getOutputStream() has already been called for this response] with root cause

The "getOutputStream() has already been called for this response" error typically occurs when you're trying to write or send data to the output stream of a response more than once. In Java servlets, the output stream is obtained from the HttpServletResponse object using the getOutputStream() method.

To resolve this issue, make sure you are not calling the getOutputStream() method multiple times within the same request. It's important to note that you can only call either getOutputStream() or getWriter() once in a single request.

If you need to write data to the output stream multiple times, you can use the getWriter() method instead of getOutputStream(). Alternatively, you can buffer your data and write it all at once to the output stream using either getOutputStream() or getWriter().

Here's an example of how to use getWriter() to write data to the response:

PrintWriter out = response.getWriter();
out.println("Your data here");

Remember to only call either getOutputStream() or getWriter() once in your code for each request.