Today, I write a java file download program. After it’s finished, everything is normal, but it always throws out java.lang.IllegalStateException Abnormal, although it does not affect the normal use, but it always makes people feel very uncomfortable. There is nothing wrong with checking the code. Finally, I checked a lot of information on the Internet and finally found out the reason.

When we upload or download files, or filter files, we may need to use the output stream of the page.
for example, when we use it in action:
for example, when we use it in action response.reset ();
　　　　 response.setContentType (”application/ vnd.ms -excel”);
　　　　OutputStream os = response.getOutputStream ();
throw an exception: java.lang.IllegalStateException

Cause analysis:
this is a problem in the servlet code generated by the web container out.write (), which is invoked in JSP. response.getOutputStream () conflicts.
that is, the servlet specification states that it cannot be called either response.getOutputStream (), and then call response.getWriter (), no matter which one is called first, an IllegalStateException will be thrown when calling the second one,

Because in JSP, the out variable is generated through the response.getWriter It is used in the program response.getOutputStream , and the out variable is used, so the above error occurs.

solve:

Method 1: add the following two sentences to the JSP file

Defects of this method:
many development projects are not caused by JSP front-end, such as freemaker, velocity and so on“ response.getOutputStream () “not in JSP, but in servlet/action

Method 2: in action, do not return to the specific result file, return null
instead