今天寫驗證碼的時候，遇到了這個問題：java.lang.IllegalStateException: getOutputStream() has already been called for this response
上網找到了以下知識，我只試了第二種方法確實好使！粘來供大家參考：
getOutputStream() has already been called for this response問題的解決
在jsp向頁面輸出圖片的時候,使用response.getOutputStream()會有這樣的提示：java.lang.IllegalStateException:getOutputStream() has already been called for this response,會拋出Exception
原因一:
JSP默認的輸出流爲PrintWriter ,即<% %>以外的東西所默認的輸出方式,如果你嘗試在JSP中使用ServletOutputStream就會引起錯誤.要嘛直接改用Servlet輸出(複寫service方法),要嘛刪除除%><%中的任何東西(包括HTML標籤,空格,回車等東西)應該就可以。
對於這樣的情況應該這樣來解決，刪除%><%之間的所有內容包括空格和換行符，最後也要消除空格和換行符，最好再加上一句response.reset()。
原因二:
在J2EE的API參考裏有這麼個:
ServletResponse的getWriter()方法裏會拋出這個異常,
IllegalStateException - if the getOutputStream method has already been called
for this response object
而它的getOutputStream()方法裏會拋出這個異常.
IllegalStateException - if the getOutputStream method has already been called for this response object
並且兩者的函數申明裏都有這麼樣的一句
Either this method or getOutputStream() may be called to write the body, not both.
Either this method or getWriter() may be called to write the body, not both.
以上說明也解釋了爲什麼在往頁面中寫入圖片的時候要使用如下循環格式
OutputStream output=response.getOutputStream();
while((len=in.read(b)) >0)
{
output.write(b,0,len);
}
output.flush();
而不是把response.getOutputStream().write()放到循環體內
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在做圖形驗證碼的時候隨便從網上找了段代碼，結果出現了這個異常，詳細內容如下：
(4361578 ms) [http-8080-Processor25] ERROR: org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/xixibar].[jsp]#invoke : Servlet.service() for servlet jsp threw exception
java.lang.IllegalStateException: getOutputStream() has already been called for this response
at org.apache.catalina.connector.Response.getWriter(Response.java:599)
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:195)
at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:124)
at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:117)
at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:182)
at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:115)
at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:75)
at org.apache.jsp.image_jsp._jspService(image_jsp.java:105)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:334)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
剛開始都沒看明白怎麼回事，後來找Google幫忙，找到如下解釋：
在tomcat5下jsp中出現此錯誤一般都是在jsp中使用了輸出流（如輸出圖片驗證碼，文件下載等），
沒有妥善處理好的原因。
具體的原因就是
在tomcat中jsp編譯成servlet之後在函數_jspService(HttpServletRequest request, HttpServletResponse response)的最後
有一段這樣的代碼
finally ...{
if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
}
這裏是在釋放在jsp中使用的對象，會調用response.getWriter(),因爲這個方法是和
response.getOutputStream()相沖突的！所以會出現以上這個異常。
然後當然是要提出解決的辦法，其實挺簡單的（並不是和某些朋友說的那樣--
將jsp內的所有空格和回車符號所有都刪除掉），
在使用完輸出流以後調用以下兩行代碼即可：
out.clear();
out = pageContext.pushBody();
