导出Excel时出现错误 java.lang.IllegalStateException: getOutputStream() has already been called

今天在做Excel导出处理，在出现保存对话框选择保存的时候控制台出现了如下的错误：

java.lang.IllegalStateException: getOutputStream() has already been called for this response
	at org.apache.catalina.connector.Response.getWriter(Response.java:610)
	at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
	at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
	at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
	at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
	at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:180)
	at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:118)
	at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:77)
	at org.apache.jsp.report.error_jsp._jspService(error_jsp.java:73)
	at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
	at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
	at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:551)
	at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:488)
	at org.apache.struts2.dispatcher.ServletDispatcherResult.doExecute(ServletDispatcherResult.java:166)
	at org.apache.struts2.dispatcher.StrutsResultSupport.execute(StrutsResultSupport.java:186)
	at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:371)
	at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:275)
	at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
	at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:546)
	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 com.yao.filter.LoginFilter.doFilter(LoginFilter.java:66)
	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:298)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:852)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
	at java.lang.Thread.run(Thread.java:662)

上午查找资料发现这个异常时因为response.getOutputStream()跟response.getWriter()相冲突造成的，网上给出的解决方法是在调用response.getOutputStream()之前加上如下两行代码：

out.clearBuffer(); 
out = pageContext.pushBody();

其中out和pageContext均为JSP页面内置对象。

但是我并没有在页面做response的输出操作。

最终找到了适合我的情况的解决方法，即把Strust2的action中的最后的 return "success"改为 return null解决。原因是每个方法返回的是一个ActionForward对象，而response是ActionForward的对象参数，所以就会使response产生冲突，所以处理Excel导出的action返回null即可。