错误There is no Action mapped for namespace / and action name—Struts2

Struts2出现错误，总是提示"There is no Action mapped for namespace / and action
name"的错误。

错误代码如下：

09:36:13,515 WARN  [Dispatcher] Could not find action or result

There is no Action mapped for namespace [/] and action name [css] associated with context path [/ChinaUnicom]. - [unknown location]

    at com.opensymphony.xwork2.DefaultActionProxy.prepare(DefaultActionProxy.java:185)

    at org.apache.struts2.impl.StrutsActionProxy.prepare(StrutsActionProxy.java:63)

    at org.apache.struts2.impl.StrutsActionProxyFactory.createActionProxy(StrutsActionProxyFactory.java:39)

    at com.opensymphony.xwork2.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:58)

    at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:534)

    at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)

    at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)

    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)

    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)

    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)

    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)

    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)

    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)

    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)

    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)

    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)

    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)

    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)

    at java.lang.Thread.run(Thread.java:619)

上网查询后发现这是一个初学者经常碰到的问题，导致错误的原因主要有两种。总结如下：

1.struts.xml文件错误。这种错误又分为以下几种：

1.1struts.xml文件名错误。一定要注意拼写问题；

1.2struts.xml文件放置路径错误。一定要将此文件放置在src目录下。编译成功后，要确认是否编译到classes目录中；

1.3struts.xml文件内容错误。下面给出一个正确的struts.xml文件以供参考。注意背景色部分。

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE struts PUBLIC

    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"

    "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>

    <package name="default" namespace="/" extends="struts-default">

        <action name="login" class="com.wanggc.struts2.sample.Struts2Action">

        <result name="success">/jsp/result.jsp</result>

        </action>

    </package>

</struts>

2.如果排除了struts.xml文件的问题，还有一种可能就是，在web.xml文件中的<welcome-file>信息中是否配置了自己工程的启动页面。如果没有配置，地址栏中要输入完成的url，如：http://localhost:8080/Struts2Sample/jsp/login.jsp，后面的资源详细信息不输入的时候也会报这个错误，我就是栽在这个问题上的(^_^)。

错误There is no Action mapped for namespace / and action
name—Struts2