Why do I get java.lang.IllegalStateException? -优快云博客

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! */

}