新手Struts2启动的时候ClassNotFoundException？

Question

部署环境是tomcat8 + struts2.3.20 + IDEA2016

这是目录树

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这是Tomcat Localhost Log的报错信息，我的包都已经正确引入了啊，为什么会ClassNotFoundException呢？

java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter

15-Oct-2016 11:11:02.905 SEVERE [RMI TCP Connection(2)-127.0.0.1] org.apache.catalina.core.StandardContext.filterStart Exception starting filter struts2
 java.lang.ClassNotFoundException: org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1333)
    at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1167)
    at org.apache.catalina.core.DefaultInstanceManager.loadClass(DefaultInstanceManager.java:518)
    at org.apache.catalina.core.DefaultInstanceManager.loadClassMaybePrivileged(DefaultInstanceManager.java:499)
    at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:118)
    at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:258)
    at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:105)
    at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4700)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5340)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717)
    at org.apache.catalina.startup.HostConfig.manageApp(HostConfig.java:1696)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at org.apache.catalina.mbeans.MBeanFactory.createStandardContext(MBeanFactory.java:484)
    at org.apache.catalina.mbeans.MBeanFactory.createStandardContext(MBeanFactory.java:433)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at javax.management.remote.rmi.RMIConnectionImpl.doOperation(RMIConnectionImpl.java:1468)
    at javax.management.remote.rmi.RMIConnectionImpl.access$300(RMIConnectionImpl.java:76)
    at javax.management.remote.rmi.RMIConnectionImpl$PrivilegedOperation.run(RMIConnectionImpl.java:1309)
    at javax.management.remote.rmi.RMIConnectionImpl.doPrivilegedOperation(RMIConnectionImpl.java:1401)
    at javax.management.remote.rmi.RMIConnectionImpl.invoke(RMIConnectionImpl.java:829)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:323)
    at sun.rmi.transport.Transport$1.run(Transport.java:200)
    at sun.rmi.transport.Transport$1.run(Transport.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.rmi.transport.Transport.serviceCall(Transport.java:196)
    at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:568)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

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web.xml内容，IDEA自动帮我新建的，没有修改过

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
    <filter>
        <filter-name>struts2</filter-name>
        <filter-class>org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>struts2</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>

Answer 1

回复：

上面的是 lib 目录（自己存放的 jar 包目录）
下面的是运行Tomcat的时候，自动构建项目时候的输出目录，有一个 lib 目录。只有这里存在依赖的 jar 包，就代表运行时候有正确的依赖包。

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不用继续贴代码了，因为你这情况跟代码无关。先说原因：缺少依赖的 jar 包。
建议：

1. 跟楼上说的那样把 struts相关的包 复制到 lib 目录。

2. 用 Maven 管理依赖，不要用 IDE 软件自带的扩展功能导入某个框架的包。不懂可以接触一下，很容易的。

3. Struts 已经不用再学了。如果不是一定要满足课业要求，直接跳过这个框架，学习别的。（因为过时很多年了）

Answer 2

你手动把jar包复制到/WEB-INF/lib目录里面试试。

Answer 3

总结

1. 对于过滤器在启动的时候出现ClassNotFoundException的原因是因为struts2的jar包没有正确引入进来，解决方法是在WEB-INF里面新建一个lib。不仅要在External-Libary里面引入jar包，而且也要在WEB-INF里面新建一个lib，按照绅士猫答主的截图，可以查看out目录里面是否有lib包。IDEA加载的流程待查。

2. 如果一次性引入struts2.3.30 lib目录下面的所有jar包，也会出现报错，原因待查。