java.lang.ClassNotFound: org.apache.struts2.dispatcher.ng.filter.Struts？

为甚麽我把struct2包放在lib 文件夹下就正常
我build path 就报错呢？
java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1676)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1521)
at org.apache.catalina.core.DefaultInstanceManager.loadClass(DefaultInstanceManager.java:415)
at org.apache.catalina.core.DefaultInstanceManager.loadClassMaybePrivileged(DefaultInstanceManager.java:397)
at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:118)
at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:252)
at org.apache.catalina.core.ApplicationFilterConfig.setFilterDef(ApplicationFilterConfig.java:372)
at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:98)
at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4542)
at org.apache.catalina.core.StandardContext$2.call(StandardContext.java:5220)
at org.apache.catalina.core.StandardContext$2.call(StandardContext.java:5215)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)