Why do we write out.println() in jsp instead of System.out.println()?

java, jsp

Solution

Because the `out` we're referring to isn't `System.out`, it's a variable in the effective method that wraps our JSP page. `System.out` writes to the servlet container's console (usually a log file); `out` is a different class entirely which writes to the output stream for the generated response.

When a JSP is turned into code, it (in theory, and with Tomcat, in fact) goes through two steps: JSP -> servlet source code, then servlet source code -> class. The entire page is put inside a method, which with Tomcat looks something like this:

public void _jspService(HttpServletRequest request, HttpServletResponse response)
    throws java.io.IOException, ServletException {

    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 {
        response.setContentType("text/html");
        pageContext = _jspxFactory.getPageContext(this, request, response,
                  "qdforumerror.jsp", true, 65536, true);
        _jspx_page_context = pageContext;
        application = pageContext.getServletContext();
        config = pageContext.getServletConfig();
        session = pageContext.getSession();
        out = pageContext.getOut();
        _jspx_out = out;

        /* =============================================
           ...your <% ... %> JSP code here, with
           any markup outside those tags converted into
           out.print("..."); statments...
           =============================================
        */
    }
    catch (Throwable t) {
        if (!(t instanceof SkipPageException)){
            out = _jspx_out;
            if (out != null && out.getBufferSize() != 0)
                try { out.clearBuffer(); } catch (java.io.IOException e) {}
            if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        }
      }
      finally {
          _jspxFactory.releasePageContext(_jspx_page_context);
    }
}

As you can see, `out` is a variable within that method, of type `JspWriter` (rather than `OutputStream` as with `System.out`).

(Side note: Code you include in `<%! ... %>` tags rather than the normal `<% ... %>` tags isn't put in the method; it's put elsewhere in the generated servlet class.)

Problem

I started learning jsp and I am seeing that, if we want to print something in jsp,we have to write `out.println()` instead of `System.out.println()`, but if we write `System.out.println()` it does not show any error but does not o/p to the browser also. I want to know why it happens? As all we know that `System` is a predefined class and `out` is the output stream connected to the console. So why we do not require to write `System` in jsp? Thanks.

Original source