ClassNotFoundException: HttpServletRequest with Struts2 in Eclipse

I have 2 versions of a project (GIT), the oldest (work perfectly) and the new one commited (don't work - issue below).

Here the error I have in Eclipse :

août 18, 2020 4:22:24 PM org.apache.catalina.core.StandardContext filterStart
GRAVE: Exception au démarrage du filtre struts2
java.lang.NoClassDefFoundError: HttpServletRequest
	at java.lang.Class.getDeclaredMethods0(Native Method)
	at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
	at java.lang.Class.privateGetPublicMethods(Unknown Source)
	at java.lang.Class.getMethods(Unknown Source)
	at org.apache.struts2.convention.PackageBasedActionConfigBuilder.getActionAnnotations(PackageBasedActionConfigBuilder.java:877)
	at org.apache.struts2.convention.PackageBasedActionConfigBuilder.buildConfiguration(PackageBasedActionConfigBuilder.java:689)
	at org.apache.struts2.convention.PackageBasedActionConfigBuilder.buildActionConfigs(PackageBasedActionConfigBuilder.java:355)
	at org.apache.struts2.convention.ClasspathPackageProvider.loadPackages(ClasspathPackageProvider.java:53)
	at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reloadContainer(DefaultConfiguration.java:274)
	at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:67)
	at org.apache.struts2.dispatcher.Dispatcher.getContainer(Dispatcher.java:967)
	at org.apache.struts2.dispatcher.Dispatcher.init_PreloadConfiguration(Dispatcher.java:435)
	at org.apache.struts2.dispatcher.Dispatcher.init(Dispatcher.java:479)
	at org.apache.struts2.dispatcher.ng.InitOperations.initDispatcher(InitOperations.java:74)
	at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.init(StrutsPrepareAndExecuteFilter.java:57)
	at org.apache.catalina.core.ApplicationFilterConfig.initFilter(ApplicationFilterConfig.java:279)
	at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:260)
	at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:105)
	at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4583)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5207)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1408)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1398)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: HttpServletRequest
	at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1333)
	at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1167)
	... 27 more

The GRAVE: Exception au démarrage du filtre struts2 can be translated in SEVERE: Exception in start of struts2 filter

I saw this answer java.lang.ClassNotFoundException: HttpServletRequest but I have already not servlet-api in the WEB-INF/lib folder but in the Tomcat/lib. And I have exactly the same .jar between the current modified project and the old one.

What can I do about this ?

The error comes from the convention plugin. This plugin is used to create additional configuration objects by convention. If you havily use annotations to define Java based configuration for struts actions then it invokes a reflection mechanism to process creating runtime configuration elements.

The NoClassDefFoundError says that you missing some classes on the classpath of the current classloader. Usually it fixed by adding missing jar to the classpath.

It could be ignored if your application used different classloader than a servlet container is used. Tomcat is a servlet container which is using multiple classloaders in the multithreaded pool. It could also have bugs the one had discovered in the 8.x version.

In many ways missing classes in the broken servlet environment leads to change/update the servlet container itself and download correct dependencies from the Maven central repository.

I finally found a solution but I hesitated to post because I don't know how I did that.

I was trying this :

Duplicate the existing Eclipse server into another folder.
Create a new server in the Servers view in Eclipse with the new folder.
Click-right on the new server -> Clean and Clean Tomcat Work Directory...
Go to Run -> Run configurations

Totally random action :
Double click on one of the server

The server instance start. Stop it. Restart your project. The problem has disappeared.

I think a clean publish is what I needed but we will never know..