An implicit variable is referenced in an EL expression

In addition to the 9 large implicit variables in the JSP (also called pre-defined variables in the previous article), you can see in the _jspservice after conversion into a servlet:

public void _jspservice (final javax.servlet.http.HttpServletRequest request, final Javax.servlet.http.HttpServletResponse response)
Throws Java.io.IOException, Javax.servlet.ServletException {

Final Javax.servlet.jsp.PageContext PageContext;
Javax.servlet.http.HttpSession session = NULL;
Final Javax.servlet.ServletContext Application;
Final Javax.servlet.ServletConfig config;
Javax.servlet.jsp.JspWriter out = null;
Final Java.lang.Object page = this;
Javax.servlet.jsp.JspWriter _jspx_out = null;
Javax.servlet.jsp.PageContext _jspx_page_context = null;

To improve efficiency, the following implicit objects are regulated in the El Expression:

(1) PageContext
The PageContext object references the current PageContext. PageContext. Class has the request, Response,session,out, and ServletContext properties (that is, getrequest,getresponse,getsession, Getout, Getservletcontext method). For example, the following expression outputs the current session ID

${pagecontext.session.id}

(2) Param and paramvalue
These objects allow us to access a basic request parameter bit (param) or an array of request parameter values (paramvalues). Therefore, the following expression outputs the value of the CustID request parameter (returns an empty string if the parameter does not exist in the current request). ${param.custid}

(3) Header and Headervalues
These objects access the main value of the HTTP request header and all values, respectively. In retrospect, you cannot use dot notation if the value after the dot number cannot be a valid name. Therefore, to access the accept header, we can use ${header. Accept}, or ${header["accept"}

Similar objects also have cookies, Initparam,pagescope,requestscope,sessionscope,applicationscope

An implicit variable is referenced in an EL expression