0(Native Method) ~[na:1.8.0_181]
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2701) ~[na:1.8.0_181]
    at java.lang.Class.getDeclaredMethods(Class.java:1975) ~[na:1.8.0_181]
    at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:641) ~[spring-core-5.0.4.RELEASE.jar:5.0.4.RELEASE]
    ... 20 common frames omitted
Caused by: java.lang.ClassNotFoundException: javax.servlet.http.HttpSession
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_181]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_181]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_181]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_181]
    ... 24 common frames omitted

當使用idea開發工具時，maven作為項目管理工具時，idea是默認不提供servlet-api.jar的，所以當javax.servlet-api的scope的值設置provide(表示該jar在編譯時需要，而在發布時不需要)時會提示找不到對應的jar包，我這里是提示不能創建對應的service，是因為我的類中用到了HttpSession，依賴于servelt-api.jar.只需要把provide注釋掉即可。

但是，注釋掉之后，又出現了其他問題：如下：
Bean 'org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration' of type [org.springframework.transaction.annotation.ProxyTransactionManagementConfiguration$$EnhancerBySpringCGLIB$$723b5875] is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)
2019-06-29 20:05:39.786 WARN 4452 --- [ main] 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.
2019-06-29 20:05:39.794 INFO 4452 --- [ main] ConditionEvaluationReportLoggingListener :

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
Disconnected from the target VM, address: '127.0.0.1:49637', transport: 'socket'
2019-06-29 20:05:39.803 ERROR 4452 --- [ 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:155) ~[spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543) ~[spring-context-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.0.RELEASE.jar:2.0.0.RELEASE]

即在啟動applicationContext容器失敗。解決辦法和上面大致一樣。如下圖：

在本地啟動一個新項目時，
報錯: java.lang.ClassNotFoundException: javax.servlet.Filter
環境intellJ idea,我同事的myeclipse就可以直接啟動

最后搜了下問題
問題原因是：spring boot項目中引入使用的是scope=provided
而默認情況下，idea是不會提供對應的servlet-api.jar包eclipse會提供

網上有人說這是idea的bug,這么說顯然是不對的，是完全沒有理解provided和compile的區別
對于scope=compile的情況（默認scope),也就是說這個項目在編譯，測試，運行階段都需要這個artifact對應的jar包在classpath中。而對于scope=provided的情況，則可以認為這個provided是目標容器已經provide這個artifact。

??????? 這么說可能不是很好理解：換個簡單的，就是你在把這個項目打成war包時，scope=provided的jar包，不會出現在WEB-INFO/lib目錄下，而scope=compile的jar包，會放到WEB-INFO/lib目錄

??????? 這樣可以避免想servlet-api.jar這樣的jar包沖突，但是如果遇到idea下默認不提供servlet-api.jar時，使用scope=provided就會缺少對應的jar包，也就是我遇到的問題找不到類