What are implicit objects? What does it mean?

Question

Whenever I study JSP and Servlets I come across word implicit objects, what does the term mean?

How they are called in my program without instantiating objects? Who instantiates implicit objects? Please elaborate.

Thanks

Answer 1

Those are objects which are already been placed in the scope by the servlet container, so that it's accessible by EL (Expression Language), such as the PageContext, HttpServletRequest#getParameter(), HttpServletRequest#getHeader() and so on. Those are just for convenience so that you don't need to use old-fahioned scriptlets to grab them.

So instead of for example

<%= pageContext.getSession().getMaxInactiveInterval() %><br>
<%= request.getParameter("foo") %><br>
<%= request.getHeader("user-agent") %><br>
<%  for (Cookie cookie : request.getCookies()) { // Watch out with NPE!
        if (cookie.getName().equals("foo")) {
            out.write(cookie.getValue());
        }
    }
%><br>

you can just do

${pageContext.session.maxInactiveInterval}<br>
${param.foo}<br>
${header['user-agent']}<br>
${cookie.foo}<br>

You see that they follows the JavaBean conventions to be accessed (i.e. you can just invoke the getters the JavaBean way). You see that I used the brace notation [] to get the user-agent, that's because the - is a reserved character in EL, so ${header.user-agent} isn't going to work, it would try to return the result of request.getHeader("user") - pageContext.findAttribute("agent") which is obviously not going to work.

For an overview of them all, check the chapter Implicit Objects in the Java EE tutorial.