session何时创建

没有 HttpServletRequest.getSession(true) ，Sesion是不会创建的， 即便Servlet又转发（或者重定向）到第二个页，只要第二个页(html)依然不是jsp页，Session就依然不会创建，如果是jsp则自动创建一个session对象，放到jsp的内置对象里面：jsp生成的servlet.java
//可以看到内置对象有session：
public void _jspService(HttpServletRequest request, HttpServletResponse response)
        throws java.io.IOException, ServletException {

    JspFactory _jspxFactory = null;
    PageContext pageContext = null;
    HttpSession session = null;
    ServletContext application = null;
    ServletConfig config = null;
    JspWriter out = null;
    Object page = this;
    JspWriter _jspx_out = null;
    PageContext _jspx_page_context = null;


    try {
      _jspxFactory = JspFactory.getDefaultFactory();
      response.setContentType("text/html;charset=UTF-8");
      pageContext = _jspxFactory.getPageContext(this, request, response,
      			null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("\r\n");
      out.write("\r\n");
      out.write("\r\n");
      out.write("<html>\r\n");
      out.write("<head>\r\n");

String selfUrl = (String) SystemParameter.getValue(ConfigClassType.SYSTEM, "SSO", "SelfUrl");

      out.write("\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("<SCRIPT language=\"javascript\">\r\n");
      out.write("<!--\r\n");
      out.write("top.window.location = '");
      out.print( request.getContextPath());
      out.write("/loginindex.do';\r\n");
      out.write("//-->\r\n");
      out.write("</SCRIPT> \r\n");
      out.write("<!-- top.window.location = '");
      out.print( request.getContextPath() + selfUrl);
      out.write("';-->\r\n");
      out.write("<!-- top.window.location = '");
      out.print( request.getContextPath());
      out.write("/loginindex.do';-->  \r\n");
      out.write("</body>\r\n");
      out.write("</html>");
    } catch (Throwable t) {
      if (!(t instanceof SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          out.clearBuffer();
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
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