Web开发中常见的java.lang.IllegalStateException错误

现象

严重: Servlet.service() for servlet default threw exception
java.lang.IllegalStateException
    at org.apache.catalina.connector.ResponseFacade.sendError(ResponseFacade.java:407)
    at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:725)
    at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:483)
    at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
    at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:76)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at filters.TomcatFormFilter_UTF8.doFilter(TomcatFormFilter_UTF8.java:79)
    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:128)
    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.Http11AprProcessor.process(Http11AprProcessor.java:859)
    at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:574)
    at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1527)
    at java.lang.Thread.run(Unknown Source)

原因：

HttpServletResponse response = ServletActionContext.getResponse();
	response.setContentType("text/html;charset=gbk");
	response.setCharacterEncoding("gbk");
	
	PrintWriter out=response.getWriter();

解决：在action中，不要return回具体的result文件或者action，而是return null。

其他参考：http://blog.sina.com.cn/s/blog_6151984a0100owod.html