Why do I get java.lang.IllegalStateException?

Why do I get java.lang.IllegalStateException?

These are the most common reasons how you can get an java.lang.IllegalStateException:

• Calling setBufferSize and content has been written.
• The response has been committed and you do any of the following:
  • Calling ServletResponse.reset() or ServletResponse.resetBuffer()
  • Calling either HttpServletResponse.sendError() or HttpServletResponse.sendRedirect().
  • Calling RequestDispatcher.forward() which includes performing a jsp:forward
  • Calling RequestDispatcher.forward() which includes performing a jsp:forward

Remember that if you call forward() or sendRedirect(), any following lines of code will still execute. For example:

	{ ... response.sendRedirect("foo.jsp"); // At this point, you should probably have a return statement otherwise // the following code will run System.out.println("After redirect! By the way ..."); System.out.println("Use commons-logging or log4j, not System.out"); System.out.println("System.out is a bad practice!"); response.sendRedirect("bar.jsp"); /* This will throw an error! */}