Servlet: Cannot forward after response has been committed

Topic:  Servlets 2.4

Application Server: JBoss 4.0.2 / Tomcat 5.5.9

Exception:

java.lang.IllegalStateException: Cannot forward after response has been committed
        at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationD
ispatcher.java:313)
        at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDis
patcher.java:301)
        at com.HelloWorldServlet.doGet(HelloWorldServlet.java:49)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:697)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Appl
icationFilterChain.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationF
ilterChain.java:173)
        at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFi
lter.java:81)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Appl
icationFilterChain.java:202)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationF
ilterChain.java:173)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperV
alve.java:213)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextV
alve.java:178)
        at org.jboss.web.tomcat.security.CustomPrincipalValve.invoke(CustomPrinc
ipalValve.java:39)
        at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(Securit
yAssociationValve.java:153)
        at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValv
e.java:59)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.j
ava:126)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.j
ava:105)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineVal
ve.java:107)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.jav
a:148)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java
:856)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.proce
ssConnection(Http11Protocol.java:744)
        at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpo
int.java:527)
        at org.apache.tomcat.util.net.MasterSlaveWorkerThread.run(MasterSlaveWor
kerThread.java:112)
        at java.lang.Thread.run(Thread.java:619)

Resolution:

If the response has already been committed (that is, the response output buffer has been flushed) in the forwarding web component (Servlet/JSP), then the forward will not take place and the invocation to RequestDispatcher.forward() will result in an IllegalStateException. The client will then receive this already committed response only.

Only one servlet can commit a response, either the forwarding servlet or the servlet where the request has been forwarded.

Either remove the call to RequestDispatcher.forward() in the forwarding Servlet or remove the println() calls on the response (the objects that write to the response) in the forwarding Servlet.