struts中的ajax配置问题
在struts.xml中配置了一个package，extends="json-default",显示unknown tag,重启了tomcat,就报错,,
报错如下，，想问一下，要让struts返回json 数据，应该怎么配置，请详细点..感激不尽

严重: Dispatcher initialization failed

Unable to load configuration. - [unknown location]

	at com.opensymphony.xwork2.config.ConfigurationManager.getConfiguration(ConfigurationManager.java:70)

	at org.apache.struts2.dispatcher.Dispatcher.init_PreloadConfiguration(Dispatcher.java:429)

	at org.apache.struts2.dispatcher.Dispatcher.init(Dispatcher.java:473)

	at org.apache.struts2.dispatcher.ng.InitOperations.initDispatcher(InitOperations.java:74)

	at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.init(StrutsPrepareAndExecuteFilter.java:51)

	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:4746)

	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5399)

	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)

	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)

	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)

	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:633)

	at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1114)

	at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1672)

	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)

	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)

	at java.util.concurrent.FutureTask.run(FutureTask.java:138)

	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)

	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)

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

Caused by: Parent package is not defined: json-default - [unknown location]

	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.buildPackageContext(XmlConfigurationProvider.java:661)

	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.addPackage(XmlConfigurationProvider.java:510)

	at com.opensymphony.xwork2.config.providers.XmlConfigurationProvider.loadPackages(XmlConfigurationProvider.java:292)

	at org.apache.struts2.config.StrutsXmlConfigurationProvider.loadPackages(StrutsXmlConfigurationProvider.java:112)

	at com.opensymphony.xwork2.config.impl.DefaultConfiguration.reloadContainer(DefaultConfiguration.java:250)