Struts2配置时常见的问题

struts2的配置问题：
  
      配置步骤：
          一：添加jar包，常用的有八个jar文件

  注意jar包的冲突问题，比如用到xml解析的时候xml-apis.jar和xerces-2-6-2就会和JDK1.6冲突，冲突时会报如下错误：

Caught exception while loading file struts-default.xml - [unknown location]
	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadConfigurationFiles(XmlConfigurationProvider.java:839)
	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadDocuments(XmlConfigurationProvider.java:131)
	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.init(XmlConfigurationProvider.java:100)
	at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reload(DefaultConfiguration.java:130)
	at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:52)
	at org.apache.struts2.dispatcher.Dispatcher.init_PreloadConfiguration(Dispatcher.java:395)
	at org.apache.struts2.dispatcher.Dispatcher.init(Dispatcher.java:452)
	at org.apache.struts2.dispatcher.FilterDispatcher.init(FilterDispatcher.java:203)
	at org.apache.catalina.core.ApplicationFilterConfig.initFilter(ApplicationFilterConfig.java:281)
	at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:262)
	at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:107)
	at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4775)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5452)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.startup.HostConfig.checkResources(HostConfig.java:1282)
	at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1446)
	at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:322)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
	at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
	at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1374)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1530)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1540)
	at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1519)
	at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.ClassCastException: org.apache.xerces.parsers.XML11Configuration cannot be cast to org.apache.xerces.xni.parser.XMLParserConfiguration
	at org.apache.xerces.parsers.DOMParser.<init>(Unknown Source)
	at org.apache.xerces.parsers.DOMParser.<init>(Unknown Source)
	at org.apache.xerces.jaxp.DocumentBuilderImpl.<init>(Unknown Source)
	at org.apache.xerces.jaxp.DocumentBuilderFactoryImpl.newDocumentBuilder(Unknown Source)
	at com.sun.org.apache.xalan.internal.xsltc.trax.SAX2DOM.<init>(SAX2DOM.java:69)
	at com.sun.org.apache.xalan.internal.xsltc.runtime.output.TransletOutputHandlerFactory.getSerializationHandler(TransletOutputHandlerFactory.java:187)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.getOutputHandler(TransformerImpl.java:392)
	at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerHandlerImpl.setResult(TransformerHandlerImpl.java:137)
	at com.opensymphony.xwork2.util.DomHelper$DOMBuilder.setup(DomHelper.java:213)
	at com.opensymphony.xwork2.util.DomHelper$DOMBuilder.<init>(DomHelper.java:198)
	at com.opensymphony.xwork2.util.DomHelper$DOMBuilder.<init>(DomHelper.java:189)
	at com.opensymphony.xwork2.util.DomHelper$DOMBuilder.<init>(DomHelper.java:175)
	at com.opensymphony.xwork2.util.DomHelper.parse(DomHelper.java:115)
	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadConfigurationFiles(XmlConfigurationProvider.java:830)
	... 23 more

   二：配置struts.xml
      配置的标签格式就不多说了，就提提需要注意的问题。
       （1）添加xml头部信息，别粗心。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
	"-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
	"http://struts.apache.org/dtds/struts-2.3.dtd">

缺少的话会是404.
      （2）<action name="LoginAction" class="com.huaxin.struts.LoginAction">  name的值必须和jsp页面中想要请求的Action一致。不一致时报如下错误

java.lang.NullPointerException
	org.apache.struts2.impl.StrutsActionProxy.getErrorMessage(StrutsActionProxy.java:69)
	com.opensymphony.xwork2.DefaultActionProxy.prepare(DefaultActionProxy.java:185)
	org.apache.struts2.impl.StrutsActionProxy.prepare(StrutsActionProxy.java:63)
	org.apache.struts2.impl.StrutsActionProxyFactory.createActionProxy(StrutsActionProxyFactory.java:39)
	com.opensymphony.xwork2.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:58)
	org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:501)
	org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:432)

因为是通过jsp中form的提交路径找到struts配置中的该Action类。
     （3）struts2中struts2配置文件文件名必须为struts.xml，在struts1中为struts—config.xml,而且该文件必须存放在项目中的src目录下，千万别放到WEB-INF下了。
   三：Action类的实现
       （1）写该类的时候在继承ActionSupport的时候记得重写execute（）方法就行了。
       （2）该类中所用到的参数名需和jsp页面表格中提交的参数名一致。比如类中有这两个属性

String username;
String password;

那么jsp中参数名必须一致。而get（），set（）方法最好用快捷键实现，如果采用以上属性名，则get（）方法必须为getUsername（） 注意大小写的错误。
     上面这些问题都是刚刚配置的时候遇到的问题，总之一切得细心细心再细心，一个字母的差错，说不定会浪费你一两个小时的时间去发现这个错误。