Tomcat, Comet and getOutputStream() has already been called for this response

Question

I am using Tomcat 6.0.32 with the Http11NioProtocol connector to implement long polling.

My comet processor is holding the HttpSevletResponse on a queue until an event is ready to send back. When it sends back the event it is writing a binary stream to an applet, so I use response.getOutputStream().write() to send data back. When writing to the stream it is synchronized.

When another web page in a second frame reloads (Independent JSP generated from SpringController in same container) occasionally I see this in the logs and the page fails to load.

Is it possible there is a concurrency error between a response generated from a comet processor and a regular servlet when both are requested at the same time?

The stack trace:

java.lang.IllegalStateException: getOutputStream() has already been called for this response
at org.apache.catalina.connector.Response.getWriter(Response.java:611)
at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:198)
at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
at javax.servlet.ServletResponseWrapper.getWriter(ServletResponseWrapper.java:112)
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.WEB_002dINF.jsp.sale_jsp._jspService(sale_jsp.java:96)

Have you tried using response.getWriter() instead? a la [This answer](http://stackoverflow.com/questions/1776142/getoutputstream-has-already-been-called-for-this-response/1776161#1776161) — Andrew, Mar 18 '11 at 15:41
I need to use getOutputStream as I am sending bytes to the applet, not text — Adam, Mar 18 '11 at 15:58

score 2 · Answer 1 · answered Dec 22 '11 at 14:26

As you can see in the stack trace, the JSP calls response.getWriter(), but the exception message says there was a call to response.getOutputStream() before. You cannot call both methods on the same response object.

This is the problem, I don't know how to solve it in your specific context..

score 0 · Answer 2 · answered Sep 22 '11 at 21:58

About the possible concurrency: I don'y think so. Your long pooling Servlet is executed in another Thread, with its own HttpRequest and its own HttpResponse.

Your problem is located in your JSP as indicated: sale_jsp.java:96
Something is already commited in the response, so you're not allowed to use getOutputStream().

And it's very very easy in a JSP, this is a simple example:

<%@ page language="java" contentType="text/html;" %>
<%@ page import="java.io.*" %>

<% OutputStream os = response.getOutputStream(); 
   os.write(......);
%>

The generated Servlet will, at least, print 3 '\n' in the output before OutputStream os = response.getOutputStream(), so you're done..
To avoid this, remove every white spaces between %> and <%, like this:

<%@ page language="java" contentType="text/html;"
    import="java.io.*"   %><%

    OutputStream os = response.getOutputStream(); 
   os.write(......);
%>

Configure your Tomcat to keep the code generated.
Have a look to the generated code for your sale.jsp , I'm sure you'll find out what's wrong.

Also, be careful with <%! you should declare some variables and share them unintentionally over ALL requests on this JSP this could lead to that kind of concurrency...

Tomcat, Comet and getOutputStream() has already been called for this response

2 Answers2