This is a common error message for a new Struts2 developer. Many Struts 2 related websites have been reported this problem. Overall, when we see a ClassNotFoundException, we should have a reflection that it is very possible that the build path is wrong somewhere.

java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.FilterDispatcher

java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.FilterDispatcher
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1680)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1526)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:269)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:422)
at org.apache.catalina.core.ApplicationFilterConfig.(ApplicationFilterConfig.java:115)
at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4072)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4726)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1057)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:840)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1057)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:463)
at org.apache.catalina.core.StandardService.start(StandardService.java:525)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:754)
at org.apache.catalina.startup.Catalina.start(Catalina.java:595)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:289)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:414)

This works for regular Java project, but does not work for Struts2 web project.

The solution is to simply copy the jar files required to the "lib" directory, then the error message will be gone. 
(remove all jars from the class path if you added them before)

Note: Actually, when the error message is something about ClassNotFoundException, we should think in the direction of build path is not correct. Also, different kinds of Java project may use different way to configure build path, e.g., an eclipse plug-in project have to use plug-in to add some library to it.

java.lang.ClassNotFoundException: org.apache.strut